Question

Assertion: ${\text{Mg}}^{2+}$and ${\text{Al}}^{3+}$ are isoelectronic but ionic radius ${\text{Al}}^{3+}$ is less than that of ${\text{Mg}}^{2+}$.
Reason: The effective nuclear charge on the outer-shell electrons in ${\text{Al}}^{3+}$ is more than that in ${\text{Mg}}^{2+}$.

• A. Both assertion and reason are true. Reason is the correct explanation of assertion.
• B. Both assertion and reason are true. Reason is not the correct explanation of assertion.
• C. Assertion is true. Reason is false.
• D. Assertion is false. Reason is true.

Answer 1

The correct option is A Both assertion and reason are true. Reason is the correct explanation of assertion.

$\text{Mg}$ has the electronic configuration: ${\text{[Ne]3s}}^2$ and ${\text{Mg}}^{2+}$ cation has the same electronic configuration as of $\text{Ne}$. Likewise, $\text{Al}$ has the electronic configuration of ${\text{[Ne] 3s}}^{2}3{\text{p}}^1$ and ${\text{Al}}^{3+}$ cation has the same configuration as of ${\text{Mg}}^{2+}$. Hence, ${\text{Al}}^{3+}$ and ${\text{Mg}}^{2+}$ are isoelectronic species, that is, they have same number of electrons.

${\text{Al}}^{3+}$ has more protons compared to ${\text{Mg}}^{2+}$. Hence, the effective nuclear charge (${\text{Z}}_{eff}$) of ${\text{Al}}^{3+}$ is higher. Thus, the ionic radius of ${\text{Al}}^{3+}$ will be smaller than ${\text{Mg}}^{2+}$.

Therefore, both assertion and reason are true. Reason is the correct explanation of assertion.