Question

The combination of which of the following gives an ionic compound with maximum covalent character:

• A. \(\text{Mg}^{2+}\) and \(\text{Cl}^{-}\)
• B. \(\text{Mg}^{2+}\) and \(\text{O}^{2-}\)
• C. \(\text{Na}^{+}\) and \(\text{Br}^{-}\)
• D. \(\text{Na}^{+}\) and \(\text{O}^{2-}\)

Hint:

According to Fajan’s rule, covalent character increases with: highly charged small cation and \[ \text{highly charged large anion}. \]

Answer 1

The Correct Option is B

Step 1: State Fajans' Rules for covalent character in ionic compounds.
Covalent character in an ionic bond increases when: (i) the cation is small and highly charged, and (ii) the anion is large and highly charged. The cation polarizes the electron cloud of the anion.
Step 2: Evaluate the polarizing power of cations.
Polarizing power = charge/size. A smaller, more highly charged cation has greater polarizing power. Options to compare: Li⁺, Na⁺, Mg²⁺, Al³⁺. Among these, Al³⁺ and Mg²⁺ have high charge; Mg²⁺ is relatively small.
Step 3: Evaluate polarizability of anions.
Larger anions with high charge are more polarizable. O^2- is small but highly charged; Cl^-, Br^-, I^- are large. However, O^2- is highly charged (2-).
Step 4: Apply Fajans' Rules to the given options.
The combination Mg²⁺ + O^2-: Mg²⁺ is a small, doubly-charged cation with high polarizing power; O^2- is a doubly-charged anion. The high charge on both means greater covalent character.
Step 5: Compare with other options.
Combinations with singly-charged ions (Na⁺, K⁺ with Cl^-) have less polarizing power and lower covalent character. Mg²⁺/O^2- shows maximum covalent character due to high charges on both ions.
Step 6: State the conclusion.
Among the given combinations, Mg²⁺ and O^2- form an ionic compound with maximum covalent character due to the high charge density of Mg²⁺ and the high polarizability of O^2-.
\[ \boxed{Mg^{2+} \text{ and } O^{2-}} \]