SO32

Meaning: SO32 is read here as SO₃²⁻, the sulfite ion, the conjugate base of bisulfite (HSO₃⁻), which comes from sulfurous acid (H₂SO₃). Valence electrons: 6 + 3(6) + 2 = 26. Sulfur is central, bonded to three oxygens, and keeps one lone pair. Four electron domains give tetrahedral electron geometry and trigonal pyramidal molecular geometry, with O–S–O angles a little under 109.5° because the lone pair repels more strongly than bonding pairs. Sulfur has oxidation state +4, so sulfite can be oxidized to sulfate (+6) and acts as a reducing agent.

Equation: Formal charge = V − L − B/2, where V is the free atom's valence electrons, L is nonbonding electrons on the atom, and B is bonding electrons (all are electron counts, so no SI unit). With three single bonds, S = 6 − 2 − 3 = +1 and each O = 6 − 6 − 1 = −1, which sums to −2.

On the MCAT: Expect questions that ask for the Lewis structure, the shape, or whether the ion is polar (it is, because the pyramid has a net dipole). Redox passages may have sulfite oxidized to sulfate. Acid-base questions may treat it as a weak base.

Common confusion: Students mix up SO₃²⁻ with neutral SO₃, which is trigonal planar with no lone pair on sulfur. The lone pair on sulfur in sulfite makes it pyramidal. Also keep sulfite (SO₃²⁻, three O) separate from sulfate (SO₄²⁻, four O, tetrahedral).