A. Elements Discovered in Nature

Elements Discovered in Nature

Most chemical elements known at present have been discovered in nature (in various ores and minerals, the earth's atmosphere, etc.) and one can say with confidence that there are no more undiscovered elements in nature, including both stable elements and those referred to as naturally radioactive ones. They can be called elements "discovered by means of analysis". They exist independently of man, his knowledge, and methods of investigation. They existed at the earliest stages of evolution of the solar system when the Earth was being formed as a planet.

How these elements were discovered is the subject of the first part of our book.

More than 90 per cent of elements occurring in nature are stable, i.e. not radioactive. They occupy boxes from 1 to 83 in the periodic table, i.e. from hydrogen to bismuth. There are two gaps in this sequence corresponding to the elements with Z = 43 (technetium) and Z = 61 (promethium). The strange properties of atomic nuclei have made all the isotopes of these elements radioactive with relatively short lifetimes; therefore, technetium and promethium have not been preserved in nature but decayed and transformed into the neighbouring stable elements.

The number of naturally radioactive elements on Earth is considerably smaller than that of stable ones. In the periodic table they begin with polonium (Z — 84) and end with uranium (Z = 92). Among them only thorium and uranium have very long half-lives; therefore, they have survived on Earth since the time of its formation and their amounts are rather noticeable. That is why uranium and thorium have been discovered as new chemical elements long before scientists succeeded in observing radioactivity. The amounts of other naturally radioactive elements (polonium, radon, radium, actinium, and protactinium) are much smaller.