Question

Which of the following is smallest in size?

• A. $N^{3-}$
• B. $O^{2-}$
• C. $F^{-}$
• D. $N{a}^{+}$

Answer 1

Answer: (D)

$N{a}^{+}$

An isoelectronic series is a group of ions that all have the same number of electrons.

For example, one isoelectronic series could include $N^{-3},O^{2-},F^{-},N{a}^{+}$. These all have ten electrons. The number of protons, though, increases as atomic number increases, so nuclear charge increases. When we consider effective nuclear charge then, the ions of greater nuclear charge attract those ten electrons more strongly and pull them in more tightly. Therefore, the radii of the ions in an isoelectronic series decrease as nuclear charge (or atomic number) increases.

So order is:

$N{a}^{+}<N^{-3}<O^{2-}<F^{-}$.