Discovery of Hydrogen
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
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.