Discovery of Polonium Element


Polonium was the first natural radioactive element discovered with the radiometric technique. Back in 1870 the main properties of polonium were predicted by D. I. Mendeleev. He wrote: “Among heavy metals we can expect to find an element similar to tellurium whose atomic weigh is greater than that of bismuth. It should possess metallic properties, and give rise to an acid whose composition and properties should be similar to those of sulphuric acid and whose oxidizing power is higher than that of telluric acid…

The oxide RO2 cannot be expected to have acidic properties which tellurous acid still has. This element will form organometallic compounds but not hydrogen compounds…”

Nineteen years had passed and Mendeleev made a significant addition to his description of dvi–tellurium (as he called the unknown element). He predicted the following properties: relative atomic mass 212; forms oxide DtO3; in a free state the element is a crystalline low–melting non–volatile metal of grey colour with a density of 9.8; the metal is easily oxidized to DtO2; the oxide will have weak acidic and basic properties: a hydride of the element, if it exists at all, must be unstable; the element must form alloys with other metals.

Below readers will see for themselves how accurate were Mendeleev’s predictions of the properties of a heavy analogue of tellurium. But these predictions had only an indirect effect on the history of polonium, if any. The discovery of polonium (and then radium) proved to be a significant milestone in the science of radioactivity and gave an impetus to its development.

As one can see from the laboratory log–book of Marie and Pierre Curie they started to study the Becquerel rays, or uranium rays, on December 16, 1897. First the work was conducted by Marie alone and then Pierre joined her on February 5, 1898. He performed measurements and processed the results. They mainly measured the radiation intensities of various uranium minerals and salts as well as metallic uranium. The results of extensive experiments suggested that uranium compounds had the lowest radioactivity, the metallic uranium exhibited a higher radioactivity, and the uranium ore known as pitchblende had the highest radioactivity. These results indicated that pitchblende, probably, contained an element whose activity was much higher than that of uranium.

As early as April 12, 1898 the Curies reported this hypothesis in the proceedings of the Paris Academy of Science. On April 14 the Curies started their search for the unknown element with the assistance of the chemist G. Bemont. By the middle of July they finished the analysis of pitchblende. They carefully measured the activity of each product successively isolated from the ore. Their attention was focussed on the fraction containing bismuth salts. The intensity of the rays emitted by this fraction was 400 times that of metallic uranium. If the unknown element really did exist it had to be present in this fraction.

Finally, on July 18 Marie and Pierre Curie delivered a report to a session of the Paris Academy of Science entitled “On a new radioactive substance contained in pitchblende”. They reported that they had managed to extract from pitchblende a very active Sulphur compound of a metal that had previously been unknown. According to its analytical properties it was a neighbour of bismuth. The Curies suggested, if the discovery could be proved, to name the new element in honour of the country where Marie had been born and brought up, that is, polonium after Poland.

The scientists emphasized that the element had been discovered with a new research method (the term “radioactivity”, which later became conventional, was first introduced in this report).

The introduction of spectral analysis made it possible to reveal the existence in natural objects of elements that could not be seen, felt or weighed. Now the history repeated itself but the role of indicator was played by radioactive radiation, which could be measured with a radiometric technique. However, the results of the Curies were not faultless. They were wrong in suggesting a chemical similarity between polonium and bismuth. Even a brief look at the periodic system shows that the existence of a heavy analogue of bismuth is hardly possible. But one must not forget that the Curies did not extract pure metal, could not determine its relative atomic mass, and, finally, did not see differences in the spectra of polonium and bismuth. This is why they actually ignored a possible analogy between polonium and tellurium.

Thus, we may regard 18 July, 1898, as the date of just a preliminary discovery of polonium as substantiation of the discovery took quite a long time. The high intensity of radiation from polonium made difficult its study. The radiation was found to consist of only alpha rays with no beta or gamma rays. A strange finding was that the activity of polonium decreased with time and the decrease was rather noticeable; neither thorium nor uranium exhibited such behaviour. This is why some scientists doubted whether polonium existed at all. The sceptics said it was just normal bismuth with traces of radioactive substances.

But in 1902 the German chemist W. Marckwald extracted the bismuth fraction from two tons of uranium ore. He put a bismuth rod into a bismuth chloride solution and observed precipitation of a highly radioactive substance on it which he took for a new element and named radiotellurium. Later he recalled: “I named this substance radiotellurium just for the time being since all its chemical properties suggested placing it into the sixth group into the still unoccupied box for the element with a somewhat higher atomic weight than that of bismuth…. The element was more electronegative than bismuth but more electropositive than tellurium; its oxide should also have basic rather than acidic properties.

All this corresponded to radiotellurium…. The expected atomic weight for this substance was about 210”. Later he said that he had got his idea for extracting polonium when analysing the periodic system.

As for the polonium discovered earlier Marckwald promptly declared it a mixture of several radioactive elements. This led to a stormy discussion of the real nature of polonium and radiotellurium. Most scientists supported the Curies. A. later comparison of the two elements revealed their identity. The discovery was credited to the Curies and the name “polonium” was retained.

Though polonium was the first of the new natural radioactive elements its symbol Po did not appear in the appropriate box in the periodic system. The atomic mass of the element was very difficult to measure. The lines of the polonium spectrum were reliably identified in 1910. It was only in 1912 that the symbol Po occupied its place in the periodic table.

For almost half a century scientists had to be satisfied to work only with polonium compounds (usually in rather small amounts). The pure metal was prepared only in 1946. High density layers of metallic polonium prepared by vacuum sublimation have a silvery colour. Polonium is a pliable low–melting metal (melting point 254oC, boiling point 962oC), its density is about 9.3 g/cm3. When polonium is heated in the air it readily forms a stable oxide; its basic and acidic properties are weakly manifested. Polonium hydride is unstable. Polonium forms organometallic compounds and alloys with many metals (Pb, Hg, Ca, Zn, Na, Pt, Ag, Ni, Be). When we compare Mendeleev’s predictions with these properties we see how close they are to the truth.

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