Valence shell electron pair repulsion (VSEPR) theory is a model for predicting the overall shape of a molecule. Molecular geometry represents the accurate three-dimensional shape of a compound that takes into account the effects of lone pairs, bond lengths, bond angles and the atomic size of elements.

**How to Determine Molecular Geometry**

A domain represents the number of surrounding elements and lone pairs on the central element. Under molecular geometry we treat surrounding elements as different from the lone pairs on the central element.

**A = Central Element** **X = Surrounding Element** **E = Lone Pair (nonbonding electrons)**

**Domain of 2 **

When a molecule has a domain of 2 then only AX_{2} is possible. An AX_{2} molecule would have a central element (A) connected to 2 surrounding elements (X) and possess a linear molecular geometry.

**Domain of 3**

A domain of 3 has AX_{3} and AX_{2}E_{1} as possible orientations. For an AX_{3} orientation the central element (A) is connected to 3 surrounding elements (X) and possess a trigonal planar molecular geometry.

For an AX_{2}E_{1} orientation the central element (A) is connected to 2 surrounding elements (X), 1 lone pair (E) and possess a bent molecular geometry.

**Domain of 4**

A domain of 4 has AX_{4}, AX_{3}E_{1 }and AX_{2}E_{2} as possible orientations. For an AX_{4} orientation the central element (A) is connected to 4 surrounding elements (X) and possess a tetrahedral molecular geometry.

For an AX_{3}E_{1} orientation the central element (A) is connected to 3 surrounding elements (X), 1 lone pair (E) and possess a trigonal pyramidal molecular geometry.

For an AX_{2}E_{2} orientation the central element (A) is connected to 2 surrounding elements (X), 2 lone pairs (E) and possess a bent molecular geometry.

**Domain of 5**

A domain of 5 has AX_{5}, AX_{4}E_{1, }AX_{3}E_{2}, and AX_{2}E_{3} as possible orientations. For an AX_{5} orientation the central element (A) is connected to 5 surrounding elements (X) and possess a trigonal bipyramidal molecular geometry.

For an AX_{4}E_{1} orientation the central element (A) is connected to 4 surrounding elements (X), 1 lone pair (E) and possess a seesaw geometry.

For an AX_{3}E_{2} orientation the central element (A) is connected to 3 surrounding elements (X), 2 lone pairs (E) and possess a T-shape geometry.

For an AX_{2}E_{3} orientation the central element (A) is connected to 2 surrounding elements (X), 3 lone pairs (E) and possess a linear geometry.

**Domain of 6**

A domain of 6 has AX_{6}, AX_{5}E_{1, }and AX_{4}E_{2} as possible orientations. For an AX_{6} orientation the central element (A) is connected to 6 surrounding elements (X) and possess an octahedral molecular geometry.

For an AX_{5}E_{1} orientation the central element (A) is connected to 5 surrounding elements (X), 1 lone pair (E) and possess a square pyramidal geometry.

For an AX_{4}E_{2} orientation the central element (A) is connected to 4 surrounding elements (X), 2 lone pairs (E) and possess a square planar geometry.

Your understanding of these molecular geometries will be important in your understand of additional Lewis structure concepts such as the electron geometry, hybridization, polarity, the intermolecular forces and other essential bonding theories.