Electrons are negatively charged particles that move around the nucleus in fixed energy levels, also known as shells. The way these electrons are arranged is called the atom’s electronic configuration or electron structure.

Electron shells

Electrons occupy the lowest available energy levels (the ones closest to the nucleus) first.

Each shell has a maximum number of electrons it can hold:

💡 Always fill the shells in order: 1st shell → 2nd shell → 3rd shell

Electronic configuration

We can represent electron structure using numbers separated by dots or commas. These show how many electrons are in each shell.

Examples:

The atomic number tells you the number of electrons, as atoms are neutral (electrons = protons).

Drawing electron shells

To draw an atom:

1. Write the atomic number to find the number of electrons
2. Use a nucleus circle in the middle
3. Draw shells as circles around the nucleus
4. Add electrons as dots or crosses in the shells

Link to the periodic table

• The period number = number of electron shells
• The group number (in Group 1–7) = number of electrons in the outer shell

Example:

• Sodium is in Group 1, Period 3
  
  • It has 3 shells
  • It has 1 electron in the outer shell

This helps explain reactivity and chemical behaviour, as elements in the same group have similar outer shell structures.