JEE Main 2023 · 6 April · morning · Numerical answer · +4 / −0
The number of species from the following which have square pyramidal structure is _________
PF₅, BrF₄⁻, IF₅, BrF₅, XeOF₄, ICl₄⁻
Answer and solution
Answer 3
Solution
Chemistry ma’am · 3 steps
Count lone pairs on the central atom: 5 bonds plus 1 lone pair is exactly what makes a square pyramid.
- Count lone pairs. Lone pairs on the central atom = 1/2(valence electrons - electrons used in bonds ± charge). A double bond to O uses 2 of the central atom's electrons, but O still takes only one position.
- Shape rule. Six electron pairs point to the corners of an octahedron. With 5 bond pairs and 1 lone pair the shape is square pyramidal. With 4 bond pairs and 2 lone pairs it is square planar.
Species Bond positions Lone pairs Shape PF₅ 5 (5-5)/2 = 0 trigonal bipyramidal BrF₄⁻ 4 (7+1-4)/2 = 2 square planar IF₅ 5 (7-5)/2 = 1 square pyramidal BrF₅ 5 (7-5)/2 = 1 square pyramidal XeOF₄ 5 (1 O + 4 F) (8-2-4)/2 = 1 square pyramidal ICl₄⁻ 4 (7+1-4)/2 = 2 square planar - So. IF₅, BrF₅ and XeOF₄ are square pyramidal, so the answer is 3.
The common slip: 5. Counting the square planar ions BrF₄⁻ and ICl₄⁻ as well gives 5, not 3. They have 2 lone pairs, so no atom sits at the apex.
The idea to keep. Find the steric number first, then remove the lone-pair positions. For 6 pairs: 6 bonds is octahedral, 5 bonds and 1 lone pair is square pyramidal, 4 bonds and 2 lone pairs is square planar.
Answer from the NTA final answer key. Solution by PrepWiser.