Zirconium oxide closely resembles aluminium oxide or alumina. For a long time the letter effectively concealed the presence of the former. Nobody suspected an unknown element in zirconium minerals known as early as the Middle Ages. Thus, zirconium, one of the most abundant metals on Earth (0.02%) remained “invisible” up to the end of the 18th century. Today the mineral zircon is the main source of zirconium; it occurs in two varieties: hyacinth and jargoon. Already in old times hyacinth was known as a precious stone owing to its beautiful colours ranging from yellow–brown to smoky green.
It was believed that the composition of hyacinth was similar to that of ruby and topaz.
Zircon was analysed more than once and every time erroneously. In 1787 the German chemist J. Wiegleb, when analysing Ceylon zircon, found only silicon dioxide and small admixtures of lime, magnesia, and iron. Earlier such a skilled chemist as T. Bergman had established that Ceylon hyacinth contained 25% silicon dioxide, 40% aluminium oxide, 13% iron oxide, and 20% lime. The element known subsequently as zirconium was safely “hidden” in aluminium oxide.
This natural camouflage was revealed in 1789 by M. Klaproth. He heated zircon powder (a sample similar to that used by T. Bergman) with alkali in a silver crucible. The alloy was then dissolved in sulphuric acid and from the solution M. Klaproth separated a new earth which he named zirconium. His analytical results demonstrated 25 per cent silica, 0.5 per cent iron oxide, 70 per cent zirconium earth. As we see, there is nothing in common with Bergman’s results. In the same year Guyton de Morveau, separating zirconium from hyacinth found in France, confirmed Klaproth’s results.
Preparing metallic zirconium turned out to be not so simple. In 1808 H. Davy tried in vain to decompose zirconium earth with electric current. It was not before 1824 that Berzelius obtained contaminated zirconium by heating a dry mixture of potassium, potassium fluoride, and zirconium in a platinum crucible. Zirconium received its name from the mineral.
In 1787 a new mineral, strontianite, was found in a lead mine near the village of Strontian in Scotland. Some mineralogists classified it as a variety of fluorite (CaF2). The majority of scientists, however, believed that strontianite was a variety of witherite (barium mineral BaCO3).
In 1790 the Scottish physician A. Crawford thoroughly studied the mineral and came to conclusion that the salt obtained by the action of hydrochloric acid in strontianite differed from barium chloride. It dissolved in water more readily and its crystals were of different shape. Crawford decided that strontianite contained a previously unknown earth.
At the end of 1791 the Scottish chemist T. Hope concerned himself with studying strontianite, and established the difference between witherite and strontianite. Hope also noted that the strontium earth reacted with water more vigorously than quicklime; it dissolved in water much more readily than barium oxide, and all strontium compounds turned the flame red. T. Hope proved that the new earth could not be a mixture of calcium and barium earths. Lavoisier suggested that the new earth was of metallic nature but only H. Davy succeeded in proving it in 1808.
The history of the discovery of strontium would be incomplete if we did not mention another scientist to whom, undoubtedly, a great deal of credit for studying strontianite should be given. He was the Russian chemist T. E. Lovits who concluded, independently of other scientists, that strontianite contained an unknown element. Lovits was the first to discover strontium in heavy spar. The method of preparing metallic strontium suggested by H. Davy did not yield a sufficiently pure product. It was only in 1924 that P. Danner (USA) obtained pure strontium by reducing its oxide with metallic aluminium or magnesium.
In the second half of the 18th century a strange bluish–white ore was discovered in Austria or, to be more exact, in the part of it that was called Siebengebirge (Seven Mountains). It was strange because there was no common opinion about its composition. The debates mainly revolved around the question whether it contained gold or not. Its names were also unusual: paradoxical gold, white gold, and finally problematic gold. Some scientists believed that there was no problem at all, and the ore, most likely, contained antimony or bismuth, or both. In 1782 the mining engineer I. Muller (later Baron von Reichenstein) subjected the ore to a thorough chemical analysis and extracted metal reguluses from it which, as it seemed to him, closely resembled antimony. But in the following year he decided that in spite of the resemblance, he was dealing with a new, previously unknown metal. Not relying upon his own opinion, the scientist consulted T. Bergman. But the sample of the ore sent to Bergman was too small to come to a definite conclusion. It was only possible to establish that Muller’s metal was not antimony, and that was the end of the matter. During the next fifteen years nobody recalled the discovery of the Austrian mining engineer. Tellurium’s real birth was still ahead.
Its second birth was promoted by the German chemist M. Klaproth. At the Berlin Academy of Sciences session on January 25, 1798, he reported about the gold–bearing ore from “Seven Mountains”. Klaproth repeated what Muller had done in his time. But if the latter was in doubt there was no doubt for M. Klaproth. He named the new element “tellurium” (from the Latin tellus for “Earth”). Although Klaporth had received the reo sample from Muller, he did not want to share the glory of the discoverer of tellurium with him; we, for our part, think that the role of the German chemist was no less important. At any rate he revived the forgotten element.
There is reason to believe that a third person was also involved in the discovery of tellurium. He was P. Kiteibel, a Professor of the Pest University in Hungry, a chemist and botanist. In 1789 he received a mineral which was assumed to be molybdenite containing silver from a colleague. P. Kiteibel extracted a new element from it. Then he established that the same element was present in problematic gold. Thus, P. Kiteibel discovered tellurium independently of other scientists. It is a pity that he did not publish at once his findings but instead sent a description of his investigation to some of his colleagues and, in particular, to the Viennese mineralogist F. Estner. M. Klaporth learned about Kiteibel’s results through F. Estner and spoke favourably of them without actually corroborating them. I. Muller wrote to M. Klaproth severalyears later and the latter found time to reproduce the results of his correspondent. After that Klaproth considered him to be the only author of the discovery, and he underlined this in his report.
For a long time tellurium was regarded as metal. In 1832 Berzelius showed its great similarity with selenium and sulphur and tellurium was once and forever classified as a non–metal.
Although tungsten is also a rare element, it was discovered (in the form of its oxide) as early as the last quarter of the 18th century. To some extent it was a matter of chance but progress in analytical chemistry also contributed to the discovery of tungsten.
The name “tungsten” appeared much earlier. In German it means “wolf’s froth”. The point is that in smelting of tin from some ores a part of smelted metal was irretrievably lost. Medieval miners believed that tin was “devoured” by the mineral that was contained in the ore like a sheep is devoured by a wolf. This mineral was named tungsten or wolframite. As time passed, tungsten attracted ever increasing attention of the scientists. In 1761 the German mineralogist I. Lemann analysed wolframite but did not find any new components in it. His compatriot P. Wolf for his part said that wolframite contained “something”. Another strange mineral, “tungsten” or “heavy stone”, was also known. It was found in 1751 by A. Cronstedt. In 1781 this mineral attracted the attention of C. Scheele who treated tungsten (calcium wolframate) with nitric acid and obtained a white substance resembling molybdic acid. An analyst par excellence, Scheele showed the difference between the two acids and, consequently, he is considered to be the discoverer of tungsten.
At the same time T. Bergman, Scheele’s compatriot, was also at the threshold of discovery. In his opinion, tungsten, due to its high density, could contain baryta earth. Studying the mineral, the scientist found a white substance in it which he called tungstic acid. But after that Bergman followed a wrong path, believing that both tungstic and molybdic acids were arsenic derivatives; however, he did not check this assumption. In 1783 two Spanish chemists, the F. and H. D’ Egluar (brothers), separated a while acid from wolframite which proved to be similar to tungstic acid. Like Bergman and Scheele, the Spanish chemists succeeded in extracting metallic tungsten.
The molybdenum story is not rich in events. It is even trivial. Only one detail is of interest: this rare element was discovered very early, namely, in 1778, when the chemical analysis was just coming of age. Molybdenum was first separated in the form of oxide. The name “molybdenum” had appeared long before the new element was discovered. It originates from the Greek names molybdena for a lead mineral (lead glance) and molybdos for “lead”, the two resembling each other. There was another mineral which also resembled these two very much; later it became known as molybdenite (molybdenum sulphide).
