Discovery of element : Francium
The element No. 87 has a place of its own in the history of radioactive elements. Though its natural abundance is extremely small it was found originally in nature. But we shall tell its story in detail in the part of the book dealing with artificial elements. This will be better for many reasons. Here the first part of the book comes to an end.
The idea of transmutation (transformation) of elements was born in distant times. The idea was upheld by alchemists for their specific aims. But all attempts to achieve transmutation proved futile. As chemistry was developing into an independent full–fledged science and accumulating knowledge of the structure and properties of matter the very feasibility of transformation of elements was questioned. By the end of the 19th century serious scientists ignored this problem though did not dare to refute it definitely.
But at the very end of the century an event happened which suggested the paradoxical idea that continuous transmutation of elements takes place in nature. This event was the discovery of radioactivity. But only a relatively small part of elements at very end of the periodic system are subjected to natural transmutation.
Radioactive transformations are independent of human will. All attempts to affect the course of natural radioactive processes failed. When the nuclear model of atomic structure was formulated it became clear that radioactivity is a nuclear phenomenon. The structural features of nuclei determine the capacity for radioactive decay.
The nuclear charge Z is the primary parameter of a chemical element. When a nucleus emits alpha or beta particles its charge changes so that the nature of the chemical element alters. One element is transformed into another. If we are dealing with a stable chemical element its nuclear charge Z will never change by itself. It will change if we can restructure its nucleus in some way, decrease or increase the number of protons in the nucleus. Only then will the nuclear charge change and artificial transmutation of a chemical element will take place.
Rutherford was the first to carry out artificial transmutation of elements. In 1919 he bombarded nitrogen with alpha particles and obtained oxygen atoms. This first in history artificial nuclear reaction can be described by the following equation: or, in a shorter form,
Alpha particles for a long time remained the only available means for conducting nuclear reactions. The energy of naturally produced alpha particles in not high; therefore, they could penetrate the nuclei of only a relatively small number of elements and such events were extremely rare. This limited the scope of artificial transmutation of elements. The situation changed significantly as a result of two discoveries made in the thirties. In 1932 the British scientist J. Chadwick discovered a neutral elementary particle proved to be a universal instrument for performing nuclear transformations since it was not repulsed by positively charged nuclei. Two years later the French physicists Irene and Frederic Joliot – Curie discovered artificial radioactivity and detected a new type of radioactive transformation, namely, positron decay, that is, emission of positrons. It became clear that radioactive isotopes could be produced artificially by means of nuclear reactions for many stable elements.
One can ask what made possible the production of artificial radioactive isotopes in large numbers? The answer is that it was the work of experimental physicists who designed fine instruments for conducting measurements, developed special techniques for performing and studying nuclear reactions and, together with chemists, found methods for isolating traces of radioactive substance. Moreover, the range of particles available for bombardment of nuclei was extended considerably when alpha particles, protons, and neutrons were joined by deuterons (nuclei of a heavy hydrogen isotope deuterium), and later by multiply charged ions of such elements as boron, carbon, nitrogen, oxygen, neon, etc. Finally, physicists have built powerful accelerators capable of accelerating charged particles to very high velocities. All these advances paved the way for artificial synthesis of new elements.