Predicting Molecular Geometry

VSEPR theory helps to determine electron-pair geometries and molecular geometries.

A series of steps is used to predict the geometry and bond angles of molecules, such as phosphorus trichloride.

The first step is to draw the Lewis structure of the molecule.

Next, count the total number of electron groups on the central atom. Around phosphorus, there are four electron groups: three bonding pairs and one lone pair.

Now determine the electron-pair geometry. The electron pair geometry is tetrahedral. However, because of the lone pair, the molecular geometry is trigonal pyramidal. The lone pair reduces the bond angle to less than 109.5°.

The same protocol is used to predict the electron-pair geometry and molecular structure for carbon dioxide.

The Lewis structure of carbon dioxide shows the two-electron groups around the carbon atom—as each double bond counts as one electron group. The two-electron groups orient themselves on opposite sides of the central carbon atom with a bond angle of 180°. The electron-pair and molecular geometries are identical because there are no lone pairs on the central atom, and carbon dioxide molecules are linear.

The Lewis structure of tellurium tetrachlorid