In 1754 the Swedish mineralogist A. Cronstedt differentiated these minerals, saying that molybdenite possessed some peculiar properties. But proof of that was required. By a lucky coincidence, the report of the molybdenite study fell into Scheele’s hands. In 1778 he performed and analysis of molybdenite. The treatment of molybdenite with strong nitric acid resulted in the formation of a bulky white mass which Scheele described as a peculiar white earth. At the same time nitric acid had no effect on graphite. Thus, the difference between graphite and molybdenite became evident. Scheele named the white earth “molybdic acid” since it had acid properties. Having calcinated molybdic acid, the swedish chemist obtained molybdenum oxide, i.e. an oxide of a new metal. This is what Scheele believed and his belief was shared by his compatriot T. Bergman.
After that it was important to extract the metal from the molybdic earth. To do that Scheele planned to calcinate the earth with coal. But for some reason he could not perform the reaction himself and asked his friend P. Hjelm to do it. In 1790 Hjelm complied with the request. However, molybdenum obtained by him was contaminated with carbon and molybdenum carbide. The credit for preparing pure molybdenum (by reduction of the oxide with hydrogen) went to J. Berzelius (1817).
Barium, as well as his analogues in the second group of the periodic table, is not encountered in nature in the native state. Sulphates and carbonates are the most typical barium minerals. One of barium minerals attracted attention of alchemists back in the early 17th century (in 1602, to be exact).
In that year V. Casciaralo, a shoemaker from Bologna, noted that heavy spar (barium sulphate), heated with coal and drying oil and then cooled to room temperature began to emit a reddish glow. The mineral, named Bologna stone, Bologna phosphorus, sunstone, and so on, was barium sulphide BaS. The unusual luminescence was immediately interpreted in many different ways. For instance, the French chemist N. Lémery wrote in his “Chemistry Course” that the ability of Bologna stone to luminesce in the dark is due to the presence of sulphur. Another mineral displaying this property is Bolduin phosphorus (anhydrous calcium nitrate).
For a long time (up to 1774) heavy spar was confused with limestone; they were believed to be two varieties of the same compound. In 1774 Scheele, studying pyrolusite together with Gahn, discovered a new compound which gave a white precipitate under the action of sulphuric acid. Scheele established that heavy spar contained an unknown earth which was named “baryta” one. By the last quarter of the 18th century barium oxide was known rather well; it was suggested that it contained an unknown metal. This belief was supported by A. Lavoisier in his “Textbook of Chemistry”. In “The Table of Simple Bodies” barite is considered a simple substance. However, only in 1808 did H. Davy succeed in preparing the new metal by the method which he had used for obtaining calcium (see chapter 5).
The name of barium originates from the Greek baros (heavy) since barium oxide and its minerals were primarily characterized by their great mass.
Manganese compounds and, in particular, its oxidepyrolusite (MnO2)–have been known from ancient times and used for making glass and pottery. In 1540 V. Biringuccio, an Italian metallurgist, wrote that pyrolusite was brown, did not melt, and gave a violet colour to glass and ceramic when added to them. Another characteristic of pyrolusite was observed–its ability to make clear yellow and green glasses.
The Austrian scientist I. Kaim seems to be the first to have obtained a small amount of metallic manganese in 1770. He heated a mixture consisting of one part of pyrolusite powder and two parts of black flux (i.e. a mixture of coal with K2CO3) and obtained bluish–white brittle crystals. Apparently, it was contaminated manganese but the scientist only concluded that the metal obtained was iron–free and did not complete his studies.
The subsequent story of manganese is associated with T. Bergman who by that time had already confirmed the discovery of nickel. He characterized pyrolusite in the following way: the mineral called “black magnesium” is a new earth; it should not be confused either with roasted lime or with “magnesium alba” (i.e. magnesium oxide). However, T. Bergman failed to separate the metal from pyrolusite, in contrast to I. Kaim.
Scheele was the third chemist who tried to separate a new element form this mineral. In 1774 he submitted his paper “On Manganese and Its Properties” to the Stockholm Academy of Sciences; in it he summed up the three years of studies of pyrolusite. In this extremely informative paper he reported the discovery of two metals (barium and manganese) and described two gaseous elements (later identified as chlorine and oxygen). Scheele established that manganese oxide differed from all earths known at the time.
There are two significant dates in the history of manganese: May 16 and June 27, 1774. On May 16 Scheele sent I. Gahn, his friend and compatriot, a sample of purified pyrolusite and asked him to decompose it. Gahn placed a mixture of pyrolusite powder, oil, and ground coal into a coal crucible and calcinated it for an hour. On the bottom of the crucible he found a regulus of the metal whose weight was only one third that of the initial pyrolusite. On June 27, having received from Gahn the sample of the new metal, Scheele wrote to his colleague that he considered the regulus obtained from pyrolusite a new semi–metal different from all other semi–metals and closely resembling iron. The term “semi–metal” was retained in chemistry and metallurgy for some time. Thus, Gahn succeeded in separating metallic manganese. It may be said that he completed the discovery of this element, although manganese obtained by him had a high carbon content (pure metal was obtained later).
In 1785 the German chemist J. Ilsemann obtained metallic manganese independently of Gahn and Scheele by heating a mixture of pyrolusite, fluorspar, lime, and coal powder. The product was intensely calcinated. The resulting manganese was, moreover, even less pure. At first the metal was named in Latin “manganesium” which derived from the old name of pyrolusite “Lapis manganesis”. When in 1808 magnesium was obtained, to avoid confusion the Latin name of manganese was changed for “manganum”.
Nickel has very much in common with Cobalt, its neighbour in the periodic table. First of all, Nickel is also of “devilish” origin. Its name derives from the German “kupfer–nickel” (“copper devil”) and belongs to the mineral described in 1694 by the Swedish mineralogist U. Hierne, who mistook it for copper ore.
When repeated attempts to smelt copper from it failed, the metallurgists decided that it must have been Nick, the evil spirit of the mountains, at his tricks.
People came to know Nickel ages ago. Back in the 3rd century B.C the Chinese made an alloy of copper, nickel, and zinc. In the Central Asian state of Bactria coins were made from this alloy. One of them is now in the British Museum in London.
Confusion about the composition of kupfernickel remained even after the mineral had been described. In 1726 the German chemist I. Link studied the mineral and established that its dissolution in nitric acid yields a green colour. He concluded that the mineral was most probably a cobalt ore with admixtures of copper. When Swedish miners found a reddish mineral which, being added to glass, did not produce a blue colour, they named it “cobold that had lost his soul”. It was also one of the nickel minerals.
That was how matters stood up to 1751. That year the Swedish mineralogist and chemist A. Cronstedt took an interest in the mineral found in a cobalt mine. In one of his experiments he immersed a small piece of iron into an acid solution of this ore. Had copper been present in the solution, it would have been deposited on iron in a free state. To his great surprise nothing of the kind happened. The solution did not contain copper. This contradicted the existing beliefs about this ore. Cronstedt began a thorough investigation of the green crystals dispersed in the ore. After a great number of experiments, he isolated a metal from kupfernickel which did not resemble copper at all. Cronstedt described this metal as solid and brittle, weakly attracted by a magnet, transforming into a black powder when heated, and yielding a wonderful green colour upon dissolution. Cronstedt concluded that, since the metal was contained in kupfernickel, the name could be retained and shortened to nickel. At present it is known that kupfernickel is nickel arsenide.
Many chemists in Europe recognized that a new element had been discovered. But some scientists held that nickel was mixture of cobalt, iron, arsenic and copper. All doubts were removed in 1775 by T. Bergman who showed that mixtures of these elements taken in any proportions did not possess the properties of Nickel.
The history of the discovery of cobalt can conveniently be started with the history of its name. Cobalt owes its name to the mineral which medieval Saxony miners named “cobold” after the evil spirit who was assumed to inhabit the mineral. This mineral closely resembled silver ore but all attempts to produce silver from it were unsuccessful.
Blue cobalt glasses, enamels, and pigments were known as early as 5,000 years ago in ancient Egypt. In pharaoh Tutankhamen’s tomb archaeologists found fragments of blue glass. It is not known whether the preparation of blue glasses and paints on the basis of cobalt compounds was due to chance or whether it was a conscious effort. At any rate the method of their preparation remained unknown for a long time. Its first mention dates back to 1679.
Cobalt was discovered by the Swedish chemist G. Brandt in 1735. In his “Dissertation on Semi–metals” Brandt wrote about a new semi–metal, cobold, discovered by him. By semi–metals the scientist meant compounds whose properties resemble those of known metals but which are not malleable. He described six semi–metals: mercury, bismuth, zinc, antimony, cobalt, and arsenic. Since the majority of bismuth ores contain cobalt, G. Brandt proposed several methods to distinguish between cobalt and bismuth.
