Nitrogen in trimethylamine is \( sp^3 \) hybridized. The C-N-C bond angle is 108°. The geometry of trimethylamine is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Pyramidal
The shape of a molecule is set by the arrangement of its atoms, while the hybridisation tells how the electron pairs are arranged. An sp3 nitrogen has four hybrid orbitals pointing to the corners of a tetrahedron. In amines, three of them form σ bonds and the fourth holds an unshared pair of electrons.
In trimethylamine, \( \ce{(CH3)3N} \), the three methyl groups take three corners and the lone pair the fourth. The four atoms N and three C then form a pyramid with nitrogen at the top. The lone pair repels the bond pairs more strongly than they repel each other, so the C–N–C angle is squeezed from 109.5° to 108°. So the geometry is pyramidal.
Why the other options are incorrect
Option (1) Tetrahedral — Incorrect. This is the arrangement of the four electron pairs; a tetrahedral shape needs four bonded atoms, as in \( \ce{(CH3)4N+} \).
Option (2) Trigonal bipyramidal — Incorrect. This needs five electron pairs (sp3d), as in \( \ce{PCl5} \).
Option (3) Square planar — Incorrect. This needs dsp2 hybridisation, as in \( \ce{[Ni(CN)4]^2-} \).
NCERT (2026-27 edition):
Structure of Amines
