A chlorine ion is smallerlarger than a phosphorus ion.

A chloride ion (Cl⁻) and a phosphide ion (P³⁻) each have 18 electrons. Because they have the same number of electrons, the ion with more protons pulls those electrons in more tightly. Chlorine has 17 protons, compared with phosphorus’s 15, so the chlorine ion is smaller.

AI-written and checked against the verified answer.