In 1744 G. Brandt found a new mineral which contained cobalt, iron, and sulphur. It proved to be cobalt sulphide Co3S4.
Later G. Brandt studied cobalt in detail. At the turn of the 18th century cobalt and its compounds were studied by T. Bergman, L. Thenard, L. Proust, and J. Berzelius, which made cobalt a well–investigated element. It must be added that for a long time many chemists did not believe in the discovery of cobalt. In 1776 the Hungarian chemist P. Padaxe said that cobalt was a compound of iron and arsenic; but he considered nickel, which had already been discovered by that time, to be a chemical element. Only by the end of the 18th century, the efforts of many scientists confirmed the discovery of G. Brandt.
Cobalt, as well as nickel, is often present (and sometimes in great amounts) in meteorites. In 1819 the Germa chemist F. Stromeyer reported the discovery of cobalt in a meteorite and somewhat later S. Tennant found nickel in the same meteorite.
One can safely say that none of the chemical elements played such an important role in the development of chemistry as oxygen. This life–giving gas enabled chemistry to make such great progress at the end of the 18th century which had never been possible before. First of all, oxygen overturned the phlogistic theory which had seemed immovable. Erroneous as it was, this theory was undoubtedly of some historical usefulness. For the time being the theory of phlogiston made it possible somehow to systematize the existing chemical conceptions and to consider various processes in nature and laboratory from a common (though erroneous) standpoint. This gave a certain purposefulness to the research. Hydrogen and nitrogen were found from the phlogistic conceptions but the study of these “varieties of air” made it possible to accumulate new facts whose explanation demanded a different approach. Figuratively speaking, chemistry needed a new look at itself, and oxygen made it possible.
In defiance of the theory of phlogiston, vague conjectures were repeatedly made that combustion of inflammable compounds and calcination of metals drew a “substance” form the air. In 1673 R. Boyle concluded that when lead and antimony are calcinated a very fine “materia ignea” passes into the metals and combines with them, increasing their weight. “…the good Robert Boyle’s opinion is false,” Lomonosov wrote 80 years later. The famous Russian scientist said that air participates in the processes of combustion–particles form the air mix with the compound being calcinated and increase its mass.
In the period when pneumatic chemistry was gaining ground, the French Chemist P. Bayan wrote a paper (1774) in which he discussed the causes for an increase in the mass of metals during calcination. He believed that a peculiar variety of air–an expansible fluid, heavier than ordinary air–was added to a metal in the process of calcination. Bayen obtained this fluid by thermal decomposition of mercury compounds. And conversely, acting on metallic mercury, the fluid transformed it into a red compound.
Bayen, unfortunately, only established the facts and did not pursue the subject further. However, you will see later that the scientist was actually dealing with oxygen. Remember two things: the data 1774 and the compound observed by Bayen–red mercury oxide. In the same year J. Priestley experimented with the same compound. Shortly before, he had discovered that in the presence of green plants fixed air, not supporting respiration, turned into ordinary air suitable for respiration by living organisms. This fact was extremely important not only for chemistry but for biology as well. Priestley proved for the first time that plants release oxygen.
In the early 1770’s so–called saltpeter gas was already known. It was liberated in the reaction of diluted nitric acid with iron shavings (it is nitrogen oxide in modern terminology). It turned out that saltpeter gas can be transformed (by its reaction with iron dust) into another variety of air supporting combustion but not supporting respiration. Thus, J. Priestley discovered another nitrogen oxide, N2O, and named it, according to the logic of the phlogistic theory, dephlogisticated saltpeter gas.
August 1, 1774, which was to become a milestone in the history of chemistry, was a usual day of hard work for J. Priestley. He placed red mercury oxide into a sealed vessel and directed on it sunbeams, focused with a big lens. The compound began to decompose yielding bright metallic mercury and a gas (several years later this gas would be named “oxygen” and become the third elemental gas). Unlike nitrogen, oxygen was not initially isolated from the atmosphere. The new air variety was extracted from a solid. The gas discovered by Priestley proved to be suitable for respiration. A candle burnt in the atmosphere of this gas much brighter than in ordinary air. Nothing was observed when the new gas was mixed with air but, being mixed with saltpeter, the gas yielded brownish vapours (known at present to be NO2 formed from NO). A similar, although not so pronounced, picture was observed when saltpeter was reacted with ordinary air. Priestley had only to say: “The new gas is a component of air.” But he was not ready yet to do so and named the new variety of air “dephlogisticated” air–something quite natural for a follower of the phlogistic theory.
After the discovery–and this is an important detail in the history of oxygen–Priestley left for Paris where he told Lavoisier and some other French scientists about his experiments. Lavoisier appreciated at once the importance of the experiment of his English colleague–he had a much clearer idea about it than Priestley. But Priestley kept talking about dephlogisticated air, still in the grip of his delusion (which is another proof of the vitality of the phlogistic theory). Unable to see the greatness of his own discovery, Priestley considered dephlogisticated air to be a complex substance. Only in 1786, influenced by the ideas of Lavoisier, did he began to view it as an elemental gas.
Thus, we owe the discovery of oxygen to P. Bayen and J. Priestley. However, a third name should be added–that of the famous Swedish chemist C. Scheele. It became widely known to the scientific community when Scheele published the book chemical Treatise About Air and Fire. Written in 1775, it appeared only two years later for which the publisher was to blame. This disappointing fact deprived Scheele of the right to be named the discoverer of oxygen although he described it as early as 1772 and his description was much more detailed and accurate than that of Bayen and Priestley. Scheele obtained oxygen (“fiery air”) in various ways, by decomposing inorganic compounds. Distillation of saltpeter with sulphuric acid yielded brown vapours.
Which became colourless at high temperatures. Scheele collected these vapours and named the new gas “fiery air” In this gas, like in Priestley’s, a candle burned much brighter than in ordinary air. Scheele believed that fiery air was a component of ordinary air. Mixing it with mephitic or corrupted air of Rutherford, Scheele prepared a mixture which did not differ at all from ordinary air. In fact, the scientist realized that atmospheric air is a mixture of gases which later were to be known as nitrogen and oxygen. However, this seems to be easy only with our superior knowledge. Scheele was deluded by his devotion to the phlogistic theory. Burning inflammable air (hydrogen) in a vessel with air, the scientist did not detect any products of the reaction of inflammable air with fiery one. His conclusion was that this reaction produced heat. Scheele reasoned that fiery air, combining with phlogiston, produces heat (which had, according to Scheele, a material nature) whose “decomposition” yields fiery air.
Scheele discovered fiery air knowing nothing about Priestley’s experiments and informed Lavoisier about it on September 30, 1774. Regretfully, Scheele’s results were published too late. Had they appeared earlier, the difficult and contradictory process of elucidating the nature of elemental gases would have been accelerated.
Their real understanding was made possible by Lavoisier, one of the most outstanding chemists of all times. During the period dominated by the phlogistic theory, vast experimental material was accumulated which led to revolutionary changes in chemistry. The main credit for this goes to A. Lavoisier owing to whom oxygen was properly understood. F. Engels wrote: “Lavoisier was able to discover in the oxygen obtained by Priestley the real antipode to the fantastic phlogiston and thus could throw overboard the entire phlogistic theory. But this did not in the least do away with the experimental results of phlogistics. On the contrary, they persisted, only their formulation was inverted, was translated from the phlogistic into the now valid chemical language and thus they retained their validity.”
Lavoisier’s road to the discovery of oxygen was much straighter than that of his contemporaries. At first the French scientist also appealed to the phlogistic theory, but the more experimental facts he obtained, the more inclined he became to discard it. By November1, 1772, he had finished the description of his experiments on the combustion of various compounds in air. He concluded that the mass of all substances, including metals, increases upon combustion and calcination.
Since these processes require a great amount of air, A. Lavoisier made another conclusion: air is a mixture of gases with different properties. A certain part of it supports combustion and becomes bonded to the burning substance. At first A. Lavoisier assumed that this type of air is similar to the fixed air of J. Black but soon he saw his error. In February 1774, the French scientist discovered that air which interacts with a substance during combustion is most suitable for respiration. Thus, A. Lavoisier met face to face with oxygen but refrained from announcing the discovery of a new gas since he was going to perform some additional experiments.
