Geometrical shapes of molecules using VSEPR theory
VSEPR theory can be used to predict actual geometrical shapes of molecules and composite ions by determining bond directions and angles relative to the central atom. VSEPR is short for Valence Shell Electron Pair Repulsion.
Shapes of molecules according to VSEPR theory
According to VSEPR theory, the electron groups surrounding the central atom in a molecule or composite ion will be located as far apart from each other as possible. This is due to the repulsion of the electrons. All electrons have the same (negative) charge, so a pair of electrons in the bonds surrounding the central atom in a molecule will repel other electron pairs.
The shape and bond angles of a covalently bonded molecule depends on the electron groups around the central atom. An electron group can either be a:
- single bond (a pair of electrons)
- double bond (2 pairs of electrons)
- triple bond (3 pairs of electrons)
- lone pair of electrons
- single electron (as in a radical)
Lone pair electrons have a more concentrated electron charge cloud than bonding pair electrons. They have wider cloud charges that are slightly closer to the nucleus of the central atom. This leads to different amount of repulsion between different types of electron pairs.
The order of repulsion is:
- lone pair–lone pair (strongest repulsion)
- lone pair–bond pair
- bond pair–bond pair (weakest repulsion)
Using VSEPR theory to determine the shape of water molecules
To predict the molecular geometry of a water molecule we use the VSEPR theory. The VSEPR theory tells us that the electron groups surrounding the central atom will repel each other as far apart as possible. From the Lewis structure of water, the oxygen atoms in the water molecule are surrounded by four electron groups (two lone pairs and two bonding pairs) as shown below:
The four electron groups in the water molecule are as far apart as possible when they are placed in a so-called tetrahedron with angles of 109.5° around the central atom. The two hydrogen atoms will be placed on two of the vertices of the tetrahedron and the two lone pairs will occupy the two other vertices.
This produces the H-O-H bonds that are not a straight line but rather a V-shape. However, in reality, the angle of the H-O-H bonds is however slightly smaller that the 109.5° of the tetrahedral but 104° because the lone pairs have a higher repulsion than bond pair electrons. Thus lone pairs will push the bond pair electrons together, making the H-O-H bond angle smaller than the tetrahedral angle of 109.5°.
Other examples of molecular geometry
The differences in electron-pair repulsion determine the shape and bond angles in a molecule. As examples, let us compare the shapes and bond angles of methane, ammonia and water
Molecular shape and bond angles of ammonia
Ammonia has three bonding pairs of electrons and one lone pair. The lone pair–bond pair repulsion is greater than bond pair–bond pair repulsion and so the bonding pairs of electrons are pushed closer together. This gives the ammonia molecule a triangular pyramidal shape, with he H-N-H bond angle is being about 107°.
Molecular shape and bond angles of methane
Since methane has four bonding pairs of electrons surrounding the central carbon atom, the equal repulsive forces of each bonding pair of electrons results in a tetrahedral structure with all H-C-H bond angles being 109.5°.
Molecular shape and bond angles of water
Water has two bonding pairs of electrons and two lone pairs. The greatest electron pair repulsion is between the two lone pairs. This results in the bonds being pushed even closer together. The shape of the water molecule is a nonlinear V shape. The H-O-H bond angle is 104.5°.
Differences between bond angles of hydrogen sulphide and water
Hydrogen sulphide (H2S) and water (H2O) have the same have the same bonding structure, same molecular shape and same number of lone pairs of electrons. What causes the difference in their bond angle?
The bonding in water is H-O-H with two lone pairs and the bonding in hydrogen sulphide is H-S-H with two lone pairs. The differences in their bond angle is caused by the differences in electronegativities of sulphur and oxygen. Electronegativity is the ability of an atom which is covalently bonded to the other atom to attract the bond pair of electrons towards itself.
Oxygen is more electronegative than sulphur and this causes the bond pairs of electrons in water to be closer to the oxygen atom than they are to the sulphur atom in hydrogen sulphide. This causes greater repulsion in the O-H bonds than in the S-H bonds, leading to H-S-H having a bond angle of 92.5° and H-O-H having 104.5°.