Electronic Structure (AQA GCSE Chemistry)

• We can represent the electronic structure of atoms using electron shell diagrams
• Electrons orbit the nucleus in shells
• Each shell has a different amount of energy associated with it
  • The further away from the nucleus, the more energy a shell has
• Electrons fill the shell closest to the nucleus 
• When a shell becomes full of electrons, additional electrons have to be added to the next shell
• The first shell can hold 2 electrons
• The second shell can hold 8 electrons 
• For GCSE, a simplified model is used that suggests that the third shell can hold 8 electrons
  • For the first 20 elements, once the third shell has 8 electrons, the fourth shell begins to fill
• 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

The electron shells

• 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

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 last notation, in this case 7, shows that chlorine has 7 outer electrons, and therefore is in group 7

The position of chlorine on the Periodic Table

Electronic Configuration of the First 20 Elements Table

Note: The GCSE model for electronic configuration is a simplified model. The third shell of electrons can actually hold up to 18 electrons. But filling these shells follows a more complicated pattern due to energy levels and stability. This is why potassium and calcium fill the third shell with 8 electrons and then put their final electrons in the fourth shell.