In October 1774, Priestley reported to Lavoisier about his discovery revealing to the French chemist the real significance of his own findings. He immediately began to experiment with red mercury oxide which was the most suitable “generator” of oxygen. In April 1775, Lavoisier made a report to the Academy of sciences: “Memoir on the Nature of the Substance Which Combines with metals upon calcination and Increases their weight”–the announcement of the discovery of oxygen. Lavoisier wrote that this type of air had been discovered almost simultaneously by Priestley, Scheele, and by him. At first he said that it was “very easily inhaled air” but then changed the name for “life–giving” or invigorating” air.
This fact alone shows how far behind Lavoisier left his contemporaries in understanding the nature of oxygen. Invigorating air became the subject of comprehensive studies. At a later stage the scientist came to the conclusion that “the most easily inhaled air” is an acid–forming principle, the most important part of all acids. Later it was shown that this belief was erroneous (when oxygen–free acids were described with hydrohalic acids as an example). But in 1779 Lavoisier thought it possible to reflect this property of the new gas in its name of “oxygen” derived from the Greek for “producing acid”.
Determination of the water composition became a major advance of the oxygen theory. In 1781 H. Cavendish observed that inflammable air upon combustion is transformed almost completely (together with dephlogisticated air) into pure water. But he published his results only in 1784. Lavoisier knew about these experiments and, after repeating them, he concluded that water is not a simple substance but a mixture of inflammable and invigorating air. Since the conclusion was made in 1783, Lavoisier is held by many to be the first one to have established the composition of water. In reality, however, H. Cavendish was the first. Determination of the composition of water made it possible to get an insight into the nature of hydrogen.
What makes the history of the discovery of oxygen interesting is that the process was not a single event. Several stages were passed: from an empirical observation of oxygen to a proper understanding of its nature as a gaseous chemical element. It should also be mentioned that the discovery of oxygen (as well as of other elemental gases) was not the doing of one man. Engels wrote: “Priestley and Scheele had produced oxygen without knowing what they had laid their hands on …. And although Lavoisier did not produce oxygen simultaneously and independently of the other two, as he claimed later on, he nevertheless is the real discoverer of oxygen vis–à–vis other who had only produced it without knowing what they had produced.”
The study of the atmosphere led to the discovery of nitrogen. Although it is associated with the name of a certain scientist and a certain date, this certainly is misleading. It is rather difficult to separate the history of nitrogen discovery from the mainstream of pneumatic chemistry; one can only think of a more or less logical sequence of events.
Very early in history man came across nitrogen compounds, for instance, saltpeter and nitric acid, frequently observing liberation of brown vapours of nitrogen dioxide. Obviously, it would be impossible to discover nitrogen by decomposing its inorganic compounds. Tasteless, colourless, odorless, and chemically rather inactive, nitrogen would have remained unnoticed.
Therefore, it is not easy to decide where to begin the story of the discovery of nitrogen. Although our choice may seem subjective, we start with 1767 when H. Cavendish and J. Priestley, another outstanding English physicist, chemist, and philosopher, set out to study the action of electric discharges on various gases. There were only few such gases at that time: ordinary air, fixed air, and inflammable air. Although the experiments did not produce definite results, it was shown later that electric discharge in humid air yields nitric acid. Later this fact proved to be useful for the analysis of the earth’s atmosphere.
In 1777 H. Cavendish reported in a private letter to J. Priestley that he had succeeded in preparing a new variety of air named by him asphyxiating or mephitic air. Cavendish repeatedly passed atmospheric air over red–hot coal. The resulting fixed air was absorbed with alkali. The residue was mephitic gas. Cavendish did not study it thoroughly and only reported the fact to Priestley. Cavendish returned to the study of mephitic air much later, did a large work but the credit for the discovery had already gone to another scientist.
When Priestley received the letter from Cavendish he was busy with important experiments and read it without due attention. Priestley burned various inflammable compounds in a given volume of air and calcinated metals; the fixed air formed during these processes was removed with the aid of limewater. The main thing which he noticed was that the volume of air decreased considerably. A reader will prompt that as a result of metal calcination or combustion of compounds the oxygen present in the apparatus was bonded and nitrogen remained. Priestley, however, had no idea about the existence of such a gas as oxygen (two years later, however, he became one of its discoverers) and, to explain the observed phenomenon, he turned to phlogiston. Priestley believed that the result of metal calcination was due exclusively to the action of phlogiston. The remaining air is saturated with phlogiston and, consequently, it can be named “phlogisticated” air; it sustains neither respiration nor combustion.
Thus, Priestley was in possession of a gas which subsequently became known as nitrogen. But this extremely important result was treated by him as something of secondary importance. The existence of “phlogisticated” air was for Priestley evidence of the fact that phlogiston did play a role in natural processes. This story shows once more how the erroneous phlogistic theory hampered the discovery of elemental gases.
So, neither Cavendish nor Priestley could understand the real nature of the new gas. The understanding came later when oxygen appeared on the scene of chemistry. The English physician D. Rutherford, the pupil of J. Black, who is considered to be the discoverer of nitrogen, did, in principle, nothing new compared with his famous colleagues. In September 1772, Rutherford published a magisterial thesis On the So–called fixed and Mephitic Air in which he described the properties of nitrogen. This gas, according to Rutherford, was absorbed neither by limewater nor by alkali and was unsuitable for respiration; he named it “corrupted” air.
Not properly discovered or understood as a gaseous chemical element, nitrogen in the seventies of the 18th century had three names which confused still more the fuzzy chemical concept muddled by the persisting influence of the phlogistic theory. Phlogisticated, mephitic, or corrupted air was yet receive its final name.
This name was proposed in 1787 by A. Lavoisier and other French scientists who developed the principles of as new chemical nomenclature. They derived the word “azote” from the Greek negative prefix “a” and the word “zoe” meaning “life”. Lifeless, not supporting respiration and combustion, that was how the chemists saw the main property of nitrogen. Later this view turned out to be erroneous: nitrogen is vitally important for plants. The name “azote”, however, remained. The symbol of the element, N, originates from the Latin nitrogenium which means “saltpeter–forming”.Cavendish studied the properties of nitrogen in detail. He was one of the first scientists to believe that phlogisticated air is a component of ordinary air. One day, in the course of his experiments Cavendish questioned the homogeneity of phlogisticated air. He passed an electric spark through its mixture with oxygen transforming the whole into nitrogen oxides which were removed from the reaction zone. But every time a small fraction of the phlogisticated air (nitrogen) remained unchanged and did not react with oxygen. It was a very small fraction, a slightly noticeable gas bubble–only 1/125 of all nitrogen taken for the phenomenon observed in 1785. The answer was found only over one hundred years later. You will read about it in chapter 9 devoted to inert gases.
Any object is called as symmetrical if it has mirror symmetry, or ‘left-right’ symmetry i.e. it would look the same in a mirror.
For example : a cube , a matchbox, a circle
Further it can be said ; a sphere is more symmetrical compare to a cube. Cube looks the same after rotation through any angle about the diameter while during the rotation of a cube, it looks similar only with certain angles like 90°, 180°, or 270° about an axis passing from the centers of any of its opposite faces, or by 120° or 240° about an axis passing from any of the opposite corners.
Similarly Molecules can also be classified as Symmetric or Asymmetric.
Symmetry Operations
An action on an object which leaves the object at same position after the action carried out. Such type of action is called as Symmetry operations.
All known molecules can classify in groups possess the same set of symmetry elements.
Such type of classification is helpful to assign the molecular properties without calculation and in the determination of polarity and degeneracy of molecular states.
It provides the systematic treatment of symmetry in chemical systems in a mathematical framework which is called as group theory.
Group theory is also helpful is some other investigations
Prediction of polarity and chirality of molecule.
In examination of bonding and visualizing molecular orbitals of molecules.
In prediction of polarization a molecule.
In investigation of vibrational motions of the molecule.
The simplest example of symmetry operations is water molecule. If we rotate the molecule by 180° about an axis which is passing through the central Oxygen atom it will look the same as before. Similarly reflection of molecule through both axis of molecule show same molecule.
There are five types of symmetry operations and five types of symmetry elements.
1. The identity (E) This symmetry operation is consists of doing nothing. In other words; any object undergo this symmetry operation and every molecule consists of at least this symmetry operation. For example; bio molecules like DNA and bromo fluoro chloro methane consist of only this symmetry operation. ‘E’ notation used to represent identity operation which is coming from a German word ‘Einheit’stands for unity.
