Introduction to VSEPR

VSEPR
Valence Shell electron Pair Repulsion
A Brief Tutorial

The Lewis Structure of a covalently bonded molecular assembly displays the distribution of electron pairs associated with each atom in the assembly. There are two kinds of atoms in any assembly, whether the assembly is a neutral molecule or a polyatomic ion:

peripheral atoms are bonded to only one other atom
central atoms are bonded to two or more other (peripheral or central) atoms

VSEPR theory deals with the geometric arrangement of electron domains around a central atom. This geometric arrangement is determined by the number of electron pairs associated with (in the valence shell of) the central atom, as shown in the Lewis Structure. There are two kinds of electron domains:

A Lone Pair (LP) domain is assigned exclusively to the valence shell of one atom.
A Bonding (B) domain is assigned to the valence shells of two atoms; the bond pair comprises a shared pair covalent bond;

The lone pair and bond pair electrons associated with a central atom determine the geometry of the molecule..

Each Lone Pair counts as one electron domain.
Each Single Bond counts as one electron domain.
Each Double Bond counts as one electron domain.
Each Triple Bond counts as one electron domain.

The electron domains associated with the central atom occupy the vertices of a geometric figure, the coordination polyhedron of the central atom:

The total number of domains (LP + B) determines the number of vertices of the coordination polyhedron.
The geometry of this polyhedron is called the Electron Domain Geometry (EDG).
There are five Electron Domain Geometries:
- Linear (2 vertices);
- Trigonal Planar (3 vertices);
- Tetrahedral (4 vertices);
- Trigonal Bipyramidal (5 vertices);
- Octahedral (6 vertices)
The number of bond domains (ignoring all Lone Pair domains) determines the Molecular Geometry (MG).

These geometries are listed at left and can be viewed with the free CHIME browser plug-in. In this list, and in the summary table below, "A" represents a central atom, "B" represents a bond domain (or equivalently, a bonded atom or group of atoms), and "X" represents a lone pair (LP) domain.

ED(A)	B(A)	LP(A)	Form	HO(A)	EDG(A)	MG	Ex.
2	2	0	AB₂	sp	linear	linear	CO₂
2	1	1	XAB	sp	linear	linear	CO
3	3	0	AB₃	sp²	trig plan	trig plan	BF₃
	2	1	XAB₂	sp²	trig plan	bent	SO₂
	1	2	X₂AB	sp²	trig plan	linear	O₂
4	4	0	AB₄	sp³	tetr	tetr	CH₄
	3	1	XAB₃	sp³	tetr	trig pyr	H₃O⁺
	2	2	X₂AB₂	sp³	tetr	bent	H₂O
	1	3	X₃AB	sp³	tetr	linear	OH^-
5	5	0	AB₅	sp³d	trig bipyr	trig bipyr	PF₅
	4	1	XAB₄	sp³d	trig bipyr	seesaw	SF₄
	3	2	X₂AB₃	sp³d	trig bipyr	T	BrF₃
	2	3	X₃AB₂	sp³d	trig bipyr	linear	XeF₂
	1	4	X₄AB	sp³d	trig bipyr	linear	XeF^-
6	6	0	AB₆	sp³d²	oct	oct	SF₆
	5	1	XAB₅	sp³d²	oct	sq pyr	BrF₅
	4	2	X₂AB₄	sp³d²	oct	sq plan	XeF₄
	3	3	X₃AB₃	sp³d²	oct	T	XeF₃^-
	2	4	X₄AB₂	sp³d²	oct	linear	XeF₂^2-
	1	5	X₅AB	sp³d²	oct	linear	XeF^3-

Symbol and Column Key
A	Central Atom
B	Atom bonded to A
X	Lone Pair on A
1	ED(A)	Total number of Electron Domains (Bonding + Lone Pairs) on A
2	B(A)	Number of Bonding Domains on A
3	LP(A)	Number of Lone Pairs on A
4	Form	Formula (including lone pairs on A)
5	HO(A)	Hybrid Orbitals used by A to form s bonds and hold Lone Pairs
6	EDG(A)	Electron Domain Geometry of A
7	MG	Molecular Geometry
8	Ex.	Example compound or ion