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

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:

Predicting an electronic configuration

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