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Pick the CORRECT trend among these statements: (A) NaF < MgF2 < AlF3: covalent character (B) NaF < MgF2 < AlF3: melting point (C) NaF < MgF2 < AlF3: lattice energy (D) NaF > MgF2 > AlF3: polarising power of the cation

1. (A) NaF < MgF2 < AlF3: covalent character order.
2. (B) NaF < MgF2 < AlF3: melting point order
3. (C) NaF < MgF2 < AlF3: lattice energy order
4. (D) NaF > MgF2 > AlF3: order of polarising power of cation.

Correct answer: (A) NaF < MgF2 < AlF3: covalent character order.

Solution

Polarising power increases Na+ < Mg2+ < Al3+, so covalent character increases NaF < MgF2 < AlF3 (A is correct). Because AlF3 is more covalent it does NOT have the highest melting/lattice point in the simple ionic sense; (D) reverses the true polarising-power order, and (B)/(C) are not the intended correct trend.

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