Electronic structures

An electronic structure 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.

Below is a table showing the maximum number of electrons an element can have for each of its electron shells. The information shown is for elements with atomic numbers 1 to 20:

Electron shellMaximum
First2
Second8
Third8

Predicting an electronic structure

The electronic structure 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:

  • two electrons occupy the first shell
  • eight electrons occupy the second shell
  • one electron occupies the third shell

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

  • each shell is shown as a circle
  • each electron is shown as a dot or a cross
Structure of a sodium atomThe electronic structure of sodium as a diagram