Which ion would you expect to have larger size and why? Mg^++ or Na^+

Both $\mathrm{Na}{}^{+}$ and $\mathrm{Mg}{}^{2+}$ are isoelectronic species, meaning they possess the same number of electrons (10 electrons) and the same electron configuration ($1s^{2}2s^{2}2p^6$).

The size of isoelectronic ions is determined by the nuclear charge (number of protons).

• $\mathrm{Na}{}^{+}$ has 11 protons.
• $\mathrm{Mg}{}^{2+}$ has 12 protons.

Since $\mathrm{Mg}{}^{2+}$ has a higher nuclear charge, the nucleus exerts a stronger attractive force on the same number of electrons compared to $\mathrm{Na}{}^{+}$. This stronger attraction pulls the electron cloud closer to the nucleus, resulting in a smaller ionic radius.

Therefore, $\mathrm{Na}{}^{+}$ is expected to have a larger size than $\mathrm{Mg}{}^{2+}$.