There are three basic steps to determining the bond angles in a molecule:

1. Write the Lewis dot structure for the molecule.

Assume that you have to determine the bond angles in ##”BF”_3##.

##”B”## is less electronegative than ##”F”##, so ##”B”## becomes the central atom.

If we have three ##”F”## atoms, that means that we are going to use all three electrons from the ##”B”##.

This gives us three bonding pairs of electrons and 0 nonbonding pairs.

Thus, the steric number, SN — the number of bonding and non-bonding electron pairs — is 3.

The SN is also known as the number of ELECTRON DOMAINS.

2. Use the steric number and VSEPR theory to determine the electron domain geometry of the molecule.

To get the VSEPR geometry, imagine that there is a sphere around the central atom.

Place the electron pairs on the surface of the sphere so that they are as far apart as possible.

This is how two to six electron domains arrange themselves on the surface of a sphere.

Three electron domains arrange themselves evenly around the equator of the sphere to give a trigonal planar shape.

3. Use the shape to determine the angles between the electron domains.

From elementary math, we know that a circle is composed of 360 °.

We divide this number by the number of electron domains and get

##(360 °)/3= 120 °##.

Thus, the bond angles in ##”BF”_3## are 120 °.

This website will be a useful help in understanding how the above method works.