Textbook question 2E.1

[Paige Lee 1A]

For part A, could someone please explain why there must be lone pairs for a 120 degree bond angle?

[Kelvin Chung 1C]

The two molecules attached to the central atom are generally more electronegative than the central atom. There must be lone pairs on the molecule with a 120 degree bond angle so that the otherwise linear molecule "bends" downward through electron repulsion with these more electronegative atoms.

[Jacey Yang 1F]

The lone pair on the central atom has a stronger repelling effect, pushing the bonding pairs closer together to create a bent shape.

[Qilan Li 4I]

Since bond angles are affected by the presence of lone pairs of electrons at the central atom, we can tell if there are lone pairs when a picture only shows bonded pairs/atoms. A lone pair of electrons at the central atom would always repel the shared pair/bonded pair which is why the 120-degree angle would be formed. If there are no lone pairs, the bonded pairs would try to be as far away from each other as possible which would be 180 degrees, and the lone pairs prevent them from doing that (however 180 degrees between bonded pairs can still have lone pairs). Sorry if that was confusing but that is my thought process and I'm not 100% if it's correct.

[Paige Lee 1A]

I'm still confused, what about trigonal planar molecules? Don't they have 120 degree bond angles, but no lone pairs?