Electronic Configurations (Edexcel GCSE Chemistry: Combined Science)

• 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, the more energy a shell has
• Electrons first occupy the shell closest to the nucleus which can hold a maximum of 2 electrons
• When a shell becomes full of electrons, additional electrons have to be added to the next shell
• The second shell and third shell can hold 8 electrons each
• 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, the outermost shell is not full and therefore these atoms react with other atoms in order to achieve a full outer shell of electrons (which would make them more stable)
• In some cases, atoms lose electrons to entirely empty this shell so that the next shell below becomes a (full) outer shell

Deducing electron configuration

• The arrangement of electrons in shells can also be explained using numbers
• Instead of drawing electron shell diagrams, the number of electrons in each electron shell can be written down, separated by full stops
• This notation is called the electronic configuration (or electronic structure)
  • E.g. Carbon has 6 electrons, 2 in the 1st shell and 4 in the 2nd shell
    • Its electronic configuration is 2.4
• Electronic configurations can also be written for ions
  • E.g. A sodium atom has 11 electrons, a sodium ion has lost one electron, therefore has 10 electrons; 2 in the first shell and 8 in the 2nd shell
    • Its electronic configuration is 2.8

• There is a clear relationship between the electronic configuration and how the Periodic Table is designed
• The number of notations in the electronic configuration will show the number of occupied 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 (for elements in groups 1 to 7)

• The electron configuration of the first twenty elements is shown below:

Electronic Configuration of the First 20 Elements Table

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