VSEPR Theory

 

Atoms and electrons within a molecule will repel one another.  Therefore, scientists developed the Valence Shell Electron Pair Repulsion (VSEPR) Theory:  In a small molecule, the pairs of valence electrons are arranged as far apart from each other as possible.  In other words, bonded atoms and unshared pairs of electrons take up room around the central atom, and try to get as far away from one another as possible.  This theory does not work for all elements (esp. the transition metals), but is a good basis for understanding

 

Molecular Geometry

With small molecules (both ionic and covalent), there are five typical shapes that exist.  Most small molecules can be classified as one of the following shapes:

 

1) Linear:  All molecules that contain 2 atoms are linear. (Ex. HCl)

Some 3 atom molecules are also linear. (Ex. CO2)  The bond angle in a linear molecule is 180 degrees.

2) Trigonal Planar:

Trigonal means triangular and planar means flat.  A trigonal planar molecule has a central atom bonded to three other atoms and no unshared pairs of electrons on the central atom. (Ex. BCl3)

The bond angle is 120 degrees.

3) Tetrahedral:

Tetra means 4 and hedral means surface.  A tetrahedral has four bonds.

The tetrahedral is 3 dimensional not 2 dimensional.  Therefore, the bond angle is not 90 degrees. Bond angle is 109.5 degrees.

4) Pyramidal:

Same as tetrahedral, but with one pair of unshared electrons.  This electron pair takes the place of one of the bonds of a tetrahedral molecule.  A pyramidal molecule typically has a central atom with three atoms bonded and an unshared pair of electrons.  The unshared pair of electrons take up a little more room (have a greater repulsion force) than a bonded atom, so the bond angle is reduced to 107 degrees.

5) Bent:

Same as tetrahedral, but with two pairs of unshared electrons.  These electron pairs take the place of two of the bonds of a tetrahedral.  A bent molecule has a central atom with two atoms attached and 2 unshared pairs of electrons.  These 2 unshared electron pairs take up still a little more room (have an even greater repulsion force) than a bonded atom, so the bond angle is reduced to 105 degrees.

 

The following diagram summarizes the information above:

 

For a printable version of this diagram, click here:  Molecular Geometry

 

Complete the worksheet:  Molecular Shape