The ions , ,

CHEMISTRY · Single correct · A Level · Chemistry · Periodicity and periodic trends · Periodicity and periodic trends

Problem

The ions

\text{O}^{2-}

\text{F}^-

\text{Na}^+

, and

\text{Mg}^{2+}

are isoelectronic (all contain 10 electrons). Which ion has the smallest ionic radius? (a)

\text{F}^-

, because it is the most electronegative element of the group, so its electron cloud is held most tightly. (b)

\text{O}^{2-}

, because its higher negative charge draws the electron cloud inward, reducing the radius. (c)

\text{Mg}^{2+}

, because it has the greatest number of protons attracting the same number of electrons. (d)

\text{Na}^+

, because losing one electron substantially decreases electron–electron repulsion, making the ion smallest.