Electron Arrangements

Electrons can have different energy levels or shells. An electron occupies an area outside the nucleus called the electron shell or level.

Each shell can only hold a certain number of electrons.

The electrons in an atom fill the lowest (smallest) shell available and only go up a level when the lower one is full. The electron arrangements for many of the elements are listed in your data booklet on page six.

You will need to be able to work out the electron arrangement for the first 20 elements.

The first shell, closest to the nucleus can only hold two electrons. The second shell holds up to 8 electrons and the third shell holds the next eight electrons.

After this the next two electrons go into the fourth shell.

For example the electron arrangement for an atom of sodium (Na) is written: 2, 8, 1 (2 electrons in the first shell, 8 in the second shell and the remaining 1 electron in the third shell). This is illustrated on the diagram below.

The red shaded circle in the middle indicates the nucleus (containing protons and neutrons) and the blue shaded circles represent the electrons and the grey circles show the first three shells.

Next section: Ions