Question 6.2: Predicting Relative Sizes of Ions Which atom or ion in each ...
Predicting Relative Sizes of Ions
Which atom or ion in each of the following pairs would you expect to be larger?
(a) S and S^{2-} (b) Cl^{-} and I^{-} (c) Ni and Ni^{2+}
STRATEGY
For each set, compare the subshell of the valence electrons and the effective nuclear charge (Z_{eff}). Larger size correlates with higher n levels of the valence electrons and smaller Z_{eff}.
(a) S^{2-} is larger than S because Z_{eff} decreases as the proton-to-electron ratio decreases. The proton-to-electron ratio in S^{2-} (16p/18e^{-}) is smaller than in S (16p/16e^{-}). Also, two additional electrons in the 3p subshell increase electron–electron repulsions, causing an increase in size.
(b) I^{-} is larger than Cl^{-}. The valence electrons in I^{-} are in the 5p subshell while the valence electrons in Cl^{-} are in the 3p subshell.
(c) Ni is larger than Ni^{2+} because two electrons are removed from the 4s orbital, which is larger than the 3d orbital. Z_{eff} also increases for Ni^{2+} because the ratio of protons to electrons increases; Ni (28p/28e^{-}) and Ni^{2+} (28p/26e^{-}).