Question

Although geometries of NH₃ and H₂O molecules are distorted tetrahedral, bond angle in water is less than that of ammonia. Discuss.

Answer 1

Explanation:

Both NH₃ and H₂O have distorted tetrahedral geometries due to the presence of lone pairs on the central atom. However, the bond angle in water is smaller than that in ammonia.

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Bond angles:

NH₃ : 107°
H₂O : 104.5°

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Reason for difference in bond angles:

The bond angle depends on the repulsion between electron pairs around the central atom. The order of repulsion is:

Lone pair–lone pair > lone pair–bond pair > bond pair–bond pair

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Ammonia (NH₃):

• Nitrogen has one lone pair and three bond pairs.
• Lone pair–bond pair repulsion reduces the ideal tetrahedral angle (109.5°) to about 107°.

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Water (H₂O):

• Oxygen has two lone pairs and two bond pairs.
• Lone pair–lone pair and lone pair–bond pair repulsions are much stronger.
• These stronger repulsions push the O–H bond pairs closer together, reducing the bond angle further to 104.5°.

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Additional factor:

Oxygen is more electronegative than nitrogen. Hence, the bond pairs in H₂O are drawn closer to oxygen, increasing repulsion with lone pairs and further decreasing the bond angle.

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Conclusion:

Although both NH₃ and H₂O have distorted tetrahedral shapes, the bond angle in water is smaller because oxygen has two lone pairs which exert greater repulsive force than the single lone pair present in ammonia.