- 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.
- 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.
The electronic configuration of chlorine as it should be written
The position of chlorine on the Periodic table which can be deduced from its electronic configuration