Electron arrangements

An electron arrangement 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:

Electron shellMaximum number of electrons
First2
Second8
Third8

Predicting an electron arrangement

The electron arrangement 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 electron arrangement can be written as 2.8.1 (each dot separates one shell from the next). This electron arrangement 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
Structure of a sodium atomThe electron arrangement of sodium as a diagram

Electron arrangements and the periodic table

The electron arrangement of an element is related to its position on the periodic table.

Electron arrangement featureLink to the periodic table
Number or numbers of circlesPeriod number
Number of electrons in outermost shellOld group number
Total number of electrons in all shellsAtomic number

Note that:

  • helium and the other elements in group 0 (IUPAC group 18) have full outer shells
  • hydrogen has 1 electron, so it is placed above the top of group 1 (but it is not in group 1)

Example

The electron arrangement of sodium is 2.8.1. This shows that sodium:

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

The electron arrangement of nitrogen is 2.5. Explain what this shows about the position of nitrogen in the periodic table.

2.5 shows that nitrogen:

  • is in period 2
  • is in group 5 (IUPAC group 15)
  • has an atomic number of (2 + 5) = 7