- 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 green box highlights the last notation which is 7, showing that a chlorine atom has 7 outer electrons.
The position of chlorine on the Periodic Table
The Electronic configuration of the first twenty elements
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.