Understand how to use the VSEPR (valence shell electron pair repulsion) model when observing bonding pairs and lone pairs for the compounds and molecules. VSEPR shows that the bonding pairs and lone pairs will not want to be close to one another in the molecule or compound because electrons repel one another. Lone pairs repel one another and a bonding pair more strongly than bonding pairs repel one another. In the VSEPR model, single, double, and triple bonds are all counted as one.
Count the number of bonding pairs that surround the central atom. For example, the molecule SO2 has two bonding pairs, even though each of those bonds from the sulfur to the oxygen is a double bond. The molecule H2O has two bonding pairs. Both bonds from the oxygen to the hydrogen are single bonds.
Count the number of lone pairs that surround the central atom. The octet rule states that the maximum number of electrons in the outer shell must be eight. However, in SO2, sulfur has an expanded octet. Therefore, sulfur has one lone pair. The compound H2O is not an expanded octet as oxygen has two lone pairs.
Determine the shape of the molecule based on the number of bonding pairs and lone pairs. If the molecule or compound has two bonding pairs and one lone pair, then it is linear bent. So, SO2 is linear bent. If the molecule or compound has two bonding pairs and two lone pairs, then it is trigonal pyramidal bent. So, H2O is trigonal pyramidal bent.
Determine the bond angle based on the molecular geometry. The bond angle is a value that must be memorized. For linear bent compounds and molecules, such as SO2, the bond angle is 118 degrees. For trigonal pyramidal bent compounds and molecules, such as H2O the bond angle is 104.5 degrees.