Q.Assertion (A): Though the central atom of both NH3 and H2O molecules are sp^3 hybridised, yet H–N–H bond angle is greater than that of H–O–H.
Reason (R): This is because nitrogen atom has one lone pair and oxygen atom has two lone pairs.

Concept: VSEPR Theory — lone pairs repel more strongly than bond pairs, compressing bond angles.

Reasoning:

1. Both NH$_3$ and H$_2$O have $sp^3$ hybridised central atoms, giving a tetrahedral electron-pair geometry.
2. NH$_3$ has one lone pair (bond angle $\approx 107^\circ$), while H$_2$O has two lone pairs (bond angle $\approx 104.5^\circ$).
3. Lone pair–bond pair repulsion is stronger than bond pair–bond pair repulsion; more lone pairs cause greater compression of the bond angle. …

The key to understanding this lies in VSEPR (Valence Shell Electron Pair Repulsion) Theory. The central idea is simple: electron pairs — whether they are bonding pairs (shared with another atom) or lone pairs (unshared) — repel each other. They arrange themselves as far apart as possible to minimise this repulsion. But not all repulsions are equal.

Lone pairs occupy more space around the central atom than bonding pairs because they are held only by one nucleus and are therefore "fatter" and more diffuse. This means a lone pair pushes harder on its neighbours. The order of repulsive strength is: lone pair–lone pair > lone pair–bond pair > bond pair–bond pair.

Now let's apply this to the two molecules.

1. Both NH₃ and H₂O are sp³ hybridised.

   In both cases, the central atom (N or O) has four electron domains around it — a combination of bonding pairs and lone pairs. For NH₃, nitrogen has three N–H bonds and one lone pair. For H₂O, oxygen has two O–H bonds and two lone pairs. The ideal tetrahedral angle for four equivalent domains is 109.5°.

2. The lone pairs compress the bond angles.

   In NH₃, the single lone pair repels the three N–H bonding pairs. This pushes the bonding pairs closer together, reducing the H–N–H angle from 109.5° to about 107°. In H₂O, there are two lone pairs. Each lone pair repels the bonding pairs and also repels the other lone pair. The combined effect is stronger: the two O–H bonds are squeezed into a smaller angle, giving an H–O–H angle of about 104.5°.

3. Why the difference is exactly as stated. …