 ## 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 understand how to use the VSEPR model is by simply doing examples, so lets 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 contribute 1 electron. 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, so BeCl2 is a linear molecule with bond angles of 1800. Example 2: What shape is a molecule of methane (CH4)?

1. C is the central atom and it is in group 4. It has 4e valency electrons
2. Four hydrogen atoms are bonded to the central atom, each contributes 1 electron. So we have 4 electrons in total from the hydrogen atoms.
3. The total number of electrons in the valency shells is 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. It has 5e valency electrons
2. Five chlorine atoms are bonded to the central atom, each contribute 1 electron. So we have 5 electrons in total.
3. The total number of electrons in the valency shells is 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. It has 6 valency electrons
2. Six fluorine atoms are bonded to the central atom, each contribute 1 electron. So we have 6 electrons in total.
3. The total number of electrons in the valency shells is 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. Simply add this to the number of valency electrons in the central atom, 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.