Question

Which of the following cations has the smallest radius?

• A. $K^{+}$
• B. $N{a}^{+}$
• C. $L{i}^{+}$
• D. $B{e}^{2+}$

Answer 1

The correct option is D

$B{e}^{2+}$

$K^{+}$ - 3 outer shells, 18 electrons and 19 protons

$N{a}^{+}$ - 2 outer shells, has 10 electrons and 11 protons

$L{i}^{+}$ - 1 shell, has 2 electrons and 3 protons

$B{e}^{2+}$ - 1 shell, has 2 electrons and 4 protons

It is pretty clear that $L{i}^{+}$, $B{e}^{2+}$ are smaller than the other 2 because of only 1 shell.

Now between these, $B{e}^{2+}$ is smaller because of higher effective nuclear charge.