shapes of molecules

Shapes of molecules

tetrahedral molecule

The shapes of molecules is a new topic for A-level, it was never really mentioned in GCSE chemistry. Stereochemistry is the term which is often used when discussing the shapes of molecules. Molecules come in all sorts of shapes and sizes, from small molecules such as water to very large biological polymers such as proteins and DNA. The shape of molecules is perhaps most important in the chemical reactions that take place in living organisms, where a small alteration in the shape of an enzyme or a drug molecule can have dramatic effects on its action or its side effects.

The overall shape of a molecule is determined by its bond angles, which are the angles between the lines representing the bonds between the nuclei of the atoms in the molecule. The molecule shown opposite has a central black atom surrounded by 4 other atoms. The shape of this molecule is tetrahedral. All the bond angles are all 109.50. To draw 3d molecules such as this tetrahedral one we use dotted or dashed lines to represent bonds which are behind the plane and a dark shaded triangular line to represent bonds sticking out in front of the plane. Solid lines show bonds which are in the plane. You can see in this tetrahedral molecule that 2 of the bonds are in the plane, one is behind and one is in front. This is shown in the image opposite.

The valence-shell electron pair repulsion model (VSEPR model)

The VSEPR model is the one we are going to use to work out the shapes of different molecules. Its name is a bit of a mouth full but it is very easy to apply and working out the shapes of molecules using this model is very straight forward. As the title suggests we need to consider the electrons in the valance shell (outer shell), that is the ones which get involved in bond formation. We already know that these electrons pair up to form covalent bonds, which involve the sharing of a pair of electrons. These electrons are attracted to the nuclei of the atoms involved in the bonds and it is this attraction that holds the electrons in place. However the pair of electrons in a covalent bond will repel other electrons involved in bonding around the central atom. As a result of this repulsion the electron pairs will get as far apart as possible while still retaining their distance from the nuclei.

trigonal planar molecule In the example shown opposite we have a molecule with the formula AB3 where there are 3 covalent bonds around the central blue atom. Each of these covalent bonds contains a pair of electrons and since they are all negatively charged they will repel each other and try to get as far apart in 3d space as possible. So how can three electron pairs arrange themselves in space to get as far apart as possible? The answer is to form a flat (planar) molecule with bond angles of 1200. The shape of this molecule is described as trigonal planar.

The shapes of molecules with single bonds between the atoms can be easily predicted using the VSEPR model. However before we start to work out the shapes of various molecules it would help if we looked at the common shapes which are found in many molecules. You should make yourself familar with these basic shapes and bond angles as they are the basis for all of the molecular shapes you are likely to be asked to work out. The basic shapes of common molecules are based on those shown in the table below:

Basic shapes of molecules

Number of electron pairs Shape of molecule Name of molecular shape
2 linear molecule linear
3 trigonal planar molecule trigonal planar
4 tetrahedral molecule tetrahedral

The tetrahedral molecule has 4 bonding pairs or 8 electrons around the central atom. In gcse chemistry we learned that 8 electrons around a central atom was the maximum number allowed to give a stable octet of electrons. However elements in period 3 and above can have an expanded valence shell which allows them to five or six pairs of electrons around the central atom. This is shown below.

Number of electron pairs Shape of molecule Name of molecular shape
5 Trigonal bipyramidal molecule trigonal bipyramidal
6 shape of an Octahedral molecule Octahedral

Equatorial and axial position in a trigonal bipyramidal molecule

axial and equatorial positions in a trigonal bipyramidal molecule

Key Points

basic outlines of molecular shapes

Practice questions

Check your understanding - Questions on shapes of molecules