Electron Shell Diagrams

  • We can represent the electronic structure of atoms using electron shell diagrams.
  • Electrons orbit the nucleus in shells and each shell has a different amount of energy associated with it.
  • The further away from the nucleus then the more energy a shell has.
  • Electrons first occupy the shell closest to the nucleus which can hold only 2 electrons.
  • When a shell becomes full of electrons they then fill the next shell.
  • The second shell and third shell can hold 8 electrons.
  • The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons.
  • In most atoms this shell is not full and they react in order to gain electrons to fill this shell.
  • In some cases, atoms lose electrons to entirely empty this shell so that the next shell below becomes the now full outer shell.

Rules of electron-shell filling, IGCSE & GCSE Chemistry revision notes

The electron shells

Electronic Configuration

  • The arrangement of electrons in shells can also be explained using numbers.
  • This method consists of writing down the number of electrons in each shell and separating each number with a comma.
  • There is a clear relationship between the outer shell electrons and how the Periodic Table is designed.
  • The number of notations in the electronic configuration will show the number of shells of electrons the atom has, showing the Period in which that element is in.
  • The last notation shows the number of outer electrons the atom has, showing the Group that element is in.
  • Elements in the same Group have the same number of outer shell electrons.

Two ways to represent electronic structure of chlorine - AQA, IGCSE & GCSE Chemistry revision notes

The electronic configuration for chlorine

  • 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.

Position-of-Chlorine-in-Periodic-Table, IGCSE & GCSE Chemistry revision notes

The position of chlorine on the Periodic Table

The Electronic configuration of the first twenty elements

Electronic Configuration Table, IGCSE & GCSE Chemistry revision notes
Electronic Configuration 1, IGCSE & GCSE Chemistry revision notes

Note: although the third shell can hold up to 18 electrons, the filling of the shells follows a more complicated pattern after potassium and calcium. For these two elements, the third shell holds 8 and the remaining electrons (for reasons of stability) occupy the fourth shell first before filling the third shell.

AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.