2. An n-fold axis of symmetry (Cn)
This symmetry operation involves the clockwise rotation of molecule through an angle of 2 π /n radian or 360º/n where n is an integer. The notation used for n-fold of axis is Cn.
For principal axis, the value of n will be highest. The rotation through 360°/n angle is equivalent to identity (E).
For example, one twofold axis rotation of water (H2O) towards oxygen axis leaves molecule at same position, hence has C2axis of symmetry. Similarly ammonia (NH3) has one threefold axis, C3 and benzene (C6H6) molecule has one sixfold axis C6and six twofold axis (C2) of symmetry.
Linear diatomic molecules like hydrogen, hydrogen chloride have C∞ axis as the rotation on any angle remains the molecule the same.
3. Improper rotation (Sn)
Improper clockwise rotation through the angle of 2π/n radians is represented by notation ‘Sn’ and called as n-fold axis of symmetry which is a combination of two successive transformations. During improper rotation, the first rotation is through 360°/n and the second transformation is a reflection through a plane perpendicular to the axis of the rotation. Improper rotation is also known as alternating axis of symmetry or rotation-reflection axis. For example; methane (CH4) molecule has three S4axis of symmetry.
4. A plane of symmetry (σ)
There are some plane in molecule through which reflection leaves the molecule same. The vertical mirror plane is labelled as σv and one perpendicular to the axis is called a horizontal mirror plane is labelled as σh , while the vertical mirror plane which bisects the angle between two C2axes is known as a dihedral mirror plane, σd. For example, H2O molecule contains two mirror planes (a YZ Reflection (σyz) and a XZ Reflection (σxz)) which are mirror planes contain the principle axis and called as vertical mirror planes (σv).
5. Center of symmetry (i) It is a symmetry operation through which the inversion leaves the molecule unchanged. For example, a sphere or a cube has a centre of inversion. Similarly molecules like benzene, ethane and SF6have a center of symmetry while water and ammonia molecule do not have any center of symmetry.
Overall the symmetry operations can be summarized as given below.
Inversion Center
The inversion operation is a symmetry operation which is carried out through a single point, this point is known as inversion center and notated by ‘ i’. This point is located at the center of the molecule and may or may not coincide with an atom in the molecule.
When we are moving each atom in a molecule along a straight line through the inversion center to a point an equal distance from the inversion center and get same configuration, we say there is an inversion center in the molecule. It can be in such molecules which do not have any atom at center like benzene, ethane.
Geometries like tetrahedral, triangles, pentagons don’t contain an inversion center. Hence a cube, a sphere contains a center of inversion but tetrahedron does not contain this symmetry operation. The molecule must be achiral for the presence of inversion center.
Some of the common examples of molecules contain center of inversion are as follow.
(a) Benzene molecule: Inversion center located at the center of molecule.
(b) 1,2-Dichloroethane: The staggered form of 1,2-Dichloroethane contains one inversion center at the center of molecule.
(c) trans-diaminedichlorodinitroplatinum complex: trans- form of some Coordination compounds like trans diaminedichlorodinitroplatinum complex contains inversion center.
Another example of coordination compound is hexacarbonylchromium complex [Cr(CO)6], where the inversion center located at the position of metal atom in complex.
(d) Ethane molecule: The staggered form of ethane contains inversion center while eclipsed form does not.
(e) Meso-tartaric acid: The anti-periplanar conformer of meso-tartaric acid has an inversion center.
(f) Dimer of D and L-Alanine: The dimer of two configurations of Alanine; D-alanine and L-alanine contains one inversion center.
(g) 18-Crown-6: An organic compound with the formula [C2H4O]6 named as 18-crown-6 (IUPAC name: 1,4,7,10,13,16-hexaoxacyclooctadecane) also contains inversion center located at center of molecule.
(h) Cyclohexane: The chair conformation of cyclohexane contains an inversion center while boat form does not.
Molecular Symmetry Examples
A molecule or an object may contains one or more than one symmetry elements, therefore molecules can be grouped together having same symmetry elements and classify according to their symmetry. Such type of groups of symmetry elements are known as point groups because there is at least one point in space which remains unchanged no matter which symmetry operation from the group is applied.
For the labelling of symmetry groups, two systems of notation are given, known as the Schoenflies and Hermann-Mauguin (or International) systems. The Schoenflies notations are used to describe the symmetry of individual molecule. The molecular point groups with their example are listed below.
Point group
Explanation
Example
C1
Contains only identity operation(E) as the C1 rotation is a rotation by 360o
Bromochlorofloromethane (CFClBrH)
Ci
Contains the identity (E) and a center of inversion center (i).
Anti-conformation of 1, 2-dichloro-1, 2-dibromoethane.
Cs
Contains the identity E and plane of reflection σ.
Contains the same symmetry elements as Dn with the addition of n dihedral mirror planes.
Ethane (D3d), Allene(D2d)
Sn
Contains the identity and one Sn axis.
CClBr=CClBr
Td
Contains all the symmetry elements of a regular tetrahedron, including the identity, four C3 axis, three-C2 axis, six dihedral mirror planes, and three S4 axis.
Methane (CH4)
T Th
Same as Td but no planes of reflection. Same as for T but contains a center of inversion .
Oh O
The group of the regular octahedron. Same as Oh but with no planes of reflection.
Sulphur hexafluoride (SF6)
Different point groups correspond to certain VSEPR geometry of molecule. Out of them some are as follow.
VSEPR Geometry of molecule
Point group
Linear
D∞h
Bent or V-shape
C2v
Trigonal planar
D3h
Trigonal pyramidal
C3v
Trigonal bipyramidal
D5h
Tetrahedral
Td
Sawhorse or see-saw
C2v
T-shape
C2v
Octahedral
Oh
Square pyramidal
C4v
Square planar
D4h
Pentagonal bipyramidal
D5h
A molecule may contain more than one symmetry operation and show symmetrical nature. Some of the examples of symmetry operation on molecule with their point group are as given below.
(a) Benzene: The point group of benzene molecule is D6h with given symmetry operations.
Inversion center: i
The Proper Rotations: seven C2axis and one C3 and one C6 axis
The Improper Rotations: S6 and S3axis
The Reflection Planes: one σh , three σvand three σd
(b) Ammonia: The point group of ammonia molecule is C3v with following symmetry operations.
The Proper Rotations: one C3axis
The Reflection Planes: three σv plane
(c) Cyclohexane: The chair conformation of Cyclohexane has D3d point group with given symmetry operations;
Inversion center: i
The Proper Rotations: Three C2axis and one C3 axis
The Improper Rotations: S6axis
The Reflection Planes: Three σdplane
(d) Methane: The point group of methane is Td (tetrahedral) with C3 as principal axis and other symmetry operations are as follows;
The Proper Rotations: Three C2 axis and Four C3axis
The Improper Rotations: Three S4axis
The Reflection Planes: Five σdplane
(e) 12-Crown-4: Thishas S6 point group with C3 and S6 axis with inversion center (i).
(f) Allene: The point group of methane is D2d with given symmetry operations.
The Proper Rotations: Three C2axis
The Improper Rotations: One S4axis
The Reflection Planes: Two σd plane
Molecular Symmetry and Group Theory
Group theory deals with symmetry groups which consists of elements and obey certain mathematical laws. Each point group is a set of symmetry operation or symmetry elements which are present in molecule and belongs to this point group. To obtain the complete group of a molecule, we have to include all the symmetry operation including identity ‘E’. A character table represents all the symmetry elements correspond to each point group. Hence we can make separate character table for each point group like C2v, C3v, D2h… etc.
For example, in the character table of C2v point group; all the symmetry elements has to written in first row and the symmetry species or Mulliken labels are listed in first column. These symmetry species specify different symmetries within one point group. For C2v, there are four symmetry species or Mulliken labels; A1, A2, B1, B2. Remember
The symmetry species for one-dimensional representations: A or B
The symmetry species for two-dimensional representations: E
The symmetry species for three-dimensional representations: T
The best example of C2v point group is water which has oxygen as center atom. The px orbital of oxygen atom is perpendicular to the plane of water molecule, hence it is not symmetric with respect to the plane σv(yz). So this orbital is anti-symmetric with respect to the mirror plane and its sign get change when symmetry operations applied. On the other hand, the s orbital is symmetric with respect to mirror plane. The symmetric and anti-symmetric nature can be represents by using mathematical sign; +1 and -1; here +1 stands for symmetric and –1 stands for anti-symmetric which are the characters in character table.
