Shell Diagrams & Electronic Configuration

Specification Point 1.19 (a):
  • Predict the electronic configurations of the first 20 elements in the periodic table as diagrams.
  • Electrons orbit the nucleus in shells (or energy levels) 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 occupy the shell closest to the nucleus which can hold only 2 electrons and which go in separately.
  • When a shell becomes full electrons then fill the next shell.
  • The second shell can hold 8 electrons and the third shell can hold eighteen electrons and the electrons organise themselves in pairs in these shells.
  • 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.
Electron shells, Edexcel GCSE Chemistry

The electron shells

Shell Diagrams & Electronic Configuration II

Specification Point 1.19 (b):
  • Predict the electronic configurations of the first 20 elements in the form: 2.8.1

Electronic configuration

  • The arrangement of electrons in shells can also be explained using numbers called the electronic configuration.
  • Lithium for example has 3 electrons, so its electronic configuration is 2, 1.

The Electronic configuration of the first twenty elements

Element                                  Atomic number                                 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

Sulfur                                                  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

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.

Number of Electrons & the Periodic Table

Specification Point 1.20:
  • Explain how the electronic configuration of an element is related to its position in the periodic table.
  • 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.

Electronic configuration of chlorine, Edexcel GCSE Chemistry

The electronic configuration of chlorine as it should be written

Chlorine on the periodic table, Edexcel GCSE Chemistry

The position of chlorine on the Periodic table which can be deduced from its electronic configuration

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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.