Bond angles

[Rylee Nelsen 3A]

I am confused as to how you determine whether an angle is less than 109.5 degrees? Could someone give me an example?

[juleschang16]

For trigonal pyramidal geometry the bond angle is slightly less than 109.5 degrees, around 107 degrees. For bent molecular geometry when the electron-pair geometry is tetrahedral the bond angle is around 105 degrees.

[905109118]

Basically, whenever there is a lone pair on top, that means they will push down on the bonds, causing them to have an angle less than 109.5.