Angular/Bent Angles

[Alan Wu]

Why if a molecule has AX2E1 or AX2E2, the bond angle is slightly less than 120 degrees and not 180 degrees?

Isn't a linear shape 180 degrees??

[Connor Ho 1B]

Yes, it is true that linear structures have 180-degree bond angles, however, AX2E and AX2E2 have bent/angular molecular shapes.

This shape is a result of the repulsion caused by the existence of the lone pair on the central atom. The repulsion is strong enough to make the attached atoms push away from the lone pair and closer to each other in result. This is why the bond angle between the two atoms is less than 180 degrees and closer to 120 degrees.

Lavelle in class demonstrated how (for example in an AX2E molecule) there are three electron densities, making the geometry seem like trigonal planar. However, lone pairs are not represented in molecular geometry, ALTHOUGH they do affect the overall geometry due to repulsion. So, when thinking about molecular geometry, the repulsion effects of the lone pair is represented (this is what causes the bent shape instead of linear) but not the lone pair itself. Only the atoms in the structure are represented.

[JohnWalkiewicz2J]

For the AX2E1 BA, if you look at a trigonal planar (AX3), you see the bond angles are 120. Now imagine substituting one of the bonding atoms with a lone pair (E), considering lone pairs have greater repulsion than bonded pairs, they would repel the two bonding pairs farther away, which would mean the BA would be <120 degrees.

[ahuang]

AX2E follows trigonal planar where one of the bonds is a lone pair, thus the angles are less than 120 degrees.