shapes of molecules

Shapes of molecules

tetrahedral molecule showing all bond angles.

Working out the shapes of molecules is a fascinating topic in 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; these are the angles between the lines representing the bonds between the nuclei of the atoms that make up 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 (the outer shell); these are after all the electrons that are 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. The two electrons in a covalent bond are attracted to the nuclei of the atoms involved in the covalent bond and it is this attraction that holds the two electrons in the bond 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 showing basic sahpe and bond angles. 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. The shape of a molecule is basically determined by the number of bonding pairs of electrons and lone pairs of electrons around the central atom. Now being negatively charged these bonding pairs and lone pairs of electrons will obviously repel each other as much as possible in 3d space. Working out the shape of a particular molecule then is simply working out how these bonding pairs and lone pairs of electrons orientate themselves to reduce this repulsion between them. The image below shows three molecules you are probably very familiar with; ammonia, water and methane.


Now each of these molecules has 8 electrons in its valence or outer shell, 2 electrons in each bonding pair and 2 electrons in each lone pair. Using the VSPER model these lone pairs and bonding pairs of electrons can reduce repulsion between them by forming a tetrahedral shaped molecule, as shown in the image. Now in a bonding pair the 2 electrons in the bond are held between the two nuclei involved in forming the bond, while in a lone pair of electrons the two electrons are being held in place by their attraction to only one nucleus. This means that a lone pair of electrons will take up more space than a bonding pair of electrons. The presence of lone pairs in a molecule will affect the bond angles in any molecule which contains them and it is important that you locate them if they are present.

3d models of ammonia, methane and water showing the bond angles and lone pairs of electrons in the molecules

For molecules that contain more than one lone pair then this will have a further effect on the bond angles present, we can summarise the repulsion between bonding pairs and lone pairs of electrons as:

Methane lone pair ↔ lone pair is greater than lone pair ↔ bonding pair is greater than bonding pair ↔ bonding pair

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 familiar 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:

The basic shapes of molecules

The basic shape of a molecule will be determined largely by the number of electron pairs or covalent bonds the central atom in a molecule makes. These electrons pairs will repel each other and try to get as far apart as possible in 3d space. The table below shows the basic shapes of molecules with up to four electron pairs around the central atom in a molecule and also the bond angles between these electron pairs.

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. The shapes and the bond angles in these molecules are shown in the table 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

In small molecules with up four electron pairs and also larger octahedral molecules all the bond angles in these molecules are the same. If you built models of these molecules and then flipped then on your desk no matter which way the molecule landed it would always look the same, simply because the bond angles are the same in all these molecules. However this is not true of trigonal bipyramidal molecules. In these molecules there are two different positions; called the axial and equatorial positions. These two different positions are shown in the image opposite:

In a molecule with a trigonal bipyramidal shape there are two different environments in which the bonded atoms can be in when they surround the central atom. These two positions are called the equatorial and the axial positions. There are 3 atoms around the middle part of the trigonal bypyramidal molecule shown. They are separated from each other by bond angles of 1200. These atoms are desribed as being in an equatorial position.
There are also two atoms shown in green which are both at 900 to the equatorial atoms. These atoms are said to be in an axial position.

Key Points

3d basic outlines of molecular shapes

Practice questions

Check your understanding - Questions on shapes of molecules