Molecular geometries and bonding

Question

1)what condition must be met if a molecule with polar bonds is non-polar?

2)what geometries will give non-polar molecules for AB2, AB3 and AB4 geometries?

Answer 1

1. The molecule must be symmetric, so that the charge on the "surface" of the molecule is the same all over. For example, CCl4. C-Cl bonds are very polar, but the molecule is symmetric, The Cl's are all evenly spaced (at bond anges of 109.5 degrees from each other) around the Carbon. If you replaced one of the Cl with H (CHCl3) then the molecule becones polar because the Charge on the H is very different to that on the Cl's

2. AB2 needs to be linear, so that both B's are pulling equally in the opposite direction. 180 degrees

AB3 - trigonal planar, where all bond angles are 120 degrees in the same plane.

AB4 - tetrahedral, see above CCl4. All bond angles are 109.5 degrees.