Bond angle changes from hybridization and steric effects in adduct formation

Steps:

• BF₃ features sp² boron in trigonal planar geometry with F-B-F at 120°.
• NH₃ has sp³ nitrogen in trigonal pyramidal geometry with H-N-H at ~107°.
• Adduct formation coordinates N lone pair to empty B orbital, making B sp³ and tetrahedral, reducing F-B-F to ~109°.
• At nitrogen, four bonds form (3H + B); BF₃ bulk exceeds lone pair space, compressing H-N-H to ~106°.

Why B is correct:

• Matches VSEPR theory: sp² to sp³ at B decreases angle via increased repulsion; steric crowding at N decreases H-N-H per experimental data.

Why the others are wrong:

• A: H-N-H decreases, not increases, due to BF₃ bulk.
• C: F-B-F decreases from 120° to 109°, not increases.
• D: Neither angle increases; both compress.

Final answer: B