A chlorine ion is than a phosphorus ion.

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

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