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# Introduction to Molecular Geometry

## Three-Dimensional Arrangement of Atoms in a Molecule

There are two electron pairs around the central atom in a molecule with linear molecular geometry, 2 bonding electron pairs and 0 lone pairs. The ideal bond angle is 180°.

Ben Mills
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

Anne Marie Helmenstine, Ph.D.