Fig. 29—Hypothetical atomic structures
Precisely the same procedure is repeated in the fifth period of eighteen elements between krypton and xenon, the rare-earth group which intervenes between strontium (Sr) and silver (Ag) corresponding to the elements in which, with increasing atomic number, the added electrons are filling up the empty orbits in the fourth shell instead of going into what is now the outer or fifth shell (see [Table XV]).
Now in considering the sixth period of thirty-two elements from xenon (Xe) to niton (Nt), a glance at [Table XV] shows that the fourth shell in xenon contained only eighteen electrons, whereas in niton there are thirty-two, i.e., there are fourteen unfilled orbits in xenon in the fourth shell; and a similar glance at the fifth shell shows
vacant orbits there. The first two elements in this group, viz., caesium (Cs) and barium (Ba), take the added electrons in
orbits, then the electrons begin to go inside until gold is reached, when the fourth and fifth shells become full and from gold (Au) to niton (Nt), as the added electrons go to the outer shell, the chemical properties again progress as from sodium to argon, or from copper to krypton.
It will be noticed that in [Fig. 30] element 72 is hafnium, the element discovered in 1923 by Coster and Hevesy[164] by means of X-ray analysis. It is because its chemical properties resemble so closely those of zirconium that it had not been found earlier by chemical means. Hevesy estimates that it represents one one hundred-thousandth of the earth’s crust, which makes it more plentiful than lead or tin.
