Assertion : The H-O-H bond angle in H_2O is smaller than the H-N-H...

In $\mathrm{H}_2\mathrm{O}$, the central oxygen atom is $sp^3$ hybridized and possesses two lone pairs and two bond pairs. The $\mathrm{H}-\mathrm{O}-\mathrm{H}$ bond angle is approximately $104.5^{\circ}$.

In $\mathrm{NH}_3$, the central nitrogen atom is also $sp^3$ hybridized but possesses one lone pair and three bond pairs. The $\mathrm{H}-\mathrm{N}-\mathrm{H}$ bond angle is approximately $107^{\circ}$.

Thus, the $\mathrm{H}-\mathrm{O}-\mathrm{H}$ bond angle is smaller than the $\mathrm{H}-\mathrm{N}-\mathrm{H}$ bond angle, making the Assertion true.

According to VSEPR theory, lone pair-lone pair repulsion is stronger than lone pair-bond pair repulsion. The presence of two lone pairs on the oxygen atom in water causes greater repulsion on the adjacent bond pairs compared to the single lone pair on the nitrogen atom in ammonia. This greater repulsion compresses the bond angle more in water than in ammonia.

Therefore, the Reason is true and is the correct explanation for the Assertion.

Answer: Both Assertion and Reason are true and Reason is the correct explanation of Assertion.