structures can be used to predict the 3-dimensional shape of a molecule by
using a simple theory known as VSEPR: Valence Shell Electron Pair
We know that electrons repel each other, since they have like charges. VSEPR
theory states that the electrons in a molecule repel each other, so the orbitals
containing those electrons are oriented as far away from each other as possible.
In the simplest example, consider the molecule BeH2, beryllium
hydride. BeH2 has two bonds, one between each hydrogen and the
beryllium, and no lone pairs. Its Lewis structure is
The electrons in the two
bonds repel each other, thus, they will be as far apart as possible. Therefore,
the hydrogens will be on opposite sides of the beryllium and the molecule is
linear, with an angle of 1800 between the two bonds. The Chime
model below shows the BeH2 molecule.
VSEPR can be used to predict the structures around atoms with more than two
things attached: for example, the methane molecule CH4 has the four
hydrogens at the tips of a tetrahedron, with the angles between the bonds =
Double and triple bonds act exactly like single bonds for VSEPR purposes.
Lone pairs affect the molecular geometry, but VSEPR does a good job predicting
structures with lone pairs as well. For a molecule with one or more lone pairs
- A lone pair acts much like a bond for arranging things around the atom. It
repels bonds much like a bond would.
- The molecular geometry is quite a bit different than the species without
the lone pair. The lone pair is not included in the molecular geometry. See
the links below for examples.
The links below will give you models of the various molecular geometries. A
useful shorthand to write the type of geometry is the
AXmEn notation, where A is the central atom, surrounded by
m X things and n lone pairs.