Final answer:
Lone pairs cause a decrease in bond angles because they repel more strongly than bonding pairs due to occupying more space around the central atom. This leads to stronger LP-LP and LP-BP repulsions, affecting the molecular geometry by reducing bond angles when lone pairs are present.
Step-by-step explanation:
The reason why an increase in lone pairs causes the size of bond angles in a molecule to decrease is because lone pairs exert a stronger repulsive force than bonding pairs. This strong repulsion results from the fact that lone pairs occupy more space around the central atom than the bonding pairs due to increased electron density, which causes the bonding pairs to be pushed closer together, thus decreasing bond angles. To answer the student's multiple-choice question correctly, option a) Lone pairs repel more strongly than bonding pairs is the right choice.
Lone pairs, by occupying more space, create a higher electron density around the central atom compared to bonding pairs. This electron density leads to a repulsion that is stronger between lone pairs (LP-LP) or between lone pairs and bonding pairs (LP-BP) than the repulsion between bonding pairs (BP-BP) themselves. Consequently, when lone pairs are present, the molecular geometry is affected, as the bonding pairs are forced into positions that reduce their repulsion from the lone pairs, which often results in smaller bond angles. An analogy can be made with the molecule CH₂O, where the electron density impacts its geometry.