Hence the symmetry operations for the px orbitals are as follow.
1. E: Symmetric hence character will be 1
2. C2:Anti-symmetric, hence character will be 1
3. σv(xz):Symmetric; character :1
4. σv(yz):Anti-symmetric, character: -1
Hence the character table for C2v point group.
C2v
E
C2
σv(XZ)
σv(YZ)
A1
1
1
1
1
z
x2, y2,z2
A2
1
1
-1
-1
Rz
xy
B1
1
-1
1
-1
x, Ry
xz
B2
1
-1
-1
1
y, Rx
yz
Similarly character can be assigned for other symmetry species. The last two columns of character table make it easier to understand the symmetric nature. For example; x in second last column of B1 symmetry indicates that the x-axis has B1 symmetry in C2v point group and the Rx notation indicates the rotation around the x-axis. Similarly the character table for C3v point group will be
C3v
E
2C3
3σv
A1
1
1
1
z
x2+y2, z2
A2
1
1
-1
Iz
E
2
-1
0
(x, y), (Ixy, Iz)
(x2-y2, xy), (xz, yz)
For doubly degenerate, the character for E will be 2 and for triply degenerate it will be 3, because in this case we have two and three orbitals respectively which are symmetric with respect to E. Some of the character tables with their point groups are as follow
a. Character table for Oh point group, for example Sulfur fluorine (SF6)
b. Character table Td point group, methane (CH4)
c. Character table for D3d point group, for example, staggered ethane
d. Character table for D6h point group, example Benzene (C6H6)
Symmetry Adapted Linear Combinations
In some molecules like water, ammonia, methane which have more than one symmetry equivalent atom, the combinations of the symmetry equivalent orbitals can transform according to a irreducible representations of the molecules point group which are refer as Symmetry Adapted Linear Combinations. For the formation of an n-dimensional representation a set of equivalent functions -f1, f2, …, fn- can be used. The representation can be expressed as a sum of irreducible representations with the use of calculation of characters for this representation and by the use of the great orthogonality theorem. The n-linear combinations of f1, …, fn which transform the irreducible representations is given by the projection operator which denoted as p^p^ Gi;
Here
p^p^ = The operator which projects out of a set of equivalent functions the Gi Irreducible representation of the point group.
In n/g factor; n = dimension of the irreducible representation
g = the order of the group
The function fj can be chosen by any one of n which belongs to the equivalent set. For example, in the C3v character table; the 2C3^C3^ represents the class composed by C13^C31^ and C13^C31^operations. With C3 class, there are three different C3^C3^ operations would also perform separately on fj which produce different results. Let’s take an example of the O-H stretches along the ‘yz’ plane as molecular plane in water molecule; the formula can be apply to tabulate the characters of the irreducible representations and list the effect of O^O^R on one of the functions at the bottom of the table.
C2v
E
C2(Z)
sv(XZ)
sv(YZ)
A1
1
1
1
1
B1
1
-1
+1
-1
B2
1
-1
-1
+1
OR(OH2)
OHa
OHb
OHb
OHa
After applying the projection operator for A1 p^p^ A1 (O-Ha) = ¼ (O-Ha + O-Hb + O-Hb + O-Ha) = 1/2 (O-Ha + O-Hb)
According to the orthogonality theorem; it shouldn’t be possible to obtain a B1 linear combination, and indeed the projection operator will be zero. p^p^ B1 (O-Ha) = ¼ (O-Ha + O-Hb + O-Hb + O-Ha)=0
Application of the B2 projection operator gives p^p^ B2 (O-Ha) = ¼ (O-Ha + O-Hb + O-Hb + O-Ha)
=1/2 (O-Ha + O-Hb)
These linear combinations relate to the symmetric (A1) and anti-symmetric (B2) stretches of water as given below.
When two equivalent real functions are involved, the correct linear combinations will be equals to the sum and difference functions. In case of degenerate representations like in case of N-H stretching vibrations in ammonia, it is more difficult to construct the symmetry adapted linear combinations.
We can create symmetry-adapted linear combinations of atomic orbitals in exactly the same way. The point group is D2h with four carbon-hydrogen sigma bonds which are symmetry-equivalent and can make four carbon-hydrogen bonding symmetry-Adapted Linear Combinations ‘s. The character table will be as follow.
Result of symmetry operations on s1
E
C2(Z)
C2(Y)
C2(X)
i
s(XY)
s(XZ)
s(YZ)
s1
s1
s3
s4
s2
s3
s1
s2
s4
There are four non-zero symmetry-adapted linear combinations can be possible.
Hydrogen is one of the most striking elements of the periodic system, its number one, and the lightest of all the existing gases. It is the element whose discovery was indispensable for the solution of many problems of chemical theory. It is an element whose atom, losing its only valence electron, becomes a “bare” proton. And, therefore, chemistry of hydrogen is, in a way, unique; it is the chemistry of an elementary particle.
Once D.I. Mendeleev called hydrogen the typical of typical elements (meaning the elements of the short periods in the system), because it begins the natural series of chemical elements.
And such a fascinating element is readily available. It can be obtained without difficulty in any school laboratory, for instance, by pouring hydrochloric acid on zinc shavings.
Even in those bygone times, when chemistry was not a science yet and when alchemists were still searching for the “philosophers’ stone”, hydrochloric, sulphuric, and nitric acids as well as iron and zinc were already known. In other words, man had in his possession all components whose reaction could give rise to hydrogen. Only a chance was needed and chemical literature of the 16–18th centuries reported that many times chemists observed how the pouring of, for instance, sulphuric acid on iron shavings produced bubbles of a gas which they believed to be an inflammable variety of air.
One of those who observed this mysterious variety of air was the famous Russian Scientist M. V Lomonosov. In 1745 he wrote a thesis, On Metallic Lustre, which said, among other things: “On dissolution of some base metal, especially iron, in acidic alcohols, inflammable vapour shots out from the opening of the flask….” (According to the terminology of those times, acidic alcohols meant acids.) Thus, M.V Lomonosov observed none other than hydrogen. But the sentence went on to read: “……which is phlogiston.” since metal dissolved in the acid liberating material ignea or “inflammable vapour”, it was very convenient to assume that dissolving metal releases phlogiston: everything fits nicely into the theory of phlogiston.
And now is the time to meet the outstanding English scientist H. Cavendish, a man fanatically devoted to science and an excellent experimenter. He never hurried with making public his experimental results and sometimes several years had to pass before his articles appeared. Therefore, it is difficult to pinpoint the date when the scientist observed and described the liberation of “inflammable air”. What is known is that this work published in 1766 and entitled “Experiments with Artificial Air” was done as a part of pneumatic chemistry research. It is also likely that the work was performed under the influence of J. Black. H. Cavendish had become interested in fixed air and decided to see whether there existed other types of artificial air. In this manner the scientist referred to the variety of air which is contained in compounds in a bound state and which can be separated from them artificially. H. Cavendish knew that inflammable air had been observed many times. He himself obtained it by the same technique: the action of sulphuric and hydrochloric acids on Iron, Zinc, and Tin, but he was the first to obtain definite proof that the same type of air was farmed in all cases–inflammable air. And he was the first to notice the unusual properties of inflammable air. As a follower of the phlogistic theory, H. Cavendish could give only one interpretation of the substance’s nature. Like M. V. Lomonosov, he identified it as phlogiston. Studying the properties of inflammable air, he was sure that he was studying the properties of phlogiston. H. Cavendish believed that different metals contain different proportions of inflammable air. Thus, to the fixed air of J. Black, the inflammable air of H. Cavendish was added. Strictly speaking, the two scientists discovered nothing new: each of them only summarized the data of previous observations. But this summing up represented considerable progress in the history of human knowledge.
Fixed air and inflammable air differed both from ordinary air and from each other. Inflammable air was surprisingly light. H. Cavendish found that phlogiston, which he had separated, had a mass. He was the first to introduce a quantity to characterize gases, that of
Hydrogen is a gas under standard conditions. Hydrogen is used for rocket fuel and explosives.
density. Having assumed the density of air to be unity, Cavendish obtained the density of 0.09 for inflammable air and 1.57 for fixed air. But here a contradiction arose between Cavendish the experimenter and Cavendish the adherent of the phlogistic theory. Since inflammable air had a positive mass, it could by no means be considered to be pure phlogiston. Otherwise, metals losing inflammable air would have to lose mass as well. To avoid the contradiction, Cavendish proposed an original hypothesis: inflammable air is a combination of phlogiston and water. The essence of the hypothesis was that at last hydrogen appeared in the composition of inflammable air.
