valence electrons

Meaning: Valence electrons are the electrons in the outermost occupied shell (highest n). They take part in bonding and set most of an element's chemistry. For main-group elements the count is 1 or 2 for groups 1 and 2 and 3 to 8 for groups 13 to 18 (helium is the exception with 2). For transition metals, the ns and (n−1)d electrons are usually treated as valence.

On the MCAT: Expect to read a count from the periodic table or a configuration (sulfur, [Ne]3s²3p⁴, has 6). It is the starting point for Lewis structures: sum the valence electrons of all atoms, then add one per negative charge or subtract one per positive charge. It also predicts ionic charge (Na → Na⁺, Cl → Cl⁻) and formal charge = valence electrons − nonbonding electrons − number of bonds.

Common confusion: Students count every electron in the configuration. Bromine, [Ar]4s²3d¹⁰4p⁵, has 7 valence electrons, not 17, because the filled 3d subshell lies inside the n = 4 shell. The octet rule is also not a hard cap: period 3 and lower atoms can expand it, as sulfur does in SF₆ (12 electrons around S, all 6 valence electrons in bonds).