Introduction to Molecular Geometry

Molecular geometry or molecular structure is the three-dimensional arrangement of atoms within a molecule. It is important to be able to predict and understand the molecular structure of a molecule because many of the properties of a substance are determined by its geometry.

The Valence Shell, Bonding Pairs, and VSEPR Model

The outermost electrons of an atom are its valence electrons. The valence electrons are the electrons that are most often involved in forming bonds and making molecules.

Pairs of electrons are shared between atoms in a molecule and hold the atoms together. These pairs are called "bonding pairs".

One way to predict the way electrons within atoms will repel each other is to apply the VSEPR (valence-shell electron-pair repulsion) model. VSEPR can be used to determine a molecule's general geometry.

Predicting Molecular Geometry

Here is a chart that describes the usual geometry for molecules based on their bonding behavior. To use this key, first draw out the Lewis structure for a molecule. Count how many electron pairs are present, including both bonding pairs and lone pairs. Treat both double and triple bonds as if they were single electron pairs. A is used to represent the central atom. B indicates atoms surrounding A. E indicates the number of lone electron pairs. Bond angles are predicted in the following order:

lone pair versus lone pair repulsion > lone pair versus bonding pair repulsion > bonding pair versus bonding pair repulsion

Molecular Geometry

Geometry	Type	# of Electron Pairs	Ideal Bond Angle	Examples
linear	AB2	2	180°	BeCl2
trigonal planar	AB3	3	120°	BF3
tetrahedral	AB4	4	109.5°	CH4
trigonal bipyramidal	AB5	5	90°, 120°	PCl5
octohedral	AB6	6	90°	SF6
bent	AB2E	3	120° (119°)	SO2
trigonal pyramidal	AB3E	4	109.5° (107.5°)	NH3
bent	AB2E2	4	109.5° (104.5°)	H2O
seesaw	AB4E	5	180°,120° (173.1°,101.6°)	SF4
T-shape	AB3E2	5	90°,180° (87.5°,<180°)	ClF3
linear	AB2E3	5	180°	XeF2
square pyramidal	AB5E	6	90° (84.8°)	BrF5
square planar	AB4E2	6	90°	XeF4