Filling orbitals (H - Ar)
In the post-16 model of the atom, the electron configuration of an element is modified to identify the various orbitals that electrons occupy. As with the pre-16 model, electrons fill up the lowest energy level first and within that energy level the orbitals are filled according to their relative energy: s [less than] p [less than] d.
In the pre-16 model of the atom, a sodium atom (proton number = 11) has the electron configuration 2.8.1.
In the post-16 model, electrons fill up from the lowest energy orbital. An orbital is filled before electrons enter the next orbital. The 11 sodium electrons are distributed as shown in the table.
| Orbital | Max. number of electrons | Number of sodium electrons | Notation |
| 1s | 2 | 2 | 1s2 |
| 2s | 2 | 2 | 2s2 |
| 2p | 6 | 6 | 2p6 |
| 3s | 2 | 1 | 3s1 |
The full electron configuration of sodium is written out as: 1s2 2s2 2p6 3s1.
(1s2 is pronounced 1 - s - 2 not 1 - s - squared).
What is the electron configuration of a chlorine atom with a atomic number of 17?
1s2 2s2 2p6 3s2 3p5. Note that the 7 valence electrons (in the pre-16 model) are contained within the 3s and 3p orbitals.
Minimising electron repulsion
Electrons are negatively charged and there exists electrostatic repulsion between electrons. Therefore, electrons adopt the electronic configuration which is most energetically stable and which minimises electron-electron repulsion.
When filling p- or d-orbitals, each orbital is singly occupied by an electron before any of the electrons are paired. This can be more clearly explained using 'box and arrow diagrams.' Each orbital is represented by a box and an electron's spin is represented by an arrow's direction.
The electron configuration for the 2nd period are shown below using box and arrow diagrams.
INSERT[UUPT_DMA_CI06_ima - box and arrow diagrams]