What is the shape of the trimethylamine molecule around the nitrogen atom?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Pyramidal
Hybridisation: nitrogen in an amine is trivalent and carries one unshared electron pair, so its orbitals are sp3 hybridised.
Bonds: three sp3 orbitals overlap with the orbitals of the three CH3 carbons.
Lone pair: the fourth sp3 orbital holds the unshared pair; it is not an atom, so it does not count in the shape.
Shape: N with three groups below it gives a pyramid; the lone pair squeezes the C–N–C angle to 108°, below 109.5°.
Why the other options are incorrect
Option (1) Tetrahedral — Incorrect. A tetrahedral shape needs four bonded groups; here one of the four sp3 orbitals holds a lone pair.
Option (2) Trigonal planar — Incorrect. All four atoms in one plane would need sp2 nitrogen with 120° angles; nitrogen here is sp3 with angles of 108°.
Option (4) Square planar — Incorrect. It needs four groups in one plane around the central atom; nitrogen has only three.
NCERT (2026-27 edition):
9.1 Structure of Amines – “Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore, sp3 hybridised and the geometry of amines is pyramidal. … Due to the presence of unshared pair of electrons, the angle C–N–E, (where E is C or H) is less than 109.5°; for instance, it is 108° in case of trimethylamine as shown in Fig. 9.1.”
Fig. 9.1: Pyramidal shape of trimethylamine