Electronic Configurations (Edexcel IGCSE Chemistry: Double 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

• You should be able to write the electron configuration for the first twenty elements

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

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

Shorthand electronic configuration

• Period: The red numbers below the electronic configuration show the number of notations which is 3, showing that a chlorine atom has 3 shells of electrons
• Group: The last notation, which is 7, shows that a chlorine atom has 7 outer electrons matching group 7 of the periodic table

The position of chlorine on the periodic table

The position of chlorine in the periodic table