The stability of an ionic bond is primarily determined by the lattice energy. Higher lattice energy leads to a more stable ionic bond. Lattice energy is directly proportional to the product of the charges on the ions and inversely proportional to the sum of their ionic radii.

Concept: Lattice energy $U=\frac{k\cdot |q_1{q}_2|}{r_0}$, where $q_1$ and $q_2$ are the charges of the ions, and $r_0$ is the internuclear distance (sum of ionic radii).

Why (B) is correct:

Among the given options, the pair Mg and F will form the most stable ionic bond. Magnesium forms a ${\mathrm{Mg}}^{2+}$ ion, and fluorine forms a ${\mathrm{F}}^{-}$ ion. The product of the charges is $|(+2)\cdot (-1)|=2$. For the other options:

• (A) Na and Cl: ${\mathrm{Na}}^{+}{\mathrm{Cl}}^{-}$. Product of charges $|(+1)\cdot (-1)|=1$.
• (C) Li and F: ${\mathrm{Li}}^{+}{\mathrm{F}}^{-}$. Product of charges $|(+1)\cdot (-1)|=1$.
• (D) Na and F: ${\mathrm{Na}}^{+}{\mathrm{F}}^{-}$. Product of charges $|(+1)\cdot (-1)|=1$.

Since the product of charges for ${\mathrm{MgF}}_2$ is 2, which is higher than 1 for the other options, ${\mathrm{MgF}}_2$ will have a significantly higher lattice energy, leading to a more stable ionic bond. Although ionic radii also play a role, the charge factor is dominant.

Option Analysis:

• (A) Na and Cl: Forms ${\mathrm{Na}}^{+}{\mathrm{Cl}}^{-}$. Charges are +1 and -1.
• (B) Mg and F: Forms ${\mathrm{Mg}}^{2+}{\mathrm{F}}^{-}$. Charges are +2 and -1. This combination has the highest product of charges, leading to the highest lattice energy.
• (C) Li and F: Forms ${\mathrm{Li}}^{+}{\mathrm{F}}^{-}$. Charges are +1 and -1. Lithium is smaller than sodium, so $\mathrm{LiF}$ would have higher lattice energy than $\mathrm{NaF}$ or $\mathrm{NaCl}$, but still less than ${\mathrm{MgF}}_2$ due to the charge difference.
• (D) Na and F: Forms ${\mathrm{Na}}^{+}{\mathrm{F}}^{-}$. Charges are +1 and -1.

Correct Answer: (B)