Electron Configuration (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Alexandra Brennan
Reviewed by: Stewart Hird
Electron Configurations of Atoms & Ions
Energy Levels
The arrangement of electrons in an atom is called the electronic configuration
Electrons are arranged around the nucleus in energy levels (shells)
There are four main energy levels: 1,2, 3 and 4
Principal quantum numbers (n) are used to number these energy levels
The lower the principal quantum number, the closer the shell is to the nucleus
E.g. The first shell which is the closest to the nucleus is n = 1
The higher the principal quantum number, the greater the energy of the shell and the further away from the nucleus
Each shell has a fixed number of electrons it can hold:
n = 1 : up to 2 electrons
n = 2 : up to 8 electrons
n = 3 : up to 18 electrons
n = 4 : up to 32 electrons
The energy levels get closer together as you move further away from the nucleus with an overlap between the third and fourth energy levels
Atomic Orbitals
The main energy levels (or shells) are split into sub-shells
Sub-shells consist of one or more atomic orbitals
These are called s, p, and d
Elements with more than 57 electrons also have an f shell
The energy of the electrons in the sub-shells increases in the order s < p < d < f
These orbitals represent a volume of space in which there is a high probability of finding an electron
Each shell contains a different numbers of orbitals:
1st energy level
Has 1 s- orbital
2nd energy level
Has 1 s-orbital
Has 3 p- orbitals
3rd energy level
Has 1 s-orbital
Has 3 p- orbitals
Has 5 d-orbitals
Each atomic orbital can be occupied by a maximum of two electrons
The orbitals have specific 3-D shapes
Summary table of electrons in shells and sub-shells
Main Energy Level (n) | Sub Shells | Number of orbitals in sub-shell | Total number of electrons in each orbital | Total number of electrons in main shell |
---|---|---|---|---|
1 | s | 1 | 2 | 2 |
2 | s | 1 | 2 | 8 |
p | 3 | 6 | ||
3 | s | 1 | 2 | 18 |
p | 3 | 6 | ||
d | 5 | 10 | ||
4 | s | 1 | 2 | 32 |
p | 3 | 6 | ||
d | 5 | 10 | ||
f | 7 | 14 |
s orbital shape
The s orbitals are spherical
The size of the s orbitals increases with increasing shell number
E.g. the s orbital of the third shell (n = 3) is bigger than the s orbital of the first shell (n = 1)
p orbital shape
The p orbitals have a dumbbell shape
Every shell has three p orbitals except for the first one (n = 1)
The p orbitals occupy the x, y and z axes and point at right angles to each other, so are oriented perpendicular to one another
The lobes of the p orbitals become larger and longer with increasing shell number
An overview of the shells, subshells and orbitals in an atom
Examiner Tips and Tricks
You only need to know the shapes of the s- and p- orbitals but must know how many s-,p- and d- orbitals there are in each energy level.
Filling electron orbitals
There are some rules to remember when assigning electrons to atomic orbitals:
Electrons fill atomic orbitals of lower energy first
Each orbital can hold a maximum of 2 electrons
Atomic orbitals with the same energy, such as the 2p orbital, fill singly before occupying shells in pairs
Remember, there is an overlap between the third and fourth energy levels
The 4s orbital is of lower energy than the 3d so it will be filled first
We can show the atomic orbitals being filled and deduce the electronic configuration for an element using the box notation
Each box represents an atomic orbital
The boxes are arranged in order of increasing energy from bottom to top
The electrons are represented by opposite arrows to show the spin of the electrons
Some examples of box notations are below:
Nitrogen
Sodium
Chlorine
Titanium
Writing electronic configurations
We can write electronic configurations using the notation below:
This tells us how the electrons in an atom or ion are arranged in their shells, sub-shells and orbitals
Hydrogen
Hydrogen has 1 single electron
The electron is in the s orbital of the first shell
Its electron configuration is 1s1
Potassium
Potassium has 19 electrons
The first 2 electrons fill the s orbital of the first shell, 1s2
They then continue to fill subsequent orbitals and sub-shells in order of increasing energy
The 4s orbital is lower in energy than the 3d subshell, so it is therefore filled first
The full electron configuration of potassium is 1s2 2s2 2p6 3s2 3p6 4s1
To simplify this electronic structure, we can use the shorthand electronic configuration
The nearest preceding noble gas to potassium is argon
This accounts for 18 electrons of the 19 electrons that potassium has
The shorthand electron configuration of potassium is [Ar] 4s1
Elements can be classified as an s-block element, p-block element and so on, based on the position of the outermost electron:
s block elements
Have their outer electron(s) in an s orbital
p block elements
Have their outer electron(s) in a p orbital
d block elements
Have their outer electron(s) in a d orbital
f block elements
Have their outer electron(s) in an f orbital
Worked Example
Write down the full and shorthand electron configuration of the following elements:
Calcium
Gallium
Mg2+
Answer 1:
Calcium has has 20 electrons so the full electronic configuration is:
1s2 2s2 2p6 3s2 3p6 4s2
The 4s orbital is lower in energy than the 3d subshell and is therefore filled first
The shorthand version is [Ar] 4s2 since argon is the nearest preceding noble gas to calcium which accounts for 18 electrons
Answer 2:
Gallium has 31 electrons so the full electronic configuration is:
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p1
The shorthand electronic configuration is:
[Ar] 3d10 4s2 4p1
Even though the 4s is filled first, the full electron configuration is often written in numerical order. So, if there are electrons in the 3d sub-shell, then these will be written before the 4s
Answer 3:
A magnesium atom has 12 electrons so its electronic configuration would be
1s2 2s2 2p6 3s2
To form a magnesium ion, it loses its two outer electrons so the electronic configuration for the ion is:
1s2 2s2 2p6
Using the shorthand, the electronic configuration is:
[Ne]
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