All courses › Chemistry 1 › Electron shells

Electron shells

The electrons lie in shells around the nucleus. In the simple model used here, the innermost shell takes 2 electrons and the next two take 8 each. The shell being filled follows the period in the periodic table. The electrons in the outermost shell are called valence electrons, and they are the ones that take part in bonds. Sodium is in period 3 and group 1, so it has three shells and one valence electron. A full outer shell is particularly stable.

2,\ 8,\ 8,\ \ldots

room in the first shells

Symbols

$period$	number of shells in use
$groups 1 and 2$	as many valence electrons as the group number

Example

Neutral sodium has 11 electrons.

The arrangement is $2 + 8 + 1 = 11$ , with one valence electron.

When sodium reacts, it is the single outer electron that is given away.

Practise atoms and the periodic table for free →

← Isotopes · Noble gases →

Part of Chemistry 1: Atoms and the periodic table.