According to VSEPR theory, the shape of a molecule AXn depends on the total number of bonding and nonbonding electron pair regions (domains) at A.
The number of non-bonding electron pair regions (domains) is equal to the number of non-bonding electron pairs.
The number of bonding electron pair regions (domains) depends on the number of atoms bonded to the central atom NOT on the type of the bond.
Replacement of bonded atoms X by nonbonding electron pairs E
The angle between adjacent AX bonds is lower if there are non-bonding electron pairs at A because these occupy more space at A than bonding electron pairs.
The number of non-bonding electron pair regions (domains) is equal to the number of non-bonding electron pairs.
The number of bonding electron pair regions (domains) depends on the number of atoms bonded to the central atom NOT on the type of the bond.
Irrespective of whether two atoms AB are joined by a single, double or triple bond, the bond is regarded as one bonding domain (region). Additional bonding pairs between atoms do not change the bond direction!
The summary below shows the possible shapes for molecules having two, three or four electron pair regions (domains) about a central atom. Replacement of bonded atoms X by nonbonding electron pairs E
does not change the arrangement of electron pair regions about atom A.
does change the shape descriptor because this describes the location of the outer atoms in the molecule.
does change the shape descriptor because this describes the location of the outer atoms in the molecule.
| AX2 | |
 | |
| linear |
| AX3 | AX2E | |
 |  | |
| trigonal planar | bent |
| AX4 | AX3E | AX2E2 |
 |  |  |
| tetrahedral | trigonal pyramidal | bent |
The angle between adjacent AX bonds is lower if there are non-bonding electron pairs at A because these occupy more space at A than bonding electron pairs.