Specification Point 1.19:
Understand how to deduce the electronic configuration of the first 20 elements from their positions in the periodic table

Deducing Electronic Configurations

Electrons move around the nucleus in different energy levels. The energy levels are called electron shells.

Electronic configuration: The arrangement of electrons into shells for an atom. This can be figured out by following some rules:

  • Electrons always occupy shells that are closest to the nucleus first
  • When a shell becomes full, electrons will fill the next shell
electron-structure, IGCSE & GCSE Physics revision notes

Electronic configuration of the first 20 elements:

Element Number of electrons Electronic configuration
Hydrogen 1 1
Helium 2 2
Lithium 3 2 · 1
Beryllium 4 2 · 2
Boron 5 2 · 3
Carbon 6 2 · 4
Nitrogen 7 2 · 5
Oxygen 8 2 · 6
Fluorine 9 2 · 7
Neon 10 2 · 8
Sodium 11 2 · 8 · 1
Magnesium 12 2 · 8 · 2
Aluminium 13 2 · 8 · 3
Silicon 14 2 · 8 · 4
Phosphorus 15 2 · 8 · 5
Sulphur 16 2 · 8 · 6
Chlorine 17 2 · 8 · 7
Argon 18 2 · 8 · 8
Potassium 19 2 · 8 · 8 · 1
Calcium 20 2 · 8 · 8 · 2
Specification Point 1.22:
Understand how the electronic configuration of a main group element is related to its position in the periodic table

Electronic Configuration & Periodic Table Position

Electronic configuration: The arrangement of electrons into shells for an atom (e.g electronic configuration of carbon is  2 . 4)

Electronic configuration and position in periodic table

  • The number of notations in the electronic configuration will show the number of shells of electrons the atom has, showing the period
  • The last notation shows the number of outer electrons the atom has, showing the group

Example:  Electronic configuration of Chlorine:

Electronic configuration of chlorine, Edexcel GCSE Chemistry

Period: The red numbers at the bottom show the number of notations which is 3, showing that a chlorine atom has 3 shells of electrons

Group: The green box highlights the last notation, which is 7, showing that a chlorine atom has 7 outer electrons

The position of Chlorine on the periodic table:

Chlorine on the periodic table, Edexcel GCSE Chemistry

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.