0.2 The atom  (Page 11/14)

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.

Sometimes people refer to Hund's rule for electron configuration. This rule simply says that electrons would rather be in a subshell on it's own then share a subshell. This is why, when you are filling the subshells you put one electron in each subshell and only if there are extra electrons do you go back and fill the subshell, before moving onto the next energy level.

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

A special type of notation is used to show an atom's electron configuration. The notation describes the energy levels, orbitals and the number of electrons in each. For example, the electron configuration of lithium is ${1\mathrm{s}}^2{2\mathrm{s}}^1$ . The number and letter describe the energy level and orbital and the number above the orbital shows how many electrons are in that orbital.

Aufbau diagrams for the elements fluorine and argon are shown in [link] and [link] respectively. Using standard notation, the electron configuration of fluorine is ${1\mathrm{s}}^2{2\mathrm{s}}^2{2\mathrm{p}}^5$ and the electron configuration of argon is ${1\mathrm{s}}^2{2\mathrm{s}}^2{2\mathrm{p}}^6$ .