Why can anions with the same number of electrons have different radii?

Why is Cl^- a smaller ion than P^-3?
Becki

Both chloride and phosphide have the same electronic configuration. But the chloride has 17 protons in its nucleus, while the phosphide has only 15. The chloride is smaller because its electrons are drawn in closer to the higher positive charge on its nucleus.

The phosphide ion has a radius of 212 pm, while the chloride has a radius of 181 pm.

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
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Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/periodic/faq/radii-of-isoelectronic-anions.shtml