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Polyatomic Ions

When we first learnt about ions, we learnt that they have the same basic structure as atoms: they have a nucleus made of protons and neutrons, and there are electrons orbiting around the nucleus in shells.

On the left is a diagram of an atom. On the right is a diagram of an ion. They both have a nucleus made up of  protons and neutrons. They both have shells around the nucleus containing electrons.

An atom and an ion. They both have the same basic structure.

The only difference is that in an atom the number of electrons is equal to the number of protons, whereas in an ion the number of electrons is different to the number of protons. This causes ions to be positively or negatively charged, whereas atoms are neutral.

However, there are actually two different groups of ions, and so far we have only learnt about one of them.

All of the ions we have learnt about so far are what are called monatomic ions. A monatomic ion is an ion which has the same basic structure as a single atom (but with an unequal number of protons and neutrons, making it charged). Mono- means 'one', so the name monatomic refers to the fact that these ions resemble one atom.

The other group of ions are called polyatomic ions. A polyatomic ion has the same basic structure as a molecule. But whereas a molecule has an equal number of electrons and protons, making it neutral, a polyatomic ion has an unequal number of electrons and protons, making it charged. Poly- means 'many', so the name polyatomic refers to the fact that these ions are made up of multiple atoms.

The diagram below shows a polyatomic ion called a carbonate ion. It is made up of a carbon atom and three oxygen atoms joined by covalent bonds. Overall, it has two more electrons than protons, giving it a relative charge of -2. Square brackets are drawn around the polyatomic ion and the charge is written outside of the brackets. This shows that -2 is the relative charge of the whole polyatomic ion.

The displayed formula of a carbonate ion. There is a carbon atom in the centre that is bonded to three oxygen atoms. One of the oxygen atoms is joined by a double bond. The other two are joined by single bonds. The whole structure is contained within square brackets and to the top right of those brackets, the charge (which is 2-) is written.

A carbonate ion.

Every polyatomic ion has a molecular formula

The diagram of a carbonate ion above shows its displayed formula. We can also represent a polyatomic ion by writing its molecular formula. To do this, we follow the same process that we would use when writing the molecular formula of a molecule, and then at the end we add the relative charge in the same way that we would for a monatomic ion.

For example, the carbonate ion is made up of 1 carbon atom and 3 oxygen atoms, and it has a relative charge of -2. Therefore, its molecular formula is:


You need to know the common polyatomic ions

There are some common polyatomic ions that you need to know the names and molecular formulae of. They are listed below:

Hydroxide: OH-

Nitrate: NO3-

Carbonate: CO32-

Sulfate: SO42-

Phosphate: PO43-

Ammonium: NH4+

Note that many of these polyatomic ions have names ending in -ate. This means that they contain oxygen. Knowing this can help you to remember which elements each polyatomic ion is made up of. For example, for sulfate (SO42-), we can split the name into sulf-, which tells us sulfur is present, and -ate, which tells us oxygen is present.


Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.


What is a polyatomic ion?


What is the molecular formula of a hydroxide ion?


What is the molecular formula of a nitrate ion?


What is the molecular formula of a carbonate ion?


What is the molecular formula of a sulfate ion?


What is the molecular formula of a phosphate ion?


What is the molecular formula of an ammonium ion?


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