MOLECULAR GEOMETRY
 Elmhurst College
Lewis Diagrams Intro. to Molecular Geometry  Organic Molecular Geometry  Chemistry Department
Electron Pair Geometry Molecular Geometry Types  Virtual ChemBook

 
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  Electron Pair Geometry

Introduction:

The shape of a molecule can be predicted based on the number and arrangement of electron pairs around a central atom. The geometry is determined by minimizing the repulsions between electron pairs in the bonds between atoms and/or lone pairs of electrons as postulated by VSEPR theory.

Valence Shell Electron Pair Repulsion (VSEPR) theory:

Principle: Electron pairs around a central atom arrange themselves so that they can be as far apart as possible from each other.

According to VSEPR theory, molecular geometry can be predicted by starting with the electron pair geometry about the central atom and adding atoms to some or all of the electron pairs. This model produces good agreement with experimental determinations for simple molecules.

Geometry Types:
The most common geometry types are defined by the number of electron pairs around an atom. Electron pairs are defined as electrons in bonds, lone pairs, and occasionally a single unpaired electron. The various geometries are shown in the graphic on the upper left.

 
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 Tetrahedral Electron Pair Geometry Examples:
In methane, ammonia, water and hydrogen fluoride, the electron pair geometry is tetrahedral. All have four pairs of electrons about the central atom (C, N, O, or F). See graphic on middle left.

In methane, C has four bonds.
In ammonia, N has 3 bonds and one lone pair of electrons.
In water, O has 2 bonds and 2 lone pairs of electrons.
In hydrogen fluoride, F as 1 bond and 3 lone pairs.

 
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 Double, Triple Bonds:
Double or triple bonds count as "one pair" of electrons for the purpose of establishing the electron pair geometry. In carbon dioxide, the two double bonds count as two pairs of electrons around the carbon atom, predicting linear geometry. See graphic on bottom left.