According to VSEPR theory, the shape of a molecule or ion of the type AXn depends on the total number of bonding and nonbonding electron pair regions at A. The summary below shows the possible shapes for molecules having three or four electron pair regions about a central atom.
Although successive replacement of bonded atoms X by nonbonding electron pairs E does not change the arrangement of electron pair regions about atom A, the shape descriptor does change because this describes the location of the outer atoms in the molecule.
The angle between adjacent AX bonds is smaller if there are non-bonding electron pairs at A because these occupy more space at A than bonding electron pairs.
A method for deducing the number of electron pair regions at a central atom for the formula of the species is illustrated below for F4B– .
Deduce the orientation of the bonding electron pair regions and the shape of the species.
Although successive replacement of bonded atoms X by nonbonding electron pairs E does not change the arrangement of electron pair regions about atom A, the shape descriptor does change because this describes the location of the outer atoms in the molecule.
| AX3 | AX2E |
 |  |
| trigonal planar | bent |
| AX4 | AX3E | AX2E2 |
 |  |  |
| tetrahedral | trigonal pyramid | bent |
The angle between adjacent AX bonds is smaller if there are non-bonding electron pairs at A because these occupy more space at A than bonding electron pairs.
A method for deducing the number of electron pair regions at a central atom for the formula of the species is illustrated below for F4B– .
Note that this method applies to cases where halogen or H are the outer atom and the bonding electron pair regions contain only two electrons (single bond).
Deduce the number of electrons at the central atom.The number contributed by the atom itself is the last digit of its Group number (adjusted for charge): 4 for B in BF4–
The number contributed by bonded atoms is one for each halogen or hydrogen: 4 for –BF4
Total electrons at the central atom: 8 for BF4–
Deduce the number and type of electron-pair regions at the central atom. The number contributed by bonded atoms is one for each halogen or hydrogen: 4 for –BF4
Total electrons at the central atom: 8 for BF4–
Total number of electron pair regions at central atom (total electrons at central atom divided by two): 4 –BF4
Number of bonding electron-pair regions: 4 for BF4– (because there are four bonds)
Number of nonbonding electron pairs regions: 0 for BF4– (because there are a total of four electron-pair regions).
Deduce the orientation of the bonding AND non-bonding electron-pair regions.Number of bonding electron-pair regions: 4 for BF4– (because there are four bonds)
Number of nonbonding electron pairs regions: 0 for BF4– (because there are a total of four electron-pair regions).
For the four electron-pair regions in BF4–, the orientation is tetrahedral.
Deduce the orientation of the bonding electron pair regions and the shape of the species.
BF4– (an example of AX4) is tetrahedral.