Angles between bonds

[Alicia Beebe]

I noticed when doing some of the homework problems that sometimes the angles were listed as exactly 120 degrees, but sometimes it was listed as "slightly less than" 120 degrees. I'm just using 120 as an example, but what determines whether its just 120 or slightly less than?

[Isabelle De Rego 1A]

The presence of lone pairs determines whether or not the bond length is slightly less. So, if a lone pair is present then the bond angle will decrease.

[octaviahuang1f]

Like what Isabelle said, a lone pair forms a region of e- density that repels more than bonded pairs. To achieve the least amount of repulsion, the rest of the bond angles become slightly smaller. Hope this helps!!