Question
Predict the geometry, hybridization, and bond angle of the ammonia molecule (NH) using VSEPR theory. Compare it with NH and explain the difference.
Solution — Step by Step
Nitrogen (Group 15) has 5 valence electrons. In NH: 3 are shared with H atoms in N-H bonds, leaving 2 as a lone pair.
Around N: 3 bond pairs + 1 lone pair = 4 electron domains.
4 electron domains → hybridization. Electron-domain geometry: tetrahedral. Molecular geometry (counting only atoms): trigonal pyramidal — because the lone pair is invisible in shape descriptions but takes up space.
Ideal tetrahedral angle is . Lone pairs repel more than bond pairs (they’re closer to the central atom), squeezing the bond angles down. NH: H-N-H angle is .
NH: N has 4 bond pairs, no lone pairs. Still , but now perfectly tetrahedral with angles — no lone pair to distort.
Final answer: NH is hybridized, trigonal pyramidal, bond angle. NH is hybridized, tetrahedral, .
Why This Works
VSEPR theory ranks electron-pair repulsions: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair. A lone pair pushes bonds closer together, shrinking bond angles below the ideal hybridization angle.
In NH, donating the lone pair to a proton converts it into a bond pair — repulsion equalises and the molecule snaps to perfect tetrahedral.
Alternative Method
Use the steric number approach. Steric number = (bond pairs) + (lone pairs). NH: SN = 4 → , geometry depends on lone pair count. Same conclusion via a slightly more compact route.
For VSEPR shortcut: count -bonds and lone pairs around the central atom. That gives steric number. SN = 2 → (linear). SN = 3 → (trigonal planar). SN = 4 → (tetrahedral). SN = 5 → . SN = 6 → .
Common Mistake
Saying NH is “tetrahedral” because it is . Hybridization gives the electron-domain geometry; the molecular geometry depends on how many of those domains are atoms vs lone pairs. NH molecules are pyramidal, not tetrahedral.
Forgetting that lone pairs squeeze bond angles. NH has , not . HO has (two lone pairs squeeze even more). Memorise: more lone pairs → smaller bond angles.