Electronic structure

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

Electrons in shells

Electrons in atoms occupy energy levels, also called electron shells, outside the nucleus. Different shells can hold different maximum numbers of electrons. The electrons in an atom occupy the lowest available energy level first. This is the shell nearest the nucleus. When this shell is full the electrons begin to occupy the next energy level.

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

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 comma, or 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

Electronic structures and the periodic table

The electronic structure of an element is linked to its position on the periodic table.

Electronic structure featureLink to the periodic table
Number of shellsPeriod number
Number of electrons in outermost shellGroup number
Numbers added togetherAtomic number

The electronic structure of sodium (2,8,1) shows that sodium, Na:

  • is in period 3
  • is in group 1
  • has an atomic number of (2 + 8 + 1) = 11