A Survey of the Atomic and Molecular Theory

Individual atoms of elements vary in number and configuration of these elementary particles, while the total number of electrons and protons is the same, so that the whole is electroneutral to the outside. Elements can be arranged in a series, according to the number of protons in the nucleus (or electrons), in which the following member has always one proton more, than the previous one. The ordinal or atomic number of elements is determined by the number of protons which in turn decides their position in the periodic system. 

In the nuclei of atoms neutrons are concentrated, in addition to protons the presence of neutrons in the nucleus does not change the ordinal number of the element, but it contributes to the growth of its atomic weight. Elements having the same number of protons, but differing in the number of neutrons, are called isotopes. Electrons, being practically particles almost without any mass, move in eliptical paths in one or several shells round the nucleus approximately in the same manner as the planets move round the sun. 

In each shell, or on each level (marked by letters K, L, M, N, 0, P, Q) only a very definite number of electrons can move as a maximum. As can be seen from Table 1, where Z is the atomic number of each element, the innermost shell is fully saturated with two electrons already. On the highest outer levels the maximum of eight electrons (the octet) can move. 

The chemical and electrochemical characteristic properties of elements are determined by the electrons in the last outer shell. Elements with outer levels filled to completion, i. e. the rare gases (helium, neon, argon, crypton, xenon and radon), are noted for the great stability of their electronic structures atoms of such elements, known for their chemical inactivity, do not show any tendency to form molecules, neither in mutual bonds nor in bonds with other atoms. 

In all other elements the outer shells are not fully occupied by electrons such 

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