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Shells

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The electrons around the nucleus of an atom are arranged in shells.

• The first shell can hold up to 2 electrons.
• The second shell can hold up to 8 electrons.
• The third shell can hold up to 18* electrons.

* Note: At GCSE level the situation is sometimes over-simplified and you are told the third shell can only hold up to 8 electrons. This is incorrect.

• The fourth shell can hold up to 32 electrons.

In summary, the capacities of the first four main shells are

2, 8, 18 and 32 respectively.

Where these numbers come from is described in more detail below.