Question 1

What is polarisation in Fajans' rules?

Accepted Answer

Polarisation is the distortion of an anion's electron cloud by the electric field of a cation. The cation attracts the cloud toward itself, increasing electron density between the ions and raising the covalent character of the bond.

Question 2

How does cation size affect polarising power?

Accepted Answer

A smaller cation generally has stronger polarising power because its charge is concentrated more strongly. In the common comparison, Na+ polarises Cl− less strongly than Mg2+, and Mg2+ polarises it less strongly than Al3+.

Question 3

Why does a larger halide ion produce more covalent character?

Accepted Answer

A larger halide ion has a more diffuse and easily deformed electron cloud. The deformability order is F− < Cl− < Br− < I−, so the sodium halides show increasing covalent character from NaF to NaI.

Question 4

What is the covalent-character order of the chlorides?

Accepted Answer

For the same chloride anion, the order is NaCl < MgCl2 < AlCl3. The trend follows the increasing polarising power Na+ < Mg2+ < Al3+.

Question 5

Is NaCl purely ionic?

Accepted Answer

No. Na+ does polarise the electron cloud of Cl−, so NaCl has appreciable covalent character. Fajans' rules describe a continuum rather than a rigid boundary between ionic and covalent bonding.

Fajans' Rules 3D Lab — Cation Size Tilts Ionic to Covalent

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