# Electronic configurations

An electronic configuration is the way in which are arranged in an .

## Electrons in shells

Different shells can hold different maximum numbers of electrons. Electrons occupy shells starting with the innermost one. They begin to occupy the next shell when a shell becomes full.

### For elements with atomic number 1 to 20:

Electron shellMax electrons
First2
Second8
Third8

## Predicting an electronic configuration

The electronic configuration of an atom can be predicted from its . For example, the atomic number of sodium is 11. Sodium atoms have 11 and so 11 electrons:

• 2 electrons occupy the first shell
• 8 electrons occupy the second shell
• 1 electron occupies the third shell

This electronic configuration can be written as 2.8.1 (each dot separates one shell from the next). This electronic configuration can also be shown as a diagram. In these diagrams:

• each shell is modelled as a circle
• each electron is modelled as a dot or a cross
The electronic configuration of sodium as a diagram