lewis structure

Meaning: A Lewis structure shows valence electrons as bonds (lines, each a shared pair) and lone pairs (dots). Total the valence electrons (adjust for charge), put the least electronegative atom in the center, join atoms with single bonds, fill outer octets, and place leftovers on the center. If it lacks an octet, form double or triple bonds. Octet exceptions include H (2 electrons), B (fewer than 8), odd-electron species like NO, and expanded octets in period 3 and below, as in SF₆.

Equation: FC = V − N − B/2, where V is the neutral atom's valence electrons, N its nonbonding electrons, and B the electrons in its bonds. FC is in units of the elementary charge e (1.60 × 10⁻¹⁹ C), and formal charges sum to the species' overall charge.

On the MCAT: Choose the best structure or resonance contributor: favor formal charges near zero, with any negative charge on the more electronegative atom. The structure also gives VSEPR geometry, polarity, and bond comparisons (more shared pairs means shorter, stronger bonds).

Common confusion: Formal charge is not oxidation state. Formal charge splits bonding electrons equally; oxidation state gives them to the more electronegative atom. Resonance forms are not interconverting molecules; the real species is one hybrid.