Chemical bonding typically involves only the outermost electrons of the atom, while the inner electrons and nucleus remain the same. Some arrangements of outermost electrons are much more stable than others.
When an atom forms chemical bonds, its arrangement of outermost electrons becomes more stable. An atom is form by a nucleus, made by protons and neutrons, and by a different number of rings that contains electrones. The outermost ring is the one that bonds with other atoms to form chemical bonds, so it is the one that could become more stable.
