20. While the number of electrons possessed by an atom can be altered by heat,
light, an electric field, or bombardment, the maximum chemical stability
occurs when the valence shell of the atom contains a maximum number of
electrons. Each main shell has a maximum number of electrons possible,
determined by the formula No = 2(N2); however, if the main shell is the
outermost shell, it is limited to eight electrons.
As shown on the periodic chart, sodium has one valence electron; and argon,
eight. Sodium reacts violently with water, while argon, under ordinary
conditions, does not form compounds with the other elements.
Helium, neon, argon, krypton, xenon, and radon (group VIII on the periodic
chart) are called inert elements, because the atoms of each have complete
valence shells and are considered to be the most stable atoms.
The requirement for chemical stability of an atom is that the valence shell
must contain its ______________________ number of electrons.