valence electrons

Picture an atom as a tiny nucleus surrounded by electrons arranged in shells, like rings of seats around a stage. Only the outermost ring of seats faces the world; the electrons sitting there are the valence electrons, and they are the ones that take part in chemistry. Everything an atom does when it bonds — sharing, giving, or taking electrons — happens with this outer ring. The inner electrons stay buried and rarely get involved.

More precisely, valence electrons are the electrons in the highest-energy, outermost shell of an atom. You can read their count straight off the periodic table's main groups: hydrogen has 1, carbon has 4, nitrogen 5, oxygen 6, the halogens 7. These numbers explain bonding habits at a glance. Carbon, with four valence electrons, is exactly halfway to a full outer shell of eight, so rather than lose or gain four it shares all four, forming four bonds. Oxygen, with six, needs only two more and so tends to form two bonds plus two lone (non-bonding) pairs.

Valence electrons matter because they are the entire currency of bonding. Lewis structures are nothing but a bookkeeping of valence electrons; the octet rule is a statement about filling the valence shell to eight; formal charge counts whether an atom is using more or fewer valence electrons than it 'owns'. Get comfortable counting valence electrons and most of the bonding model becomes simple arithmetic. A caveat: this neat group-number rule works beautifully for the light main-group elements that dominate organic chemistry, but breaks down for transition metals.