## Q. 7.PS.12

Trends in Ionic Sizes

For each of these pairs, choose the smaller atom or ion:
(a) A $Cu$ atom or a $Cu^{2+}$ ion
(b) A $Se$ atom or a $Se^{2-}$ ion
(c) A $Cu^+$ ion or a $Cu^{2+}$ ion

## Verified Solution

(a) $Cu^{2+}$ ion           (b) $Se$ atom           (c) $Cu^{2+}$ ion

Strategy and Explanation Consider the differences in the sizes of atoms and their corresponding ions based on electron loss or gain.
(a) A cation is smaller than its parent atom; therefore, $Cu^{2+}$ is smaller than a $Cu$ atom.
(b) Anions are larger than their parent atoms, so a $Se^{2-}$ ion is larger than a selenium atom.
(c) Both copper ions contain 26 protons, but $Cu^{2+}$ has one fewer electron than does $Cu^+$.
Correspondingly, the radius of $Cu^{2+}$ is smaller than that of $Cu^+$.