Concept: Covalent Character in Ionic Compounds

Covalent character in ionic compounds arises due to polarization of anions by cations. According to Fajans' rules, covalent character increases with:

1. Smaller cation size
2. Larger anion size
3. Higher charge on cation

We compare the given chlorides (common anion Cl^-) based on cation properties:

Step 1: List cations and their properties

• Sn²⁺: Ionic radius ~118 pm, charge +2
• Fe²⁺: Ionic radius ~78 pm, charge +2
• Al³⁺: Ionic radius ~53.5 pm, charge +3
• Mg²⁺: Ionic radius ~72 pm, charge +2

Step 2: Apply Fajans' rules

Smaller cation size and higher charge increase polarization power. Compare polarization power using charge/size ratio:

Sn²⁺: $\frac{2}{118}\approx 0.017$

Fe²⁺: $\frac{2}{78}\approx 0.026$

Al³⁺: $\frac{3}{53.5}\approx 0.056$

Mg²⁺: $\frac{2}{72}\approx 0.028$

Step 3: Compare polarization power

Al³⁺ has the highest charge/size ratio (0.056), indicating strongest polarization ability.

Final Answer

AlCl₃ shows the maximum covalent character due to the small size and high charge (+3) of Al³⁺.

Related Topics

• Fajans' rules
• Ionic and covalent bonding
• Polarization and polarizing power
• Periodic trends in ionic compounds

Key Formulae and Theory

Polarizing power ∝ Charge / (Ionic radius)²

Fajans' rules predict covalent character when:

• Cation is small and highly charged
• Anion is large
• Cation has pseudo noble gas configuration

Al³⁺ is small (53.5 pm) with high charge (+3), making AlCl₃ predominantly covalent.