Which of the following is least ionic?

Solution

Chemistry ma’am · 4 steps

Same anion everywhere, so check the cation: Ag⁺ with its 18-electron shell polarises most.

1. Fajans' rules. An ionic bond becomes partly covalent when the cation distorts (polarises) the electron cloud of the anion. The anion is Cl⁻ in all four, so only the cation decides the answer.
2. What polarises more. A cation polarises more when it is small, highly charged, or has an 18-electron outer shell (ns² np⁶ nd¹⁰). The d electrons shield the nuclear charge poorly, so the anion feels a stronger pull than it would from a noble-gas cation of similar size.

   Cation	Outer shell	Polarising power	Ionic character
   K⁺	noble gas, 8 electrons	very low (large, +1)	highest
   Ba²⁺	noble gas, 8 electrons	low (large, +2)	high
   Co²⁺	partly filled 3d⁷	moderate	fairly high
   Ag⁺	18 electrons, 4d¹⁰	high	lowest

3. Evidence. The other three chlorides dissolve in water as ions. AgCl is almost insoluble, which is a typical sign of strong covalent character.
4. So. Ag⁺ polarises Cl⁻ the most, so AgCl is the least ionic. That is option (d).

The idea to keep. When the anion is the same, judge covalent character by the cation. Small size, high charge and an 18-electron shell (like Ag⁺, Cu⁺, Zn²⁺) all raise polarising power.

Answer from the NTA final answer key. Solution by PrepWiser.