Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

PERIODIC CLASSIFICATION OF THE ELEMENTS. 13

These electrons held in common occupy a position between the

"kernels" of the two atoms and by their joint attraction for the

kernels hold the molecule together.

The kernel of an atom consists of the nucleus and all of the

layers except the outer layer. Those electrons in the outer layer

which belong to the neutral atom itself are usually referred to as

the "valence electrons."

The periods of the Periodic Classification comprise the elements

coming between any two of the successive inert gases. The first

two periods are short periods and comprise 8 elements each. The

properties of all elements of these periods are clearly seen to be

due to the tendency to revert to the structure of one or the other

of the nearest inert gases. The kernels of these elements are

invariable.

The elements near the beginning and end of the long periods

show clearly marked similarity to elements in the similar positions

in the short periods, their properties being derived from the ability

of the atom to acquire the configuration of the inert gas atom. In
the middle of the long periods, however, the number of electrons

to be gained or lost to acquire the inert gas structure is too great.

The number of electrons in the valence layer usually drops to two

or three and the rest of the electrons enter some one of the kernel
layers. For example, praseodymium Pr 69 might be expected to

revert to the form of Xe 64 2-8-18-18-8 and become quinque-

valent. As a matter of fact two electrons pass into the fourth

kernel layer to give Pr 69 2-8-18-20-8-3, the 3 electrons of the
outer layer determining the valence. The tervalent ion Pr+++

is formed when three electrons are given to an element like chlo-

rine. The elements praseodymium to lutecium, 59-71, constitute

the so-called rare earth series. All have the nearly invariable
valence of three. They differ from one another simply by one

additional electron in the fourth kernel layer for each successive

element. They constitute what is called a "transition series."

With lutecium, however, the transition is complete and we have

the structure LU71 2-8-18-32-8-3 with stable kernel layers. In
the next element hafnium Hf 72 , the additional electron has to

remain in the valence layer as it cannot get into the layers con-

taining the stable numbers of 32 or 8. Hafnium has an invariable

valence of 4. Within the long periods are clearly recognizable