Electron Configuration (Oxford AQA International A Level Chemistry)

Revision Note

Alexandra Brennan

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

The arrangement of principle quantum shells
Electrons are arranged in shells, which are numbered by principal quantum numbers

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

Atomic Structure Summary, downloadable AS & A Level Chemistry revision notes

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

Atomic Structure Ground-State, downloadable AS & A Level Chemistry revision notes
The lowest energy level is filled first
  • 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

Nitrogen Electronic configuration

Sodium

Sodium electronic configuration

Chlorine

chlorine electronic configuration

Titanium

Atomic Structure Electron in Box Notation, downloadable AS & A Level Chemistry revision notes

Writing electronic configurations

  • We can write electronic configurations using the notation below:

Atomic Structure Notation Electron Configuration, downloadable AS & A Level Chemistry revision notes
The electronic configuration of hydrogen
  • 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

Atomic Structure Electronic Configuration & Periodicity, downloadable AS & A Level Chemistry revision notes

Worked Example

Write down the full and shorthand electron configuration of the following elements:

  1. Calcium

  2. Gallium

  3. 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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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.