The evident conclusion is that Cavendish, like his predecessors, did not understand the nature of inflammable air, although he had weighed it, described its properties, and considered it to be an independent kind of artificial air. In a word, Cavendish, unaware of the fact, studied “phlogiston” obtained by him as he would have studied a new chemical element. But Cavendish could not perceive that inflammable air was a gaseous chemical element–so strong were the chains of the phlogistic theory. And having found that the real properties of inflammable air contradicted this theory, he came up with a new hypothesis, as erroneous as the theory itself.
Therefore, strictly speaking, the phrase “hydrogen was discovered in 1766 by the English scientist H. Cavendish” is meaningless. Cavendish described the processes of preparation and the properties of inflammable air in greater detail than his predecessors. However, he “knew not what he was doing”. The elementary nature of inflammable air remained beyond his grasp. It was not the scientist’s fault, however; chemistry had not yet matured enough for such an insight. Many years have passed before hydrogen became, at last, Hydrogen and occupied its proper place in chemistry.
Its Latin name hydrogenium stems from the Greek words hydr and gennao which mean “producing water”. The name was proposed in 1779 by A. Lavoisier after the composition of water had been established. The symbol H was proposed by J. Berzelius.
Hydrogen is a unique element in the sense that its isotopes differ in their physical and chemical properties. At one time this difference prompted some scientists to consider hydrogen isotopes as independent elements and to find for them special boxes in the periodic table. Therefore, the history of the discovery of hydrogen isotopes is of special interest, as a continuation of the history of hydrogen itself.
The discovery of Isotopes of Hydrogen:
The search for hydrogen isotopes began in the twenties of this century but all attempts were unsuccessful, resulting in the belief that hydrogen had no isotopes.
In 1931 it was suggested that hydrogen, nevertheless, contains a heavy isotope with a mass number of 2. Since this isotope had to be twice as heavy as hydrogen, the scientists tried to isolate heavy hydrogen by physical methods.
In 1932 the American scientists Urey, Brickwedde, and Murphy evaporated liquid hydrogen and, studying the residue by spectroscopy, found a heavy isotope in it. In the atmosphere it was discovered only in 1941. The name “deuterium” originates from the Greek word deuteros which means “second, another one”.
The next isotope with a mass number of 3, tritium (from the Greek–the third is radioactive and was discovered in 1934 by English scientists M. Oliphant, P. Hartec, and E. Rutherford. The name “protium” was assigned to the main hydrogen isotope. This is the only case when isotopes of the same element have different names and symbols (H, D and T). 99.99 per cent of all hydrogen is protium; the rest is deuterium with only traces of tritium.
Zinc is also one of the elements whose compounds have been known to mankind from time immemorial. Its best known mineral was calamine (zinc carbonate). Upon calcination it yielded zinc oxide, which was widely used, for instance, for treating eye diseases.
Although zinc oxide is comparatively easily reduced to free metal, it was obtained in a metal state much later than copper, iron, tin, and lead. The explanation is that reduction of zinc oxide with coal requires high temperature (about 1100oC). The boiling point of the metal is 906oC; therefore, highly volatile zinc vapour escapes from the reaction zone. Before metallic zinc was isolated, its ores were used for making brass, an alloy of zinc and copper. Brass was known in Greece, Rome, India, and China. It is an established fact that Romans produced brass for the first time during the reign of Augustus (B.C.20-A.D.14). Interestingly, the Roman method of preparing brass was still used up to the 19th century.
It is impossible to establish when metallic zinc was obtained. In ancient Dacian ruins an idol was found containing 27.5 percent of zinc. Zinc was possibly obtained during brass production as a side product.
In the 10-11th centuries the secret of zinc production was lost in Europe and zinc had to be imported from India and China. It is believed that China was the first country to produce zinc on a large scale. The production process was extremely simple. Earthenware filled with calamine were tightly closed and piled into a pyramid. The gaps between the posts were filled with coal and the posts were heated to red heat. After cooling the pots, where zinc vapours condensed, were broken and metal ingots were extracted.
Europeans rediscovered the secret of zinc production in the 16th century when zinc had already been recognized as an independent metal. During the next two centuries many chemists and metallurgists worked on with methods of zinc extraction. A great deal of credit should go to a. Marggraf who published in 17
46 a large treatise, Methods of Extraction of Zinc from Its Native Mineral Calamine. He also found that lead ores from Rammelsberg (Germany) contained zinc and that zinc could be obtained from sphalerite, natural zinc sulphide.
The name “zinc” originates from the Latin word denoting leucoma or white deposit. Some scholars relate “zinc” to the German word zink, which means lead.
Bismuth has been known to mankind for centuries but for a long time it was confused with antimony, lead and tin. Paracelsus, for instance, said that there were two varieties of antimony–a black one used for the purification of gold and very similar to lead, and a white one named bismuth and very similar to lead, and a white one named bismuth and resembling tin; a mixture of these two varieties resembles silver. Form the chemical standpoint this confusion can easily be explained. Antimony and bismuth are analogues of each other and have common features with lead and tin, the elements of the previous group.
Agricola, unlike Paracelsus, gave a rather detailed description of bismuth and of the process of its extraction from ores mined in Saxony. Miners thought that bismuth, as well as tin, was a variety of lead and that bismuth could be transformed into silver.
In Central Russia bismuth has been known since the 15th century. With the development of book-printing bismuth, along with antimony, began to be used for casting typographical types. In literature few elements have such a great number of names as bismuth. E. von Lippmann in his book History of Bismuth from 1480 to 1800 gives twenty one names of this metal used in Europe. A sufficiently clear idea of bismuth as an independent metal was formed only in the 18th century.
Antimony and its compounds have been known from times immemorial. Some scholars say that metallic antimony was used in South Babylon for making vessels about 3400 years B.C. but in antiquity antimony was mainly used for making cosmetics such as rouge and black paint for eye brows. In Egypt, however, antimony was apparently unknown or almost unknown. This is borne out by finds from Egyptian burial sites, particularly, by painted mummies.
In antiquity antimony was confused with lead. It was only in alchemical literature of the Renaissance period that antimony was given a sufficiently accurate description. For example, G. Agricola clearly pointed out that antimony is a metal different from other metals. Basilius Valentinus devoted to antimony a whole treatise, Triumphal Carraige of Antimonium, in which he described the uses of antimony and its compounds.
There are several interpretations of the Latin name of antimony antimonium. Most likely it originates from the Greek word antimonos, which means “an enemy of solitude”, and underlines simultaneous occurrence of antimony and other minerals.
Arsenic compounds, namely its sulphides As2S3(orpiment) and As4S4(realgar or sandarac), were well known to Greeks and Romans. Orpiment was also known under the name of “arsenic”. Pliny the Elder and Dioscorides mentioned the toxicity of these compounds; Dioscorides noted calcination of “arsenic” to obtain white arsenic (oxide).
Arsenic is sometimes found in nature in native state and is fairly easily extracted from its compounds. It is not known who was the first to produce elemental arsenic. Usually its discovery is ascribed to the alchemist. Albert the great. Paracelsus described the process of preparing metallic arsenic by the calcination of “arsenic” with egg-shells. According to some reports, metallic arsenic was known much earlier but it was considered to be a variety of native mercury. This is due to the fact that arsenic sulphide resembles one of mercury minerals and the extraction of arsenic from its ores is rather simple.
In the Middle Ages arsenic was known not only in Europe but in Asia as well. Chinese alchemists could extract arsenic from its ores. Medieval Europians had no way of knowing whether death of a person was caused by arsenic poisoning but Chinese alchemists had a method of making sure. Unfortunately, their method of analysis is unknown. In Europe the test for estimating arsenic content in human body and the food eaten before death was developed by D. Marsh. This test is very sensitive and is still used.
Since arsenic sometimes accompanies tin, there are reported cases (for instance, in Chinese literature) when people were poisoned by water or wine kept for some time in new tin vessels.
For a long time people confused white arsenic, or its oxide, with arsenic itself believing the two to be the same substance. The confusion was eliminated at first by H. Brand and then by A. Lavoisier who proved that arsenic is an independent chemical element.
