Which of the following is least ionic?

🧠 Fajan's rules: ionic character decreases when the cation has high polarising power (small size, high charge, pseudo-noble-gas d¹⁰ configuration). More polarisation → more covalent → less ionic.

| Salt | Cation | Configuration | Ionic character | |---|---|---|---| | KCl | $\ce{K+}$, large | noble gas | high | | $\ce{BaCl2}$ | $\ce{Ba^{2+}}$, large 2+ | noble gas | moderate | | $\ce{CoCl2}$ | $\ce{Co^{2+}}$, small 2+ | d⁷ | moderate | | AgCl | $\ce{Ag+}$, small | d¹⁰ (18-electron config.) | least |

$\ce{Ag+}$ has the pseudo-noble-gas (18e, d¹⁰) configuration. Despite being only +1, the filled d shell screens nuclear charge poorly, making $\ce{Ag+}$ unusually polarising. AgCl is the most covalent of the four.

$\boxed{\text{Answer: (b) AgCl}}$