**Valence shell electron pair repulsion** (VSEPR) theory is a model for predicting the overall shape of a molecule. VSEPR theory postulates that a given molecule will adopt an ideal shape to reduce the repulsion between the surrounding elements and the lone pairs on the central element.

When drawing a compound you have to take into account two different systems for Lewis Structures: Molecular geometry and electronic geometry.

**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)**

For example, a molecule with a designation of AX_{2}E_{3} would mean it has a central element (A), two surrounding elements (X) and three lone pairs (E) on the central element.

**Predicting Molecular Geometry**

Based on molecular geometry when the domain is equal to 2 then AX_{2} is possible. This molecule would be linear and possess a bond angle of 180^{o}.

Molecular Geometry (Domain of 2)

When the domain is equal to 3 then AX_{3} and AX_{2}E_{1} are possible. As the central element gains lone pairs the idealized bond angle will be compressed. AX_{3} is trigonal planar and possesses an idealized bond angle of 120^{o}, while AX_{2}E_{1} is bent and has its bond angle compressed to less than 120^{o}.

Molecular Geometry (Domain of 3)

When the domain is equal to 4 then AX_{4}, AX_{3}E_{1 }and AX_{2}E_{2} are possible. AX_{4} is tetrahedral, AX_{3}E_{1} is trigonal pyramidal and AX_{2}E_{2} is bent.

Molecular Geometry (Domain of 4)

When the domain is equal to 5 then AX_{5}, AX_{4}E_{1, }AX_{3}E_{2}, and AX_{2}E_{3} are possible. AX_{5} is trigonal bipyramidal, AX_{4}E_{1} is seesaw, AX_{3}E_{2} is T-shaped and AX_{2}E_{3} is linear.

Molecular Geometry (Domain of 5)

When the domain is equal to 6 then AX_{6}, AX_{5}E_{1, }and AX_{4}E_{2} are possible. AX_{6} is octahedral, AX_{5}E_{1} is square pyramidal, and AX_{4}E_{2} is square planar.

Molecular Geometry (Domain of 6)

**Electronic Geometry**

Under electronic geometry we treat surrounding elements and lone pairs on the central element as the same.

**A = Central Element** **X = Surrounding Element** **& Lone Pairs on Central Element**

For example, a molecule with a designation of AX_{2}E_{3} would mean it has a central element (A), two surrounding elements (X) and three lone pairs (E) on the central element. Following its electronic geometry we would re-evaluate it as AX_{6. }

**Predicting Electronic Geometry**

By treating surrounding elements and lone pairs on the central elements as the same we greatly reduce the number of shapes. However, notice that the compression of bond angles hasn’t changed because the behavior of lone pair repulsion still occurs.

Compounds with a domain of 2 are still seen as being linear.

Electronic Geometry (Domain of 2)

Following the electronic geometry, all compounds with a domain of 3 are now seen as trigonal planar.

Electronic Geometry (Domain of 3)

Compounds with a domain of 4 are tetrahedral.

Electronic Geometry (Domain of 4)

Compounds with a domain of 5 are trigonal bipyramidal.

Electronic Geometry (Domain of 5)

Compounds with a domain of 6 are octahedral.

Electronic Geometry (Domain of 6)

The electronic geometry can be seen as the simple geometry and is useful in determining hybridization (s, sp, sp^{2}, sp^{3}, sp^{3}d, sp^{3}d^{2}).

**More Than Just Shapes**

The importance of these geometries goes beyond all the cool shapes. The overall polarity of a compound is based on both bond polarity and molecular shape. The way these molecules are formed is also based on our understanding of valence electrons, electron configurations and periodic trends.