# Working out the shapes of molecules using VSEPR theory

The 5 basic outlines for the shapes of all the molecules you are likely to meet are shown below. To work out the shape of a molecule using the valence shell electron pair repulsion theory (VSEPR) it is a simple matter of identifying the central atom in a molecule and working out the number of bonding and lone pairs (non-bonding pairs) of electrons that are present in the molecule. If you are not familiar with the basic shapes shown below then you should visit the page on shapes of molecules before reading this page.

## Using VSEPR rules to work out the shapes of molecules

The best way I think to learn how to use the VSEPR model is by simply doing examples; so let's get started!

Carry out the following steps in order to find the shape of the molecule:

1. Identify the central atom and the number of valency electrons it has. This is easily done, just use the periodic table to find what group the central atom is in and this will give the number of valency electrons.
2.  group in periodic table 1 2 3 4 5 6 7 8 number of valency electrons 1 2 3 4 5 6 7 8
3. Count the number of atoms bonded to the central atom; each atom bonded to the central atom will contribute 1 electron each to form a covalent bond.
4. Add up the total number of electrons and divide by 2 to get the number of electron pairs.

Example 1: What shape is a molecule of beryllium dichloride (BeCl2)?

Simply work through the rules listed above:

1. Be is the central atom and it is in group 2 in the periodic table. It has 2 valency electrons in its outer shell.
2. Two chlorine atoms are bonded to the central atom; each chlorine atom will contribute 1 electron in forming a covalent bond to the beryllium atom. So we have 2 electrons in total from the chlorine atoms.
3. This gives a total number of electrons in the valency shells as 4 electrons; dividing by 2 gives 2 electron pairs. A molecule with two bonding pairs of electrons will be linear. This will allow the electrons in the two covalent bonds to get as far apart as possible to minimise the repulsion between them; so BeCl2 is a linear molecule with bond angles of 1800.

Example 2: What shape is a molecule of methane (CH4)?

1. Carbon is the central atom and carbon is in group 4 of the periodic table. It has 4 valency electrons
2. Four hydrogen atoms are bonded to the central atom with each hydrogen atom contributing 1 electron. So we have 4 electrons in total from the hydrogen atoms.
3. The total number of electrons in the valency shells is therefore 8 electrons; dividing by 2 gives 4 electron pairs; so the shape of a CH4 molecule will be based on a tetrahedral structure with bond angles of 109.50.

Example 3: What shape is a molecule of phosphorus pentachloride (PCl5)?

1. P is the central atom and it is in group 5 of the periodic table. It has 5 valency electrons
2. Five chlorine atoms are bonded to the central atom and each chlorine atom will contribute 1 electron when it forms a covalent bond to the phosphorus atom. So we have 5 electrons in total.
3. The total number of electrons in the valency shells is therefore 10 electrons; dividing by 2 gives 5 electron pairs; so the shape of a PCl5 molecule will be based on a trigonal bipyramidal structure with bond angles of 1200 and 900.

Example 4: What shape is a molecule of sulfur hexafluoride (SF6)?

1. S is the central atom and it is in group 6 of the periodic table. It has 6 valency electrons in its outer shell.
2. Six fluorine atoms are bonded to the central sulfur atom and each fluorine atom will contribute 1 electron to each of the covalent bonds formed with the sulfur atom. So we have 6 electrons in total.
3. The total number of electrons in the valency shells is therefore 12 electrons; dividing by 2 gives 6 electron pairs; so the shape of a SF6 molecule will be based on an octahedral structure with bond angles of 900.

## Key Points

• To find the shape of a molecule simply identify the central atom; this is usually obvious from the formula of the substance.
• Each atom bonded to the central atom will contribute one electron to the covalent bond formed. Simply add this to the number of valency electrons in the central atom and this will give you the number of electrons in the outer shell of the molecule
• Simply divide your total number of electrons by 2 since each covalent bond contains 2 electrons. This will give you the number of electron pairs in the molecule.