Shapes of Molecules (VSEPR)

Shape is determined by repulsion between electron pairs.

• lone pair/lone pair repulsion has the strongest repulsion
• lone pair/bonded pair has the second strongest repulsion
• bonded pair/ bonded pair has the least repulsion

e.g. BF2

2 bonded pairs, 0 lone pairs. Bond angle of 180 degrees.

e.g. BF3

3 bonded pairs, 0 lone pairs. Bond angle of 120 degrees.

4 bonded pairs, 0 lone pairs. Bong angles of 109.5 degrees.e.g. CH4 

6 bonded pairs, 0 lone pairs. Bond angles of 90 degrees.e.g. SF6

e.g. H2O. 2 bonded pairs, and 1 or more lone pairs. Polar, because the lone pair= negative.

Lone pairs remove 2.5 degrees of a bond angle.(H2O = tetrahedral 109.5 minus 2 x 2.5 degrees= 104.5 degrees)

e.g. NH3

3 bonded pairs, 1 lone pair. Polar, because lone pair= negative.

The lone pair repels the bonded pairs down towards eachother. Bong angles 107 degree (like tetrahedral 109.5 minus 2.5 degrees)