Electronic Structure — AQA GCSE Chemistry
The arrangement of electrons in shells determines how an element behaves and where it sits in the periodic table.
Electron shells (energy levels)
Electrons occupy shells (energy levels) around the nucleus. The shells fill from the inside out, and each shell holds a maximum number of electrons:
- 1st shell: up to 2 electrons,
- 2nd shell: up to 8 electrons,
- 3rd shell: up to 8 electrons (for the first 20 elements).
Writing electronic structures
The electronic structure is written as numbers separated by commas, e.g.:
- Sodium (11 electrons) = 2, 8, 1
- Chlorine (17 electrons) = 2, 8, 7
- Argon (18 electrons) = 2, 8, 8
You can also draw them as electrons on circles (shells) around the nucleus.
Link to the periodic table
- The number of occupied shells = the period (row) number.
- The number of electrons in the outer shell = the group number (for groups 1–7; group 0 has a full outer shell).
For example, sodium (2,8,1) is in period 3 (three shells) and group 1 (one outer electron).
Exam tips
- Fill shells 2, 8, 8 for the first 20 elements.
- Write electronic structures correctly (e.g. 2,8,1).
- Outer electrons = group; number of shells = period.
- The outer-shell electrons determine an element's chemical reactions.