JEE Main 2020 · 5 September · morning · Single correct · +4 / −1
The structure of PCl₅ in the solid state is :
- (a) square pyramidal
- (b) tetrahedral [PCl₄]⁺ and octahedral [PCl₆] –
- (c) square planar [PCl₄]⁺ and octahedral [PCl₆] –
- (d) trigonal bipyramidal
Answer and solution
Answer (b) tetrahedral [PCl₄]⁺ and octahedral [PCl₆] –
Solution
Chemistry ma’am · 3 steps
In the solid, PCl₅ splits into two ions, so apply VSEPR to each ion separately.
- Gas versus solid. As a gas or liquid, PCl₅ is a single trigonal bipyramidal molecule. In the solid it is an ionic crystal: one Cl⁻ moves from one PCl₅ to another, giving [PCl₄]+[PCl₆]⁻.
2PCl₅ → [PCl₄]⁺ + [PCl₆]⁻
Ion Electrons on P Bond pairs, lone pairs Shape [PCl₄]⁺ 5 - 1 = 4 4 bp, 0 lp (sp³) tetrahedral [PCl₆]⁻ 5 + 1 = 6 6 bp, 0 lp (sp³d²) octahedral - Why it splits. A tetrahedral cation and an octahedral anion pack together more efficiently than the bulky trigonal bipyramids do. The ionic attraction makes the crystal more stable.
- So. Solid PCl₅ is made of tetrahedral [PCl₄]⁺ and octahedral [PCl₆]⁻ ions, which is option (b).
The idea to keep. For an ion, add electrons for a negative charge and subtract them for a positive one before counting pairs. Then four pairs with no lone pairs is tetrahedral, and six is octahedral.
Answer from the NTA final answer key. Solution by PrepWiser.