3.3 Electronic structure  (Page 2/5)

This diagram also helps us when we are working out the electron configuration of an element. The electron configuration of an element is the arrangement of the electrons in the shells and subshells. There are a few guidelines for working out the electron configuration. These are:

• Each orbital can only hold two electrons . Electrons that occur together in an orbital are called an electron pair .
• An electron will always try to enter an orbital with the lowest possible energy.
• An electron will occupy an orbital on its own, rather than share an orbital with another electron. An electron would also rather occupy a lower energy orbital with another electron, before occupying a higher energy orbital. In other words, within one energy level, electrons will fill an 's' orbital before starting to fill 'p' orbitals.
• The s subshell can hold 2 electrons
• The p subshell can hold 6 electrons

In the examples you will cover, you will mainly be filling the s and p subshells. Occasionally you may get an example that has the d subshell. The f subshell is more complex and is not covered at this level.

The way that electrons are arranged in an atom is called its electron configuration .

Electron configuration

Electron configuration is the arrangement of electrons in an atom, molecule or other physical structure.

An element's electron configuration can be represented using Aufbau diagrams or energy level diagrams. An Aufbau diagram uses arrows to represent electrons. You can use the following steps to help you to draw an Aufbau diagram:

1. Determine the number of electrons that the atom has.
2. Fill the 's' orbital in the first energy level (the $1\mathrm{s}$ orbital) with the first two electrons.
3. Fill the 's' orbital in the second energy level (the $2\mathrm{s}$ orbital) with the second two electrons.
4. Put one electron in each of the three 'p' orbitals in the second energy level (the $2\mathrm{p}$ orbitals) and then if there are still electrons remaining, go back and place a second electron in each of the $2\mathrm{p}$ orbitals to complete the electron pairs.
5. Carry on in this way through each of the successive energy levels until all the electrons have been drawn.

You can think of Aufbau diagrams as being similar to people getting on a bus or a train. People will first sit in empty seats with empty seats between them and the other people (unless they know the people and then they will sit next to them). When all the seats are filled like this, any more people that get on will be forced to sit next to someone or stand. As the bus or train fills even more the people have to stand to fit on.

An Aufbau diagram for the element Lithium is shown in [link] .