Working out the shapes of ions using VSEPR theory

This page follows on from an earlier page on using VSEPR to work out the shapes of simple molecules. Before trying to work out the shapes of ions you should already know the 5 basic outlines for the shapes of all the molecules you are likely to meet, these are shown below for reference. To work out the shape of ions 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 and then taking in account the size of the charge present on the ion.

Using VSEPR rules to work out the shapes of ions

The best way I think to understand how to use the VSEPR model to work out the shapes of ions is by simply doing examples, so let's get started!

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

1. Identify the central atom and the number of valency electrons it has. This is easily done using just use the periodic table to find what group the central atom is in and this will give the number of valency electrons.

group in periodic table where element is found	1	2	3	4	5	6	7	8
number of valency electrons	1	2	3	4	5	6	7	8

2. 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.
3. Add up the total number of electrons and divide by 2 to get the number of electron pairs. Some of these electron pairs could be bonding pairs of electrons and some could be lone pairs or non-bonding pairs of electrons.
4. To work out the shapes of ions. Remember that a cation; that is a positively charged ion is formed when a species loses an electron and anions; that is negatively charged ions are formed by the addition of electrons to a molecule.

Shapes of ions

Simply follow the rules above and those we used previously to work out the shape of ions.

Example 1: What is the shape of an ammonium ion, NH₄⁺?

As before using VSEPR rules:

1. Nitrogen is the central atom and it is in group 5. It has 5 valency electrons.
2. Four hydrogen atoms are covalently bonded to the central atom and each contributes 1 electron. So we have 4 electrons in total.
3. The molecule has a charge of +1; this means it has lost an electron.
4. The total number of electrons in the valency shells is therefore 8 electrons; dividing by 2 gives 4 electron pairs; so the shape will be based on a tetrahedral structure. There are 4 bonding pairs of electrons from the 4 N-H bonds and this means there are NO lone pairs of electrons in this molecule.

Example 2 - What is the shape of the BrF₂⁺ ion?

1. Bromine is the central atom and it is in group 7 of the periodic table. It has 7 valency electrons
2. Two fluorine atoms are covalently bonded to the central atom bromine atom and each contributes 1 electron. So we have 2 electrons in total.
3. The molecule has a charge of +1; this means it has lost an electron.
4. The total number of electrons in the valency shells is 8 electrons; dividing by 2 gives 4 electron pairs; so the shape will be based on a tetrahedral structure. There are 2 bonding pairs of electrons from the 2 Br-F covalent bonds so this means there are two lone pairs of electrons.

Example 3: What is the shape of an amide ion, NH₂^-?

As before using VSEPR rules:

1. Nitrogen is the central atom and it is in group 5 of the periodic table. It has 5 valency electrons.
2. Two hydrogen atoms are covalently bonded to the central atom nitrogen atom and each contributes 1 electron. So we have 2 electrons in total.
3. The molecule has a charge of -1; this means it has gained an electron.
4. The total number of electrons in the valency shells is 8 electrons; dividing by 2 gives 4 electron bonding pairs, so the shape will be based on a tetrahedral structure, there are 2 bonding pairs of electrons from the 2 N-H bonds, this means there are 2 lone pairs of electrons in this molecule.

Example 4: What is the shape of thallium (III) bromide, TlBr₃^2-?

As before using VSEPR rules:

1. Thallium is the central atom and it is in group 3 of the periodic table. It has 3 valency electrons.
2. Three bromine atoms are covalently bonded to the central atom and each contributes 1 electron. So we have 3 electrons in total.
3. The molecule has a charge of -2; this means it has gained 2 electrons.
4. The total number of electrons in the valency shells is 8 electrons; dividing by 2 gives 4 electron pairs; so the shape will be based on a tetrahedral structure. There are 3 bonding pairs of electrons from the 3 Tl-Br bonds so this means there is one lone pair of electrons in this ion.

Example 5: What shape is the tetrafluorobromate(V) ion (BrF₄^-)?

As before using VSEPR rules:

1. Bromine is the central atom and it is in group 7 of the periodic table. It has 7 valency electrons.
2. Four fluorine atoms are covalently bonded to the central atom and each contributes 1 electron. So we have 4 electrons in total.
3. The molecule has a charge of -1; this means it has gained an electron.
4. The total number of electrons in the valency shells is therefore 12 electrons; dividing by 2 gives six electron pairs; so the shape will be based on an octahedral structure. There are 4 bonding pairs of electrons from the 4 Br-F bonds so this means there are 2 lone pairs of electrons in this molecule.

• 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 the number of bonded atoms 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 bonding electron pairs in the molecule.
• If the ion has a positive charge subtract the size of the charge from the number of valency shell electrons. If the ion has a negative charge add the size of this charge to the number of valency electrons.
• If the ion has lone pairs or non-bonding pairs of electrons these are NOT used in determining the overall shape of the ion. However they will influence bond angles present in the ion since lone pairs of electrons take up more space than bonding pairs of electrons.

Check your understanding - Questions on shape of molecules and ions