anion

Meaning: An anion forms when a neutral atom or molecule gains electrons, so it has more electrons than protons. Nonmetals with high electronegativity and high electron affinity, such as the halogens (Cl → Cl⁻) and oxygen (O → O²⁻), readily form monatomic anions. Polyatomic anions include OH⁻, NO₃⁻, SO₄²⁻ and PO₄³⁻. Adding electrons increases electron-electron repulsion while the nuclear charge stays the same, so the electron cloud expands and an anion has a larger radius than its neutral atom. Among isoelectronic species, the one with more protons is smaller.

On the MCAT: Expect questions on ionic radius trends (S²⁻ > Cl⁻ > K⁺ > Ca²⁺ for an isoelectronic series), naming ionic compounds (-ide endings for monatomic anions, -ate and -ite for oxyanions), and charge balance in salts. In electrochemistry, anions migrate toward the anode in both galvanic and electrolytic cells. In a galvanic cell, a salt bridge supplies anions to the anode half-cell to offset the positive charge produced there. Solubility and acid-base passages use anions as conjugate bases, such as Cl⁻ from HCl or CH₃COO⁻ from acetic acid.

Common confusion: Students link anion with anode because both start with "a", then assume anions must move toward the anode only because the anode is positive. The anode is the site of oxidation in every cell, and its sign changes (negative in galvanic, positive in electrolytic). Anions go toward the anode in both. A mnemonic: anions go to the anode; cations go to the cathode.