VSEPR Theory: Molecular Shapes for Five and Six Electron Domains

Shapes of Molecules and the Theories of Chemical Bonding

The VSEPR Model for Five and Six Electron Domains

The Valence Shell Electron Pair Repulsion (VSEPR) model is used to predict the shapes of molecules based on the number of electron domains (bonding and nonbonding pairs) around a central atom. This section focuses on molecules with five and six electron domains, which adopt trigonal bipyramidal and octahedral geometries, respectively.

• Electron Domain: A region where electrons are likely to be found, including both bonding pairs and lone pairs.

• Bonding Domain: An electron domain involved in bonding between atoms.

• Nonbonding Domain: An electron domain consisting of a lone pair on the central atom.

Trigonal Bipyramidal Geometry (Five Electron Domains)

When a central atom has five electron domains, the domains arrange themselves in a trigonal bipyramidal geometry to minimize electron repulsion. This geometry has two distinct positions: axial and equatorial.

• Axial Positions: Two positions aligned perpendicular to the plane of the other three domains.

• Equatorial Positions: Three positions forming a triangle in a single plane, 120° apart.

• Bond Angles: 90° between axial and equatorial positions, 120° between equatorial positions.

• Example: PF5 (Phosphorus pentafluoride) is a classic example of a trigonal bipyramidal molecule.

Effect of Nonbonding Domains on Molecular Shape

Nonbonding (lone pair) domains occupy more space than bonding domains, affecting the molecular geometry. Their placement (axial or equatorial) influences bond angles and overall shape.

• Seesaw Geometry: Four bonding domains and one nonbonding domain result in a seesaw shape. The lone pair prefers an equatorial position to minimize repulsion.

• T-shaped Geometry: Three bonding domains and two nonbonding domains result in a T-shaped molecule. Both lone pairs occupy equatorial positions.

• Linear Geometry: Two bonding domains and three nonbonding domains result in a linear molecule, with all lone pairs in equatorial positions.

• Example: SF4 (seesaw), ClF3 (T-shaped), XeF2 (linear).

Octahedral Geometry (Six Electron Domains)

When a central atom has six electron domains, the domains arrange themselves in an octahedral geometry. All positions are equivalent, and the bond angles are 90°.

• Octahedral Geometry: Six bonding domains, all positions equivalent, bond angles of 90°.

• Square Pyramidal Geometry: Five bonding domains and one nonbonding domain; the lone pair occupies one position, resulting in a square pyramidal shape.

• Square Planar Geometry: Four bonding domains and two nonbonding domains; both lone pairs occupy positions opposite each other, resulting in a square planar shape.

• Example: SF6 (octahedral), BrF5 (square pyramidal), XeF4 (square planar).

Table: Electron Domain Geometries and Molecular Shapes for Five and Six Domains

Electron Domains	Bonding Domains	Nonbonding Domains	Molecular Shape	Example
5	5	0	Trigonal Bipyramidal	PF5
5	4	1	Seesaw	SF4
5	3	2	T-shaped	ClF3
5	2	3	Linear	XeF2
6	6	0	Octahedral	SF6
6	5	1	Square Pyramidal	BrF5
6	4	2	Square Planar	XeF4

Lewis Structures and Molecular Geometry

To confirm the molecular geometry, draw the Lewis structure for each molecule and count the number of bonding and nonbonding domains around the central atom. The arrangement of these domains determines the molecular shape according to the VSEPR model.

• Lewis Structure: A diagram showing the arrangement of valence electrons among atoms in a molecule.

• Application: Use the Lewis structure to identify electron domains and predict geometry.

Key Equations

• Bond Angles in Trigonal Bipyramidal: $120^{\circ}$ (equatorial-equatorial), $90^{\circ}$ (axial-equatorial)

• Bond Angles in Octahedral: $90^{\circ}$ (all adjacent positions)

Additional info: The notes infer that lone pairs prefer equatorial positions in trigonal bipyramidal geometries due to reduced electron repulsion, and that molecular shapes are determined by the number and arrangement of bonding and nonbonding domains.