The Valence Shell, Bonding Pairs, and VSEPR ModelThe 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 GeometryHere 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
|Geometry||Type||# of Electron Pairs||Ideal Bond Angle||Examples|
|trigonal bipyramidal||AB5||5||90°, 120°||PCl5|
|trigonal pyramidal||AB3E||4||109.5° (107.5°)||NH3|
|square pyramidal||AB5E||6||90° (84.8°)||BrF5|