Amongst NaCl, MgCl₂, AlCl₃, in which compound the percentage ionic character in the bonds is lowest?

To determine which of the three chlorides has the lowest percentage of ionic character, we compare how strongly the cations polarize the chloride anion.

Fajans’ rules state that covalent (or less ionic) character increases when:

1. The cation has a high charge.
2. The cation has a small ionic radius (high charge density).
3. The anion is large and easily polarizable (Cl⁻ is the same in all three compounds, so this factor does not differentiate them).

Now examine each cation:

• $\mathrm{Na}⁺$: charge = +1, relatively large radius → low polarizing power → bond is mostly ionic.

• $\mathrm{Mg}²⁺$: charge = +2, smaller radius than Na⁺ → stronger polarizing power than Na⁺, so the Mg–Cl bonds have a bit more covalent character.

• $\mathrm{Al}³⁺$: charge = +3, even smaller radius → very high charge density → strongest polarizing power among the three. According to Fajans’ rules, Al³⁺ will draw electron density from Cl⁻ toward itself, giving the Al–Cl bonds the greatest covalent (least ionic) character.

Therefore, the compound with the lowest percentage of ionic character is AlCl₃.