4. What effect does the presence of lone-pair electrons have on the bond angles in a molecule?

• A. Lone-pair electrons have no effect on the bond angles.
• B. Lone-pair electrons always result in bond angles of 120 degrees.
• C. Lone-pair electrons decrease the bond angles compared to the ideal geometry.
• D. Lone-pair electrons increase the bond angles compared to the ideal geometry.

Answer: (C)

Verified step by step guidance

1. Understand that lone-pair electrons are pairs of valence electrons that are not involved in bonding but occupy space around the central atom.
2. Recall that electron pairs, whether bonding or lone pairs, repel each other according to the VSEPR (Valence Shell Electron Pair Repulsion) theory, but lone pairs repel more strongly than bonding pairs because they are localized closer to the nucleus.
3. Recognize that this stronger repulsion from lone pairs pushes the bonding pairs closer together, which results in a decrease in the bond angles compared to the ideal geometry where only bonding pairs are considered.
4. Consider an example such as ammonia (NH₃), where the ideal tetrahedral angle is about 109.5°, but the presence of one lone pair reduces the H–N–H bond angles to slightly less than 109.5°.
5. Conclude that the presence of lone-pair electrons decreases bond angles relative to the ideal geometry due to their stronger repulsive effect on bonding electron pairs.