chlorine

Meaning: Chlorine (Cl, Z = 17) is a period 3 halogen with valence configuration 3s²3p⁵. One electron short of an octet, it readily gains an electron to form Cl⁻. It is highly electronegative (below F and O) and a strong oxidizing agent. Elemental chlorine is diatomic Cl₂, a gas at room temperature. Because it is in period 3, it can expand its octet, as in ClO₄⁻, where its oxidation state is +7.

Equation: Cl₂ + 2e⁻ → 2Cl⁻, E° = +1.36 V, where e⁻ is an electron and E° is the standard reduction potential in volts.

On the MCAT: Periodic-trend questions ask you to rank Cl against F, Br, S or Ar. Cl⁻ is larger than the Cl atom and is isoelectronic with Ar and K⁺. In electrolysis of brine, Cl⁻ is oxidized to Cl₂ at the anode. In organic chemistry, Cl₂ with light gives radical halogenation (less selective than Br₂), and Cl⁻ is a good leaving group.

Common confusion: Fluorine is more electronegative, but chlorine has the more negative (more exothermic) electron affinity, because the very small F atom has strong electron-electron repulsion. Electronegativity describes pull on shared electrons in a bond; electron affinity is the energy change when a gaseous atom gains an electron.