Arsenic oxide has for a long time been used to kill rodents and insects. The symbol As originates from the Latin word arsenicum whose etymology is obscure.
There is a science-fiction story by a Russian scientist I.A.EfremovThe Lake of the Mountain Spirits. Anybody who visited the lake in a sunny weather died. People living in the area were sure that the lake was inhabited with evil spirits who hated all visitors. When geologists reached the lake high in the mountains, they were amazed to learn that the lake contained not only water, but also native mercury element. And the “evil spirits” were nothing but element mercury vapour; in hot weather they rose above the surface of small and large mercury pools surrounding the lake.
Indeed, mercury is often found in native state, sometimes in most unexpected places. For instance, in some mountain regions of Spain, mercury was found at bottoms of wells. In antiquity mercury was known China and India. Mercury was also found in excavations of Egyptian tombs dating from about the Middle of the cinnabar was the only mercury containing mineral known in antiquity. Theophrastos (300 B.C.) described the process of extracting mercury from cinnabar by treating it with copper and vinegar. Man discovered mercury in ancient times owing to the fact that it is comparatively easily liberated from cinnabar at a sufficiently high temperature.
Mercury occurs in deposits throughout the world mostly as cinnabar (mercuric sulfide). The red pigment vermilion is obtained by grinding natural cinnabar or synthetic mercuric sulfide.
The world’s biggest mercury deposit is at Almaden (Spain). Exploitation of this deposit began at the time of the Roman Empire, and Romans extracted 4.5 tons of mercury annually. In antiquity mercury had many uses. Mirrors were made with amalgamated mercury; mercury and its compound were used as medicines. Cinnabar was mainly used as a pigment; and not for producing pure mercury. Before the invention of the galvanization process, mercury had been used in gilding and silvering processes. Amalgam of the metal was applied to a metal plate and heated to a high temperature. When mercury evaporated a thin coat of gold or silver remained on the plate. But this process was very unhealthy. Mercury played an important role in studies of gases; it was used in gas pumps and gas vessels.
Aristotle named mercury “liquid silver” and Dioskorides named in “silver water”. From this comes the Latin name of mercury –hydrargium.
Tin typically occurs in natures in the form of the mineral cassiterite. It is believed that man discovered tin about 6-6.5 thousand years ago, i.e. in the same period as copper. Tin was widely known in the Mediterranean countries, Persia, and India. Egyptians imported tin for the production of bronze form Persia.
In his book Ancient Egyptian Materials and their Production A. Lukas writes that although in Egypt tin ores were not known, the oldest known tin articles were found in burial sites of the 18th dynasty (1580-1350 B.C.) (in particular, a ring and a vessel). Tin was known not only in the countries of the Mediterranean.
Julius Caesar mentioned production of tin in central regions of Britain. Cortez, when he arrived in South America in 1519, found that tin coins were widely circulating in Mexico. However, the time of discovery of tin in America in not known.
Tin crystalline structure : Tin has a highly crystalline structure and when a tin bar is bent, a ‘tin cry’ is heard, due to the breaking of these crystals.
In antiquity tin was used not only as a component of bronze but also for making crockery and jewelry. Pliny the Elder and Dioskorides mention tinning of copper plates to protect them from corrosion.
Up to the 13th century England was the only country in Europe where tin was produced. Tin was fairly expensive. In mid-16th century its cost was equal to that of silver and it was used for manufacturing luxury goods. Then, as its production increased, it found many applications, for instance, for making tin plate.
The Latin for tin (stannum) stems from the Sanscritstan which means “solid”. The chemical symbol Sn originates from the Latin name.
Lead is very rarely encountered in a native state but is smelted fairly easily from ores. Lead become known to Egyptians simultaneously with iron and silver and was produced as early as the second millenium B.C. in India and china, In Europe production of lead began somewhat later although in the 6th century. B.C records we find mention of lead which was brought
Lead is a naturally occurring metal but its natural status doesn’t mean it’s healthy. In fact, lead is extremely toxic to humans and affects the liver, kidneys, reproductive system, and nervous system
to the Tyre trade fair. Lead was produced in great amounts during the reign of Hammurabi in Babylon. For a long time lead was confused with tin. Tin was named “plum bum album” and lead–“plumbum nigrum”. Only in the Middle Ages were they recognized as different metals.
Greeks and Phoenicians started many lead mines in Spain which later were taken over by Romans. In ancient Rome lead was widely used: for making crockery, styluses, and pipes for the famous Roman water-main. Lead was also used for manufacturing white lead. The island of Rhodes was the biggest exporter of white lead.
Lead Preparation :
The process of its preparation is still used as follows: lead pieces are immersed into vinegar and the salt thus obtained is boiled with water for a long time.
But red lead was first obtained unexpectedly. When a fire broke out in the Greek port of Piraeus barrels with lead were enveloped in flames. After the fire had been extinguished, red substance was found in the charred barrels–it was red lead.
Although in Russia lead has been known for a long time, up to the 18th century the process of lead production was very primitive. After the invention of firearms lead was used for making bullets and the military importance of lead is still great. But in addition to its “military” uses” lead has many peaceful ones; for instance, typographical types are made of its alloy with antimony. Lead is also used for protection against radiation in experiments.
Greeks named leadmolibdos; its chemical symbol Pb originates from Latin plumbum.
magnesium” is a new earth; it should not be confused either with roasted lime or with “magnesium alba” (i.e. magnesium oxide). However, T. Bergman failed to separate the metal from pyrolusite, in contrast to I. Kaim.
and established that its dissolution in nitric acid yields a green colour. He concluded that the mineral was most probably a cobalt ore with admixtures of copper. When Swedish miners found a reddish mineral which, being added to glass, did not produce a blue colour, they named it “cobold that had lost his soul”. It was also one of the nickel minerals.
fragments of blue glass. It is not known whether the preparation of blue glasses and paints on the basis of cobalt compounds was due to chance or whether it was a conscious effort. At any rate the method of their preparation remained unknown for a long time. Its first mention dates back to 1679.
element. It must be added that for a long time many chemists did not believe in the discovery of cobalt. In 1776 the Hungarian chemist P. Padaxe said that cobalt was a compound of iron and arsenic; but he considered nickel, which had already been discovered by that time, to be a chemical element. Only by the end of the 18th century, the efforts of many scientists confirmed the discovery of G. Brandt.
chemistry to make such great progress at the end of the 18th century which had never been possible before. First of all, oxygen overturned the phlogistic theory which had seemed immovable. Erroneous as it was, this theory was undoubtedly of some historical usefulness. For the time being the theory of phlogiston made it possible somehow to systematize the existing chemical conceptions and to consider various processes in nature and laboratory from a common (though erroneous) standpoint. This gave a certain purposefulness to the research. Hydrogen and nitrogen were found from the phlogistic conceptions but the study of these “varieties of air” made it possible to accumulate new facts whose explanation demanded a different approach. Figuratively speaking, chemistry needed a new look at itself, and oxygen made it possible.
compound observed by Bayen–red mercury oxide. In the same year J. Priestley experimented with the same compound. Shortly before, he had discovered that in the presence of green plants fixed air, not supporting respiration, turned into ordinary air suitable for respiration by living organisms. This fact was extremely important not only for chemistry but for biology as well. Priestley proved for the first time that plants release oxygen.
instance, saltpeter and nitric acid, frequently observing liberation of brown vapours of nitrogen dioxide. Obviously, it would be impossible to discover nitrogen by decomposing its inorganic compounds. Tasteless, colourless, odorless, and chemically rather inactive, nitrogen would have remained unnoticed.
The residue was mephitic gas. Cavendish did not study it thoroughly and only reported the fact to Priestley. Cavendish returned to the study of mephitic air much later, did a large work but the credit for the discovery had already gone to another scientist.
components whose reaction could give rise to hydrogen. Only a chance was needed and chemical literature of the 16–18th centuries reported that many times chemists observed how the pouring of, for instance, sulphuric acid on iron shavings produced bubbles of a gas which they believed to be an inflammable variety of air.
containing 27.5 percent of zinc. Zinc was possibly obtained during brass production as a side product.
preparing metallic arsenic by the calcination of “arsenic” with egg-shells. According to some reports, metallic arsenic was known much earlier but it was considered to be a variety of native mercury. This is due to the fact that arsenic sulphide resembles one of mercury minerals and the extraction of arsenic from its ores is rather simple.
