Chemistry: Electron Configuration

Atoms have layers of electron shells the amount of electrons in each shell, from innermost to outermost shell is written, as an electron configuration.

There are two ways to write electron configurations in the full from or the condensed form.

Each shell or "orbital" has a certain amount of electrons it can hold and are divided into lettered sections: s which can hold 2 electrons, p which can hold 6 electrons, d which can hold 10 electrons and f orbitals which can hold 14 electrons. The shells are filled in this order:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d.

This is the Aufbau principle.

Lets do chlorine as an example.

Chlorine has an atomic number of 17 which means it has 17 electrons.

First we fill the 1s orbital which in now 1s2 (normally the number of electrons would be by the top), we now have 15 more electrons left put in shells.

Then we fill 2s making it 2 leaving 13 electrons left.

Then we fill 2p making it 2p6 , because p orbitals hold 6 electron.

Then we fill 3s make it 3s2 we now have 5 left leaving 3p with only 5 electrons: 3p5.

So the full form is: 1s2, 2s2, 2p6 , 3s2 , 3p5.


To get the condensed form go back to the last noble gas, which in this case is neon. The orbitals of neon (1s2, 2s2, 2p6 ) are the first three of chlorine 1s2, 2s2, 2p6 , 3s2 , 3p5. So the first three orbital can be abbreviated to [Ne], then add the two extra orbitals from chlorine to get [Ne] 3s2 , 3p5.

That's the condensed version.

Sources: Chemistry Workbook for Dummies.