AQA A Level Chemistry

Revision Notes

1.1.4 Shells and Orbitals

Electron Shells


  • The arrangement of electrons in an atom is called the electron configuration
  • Electrons are arranged around the nucleus in principal energy levels or principal quantum shells
  • Principal quantum numbers (n) are used to number the energy levels or quantum shells
    • The lower the principal quantum number, the closer the shell is to the nucleus
      • So, 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 principal quantum number 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 principal quantum shells are split into subshells which are given the letters s, p and d
    • Elements with more than 57 electrons also have an f shell
    • The energy of the electrons in the subshells increases in the order s < p < d
  • The order of subshells appear to overlap for the higher principal quantum shells as seen in the diagram below:

Atomic Structure Principal-Quantum-Subshells, downloadable AS & A Level Chemistry revision notes

Electrons are arranged in principal quantum shells, which are numbered by principal quantum numbers


  • Subshells contain one or more atomic orbitals
  • Orbitals exist at specific energy levels and electrons can only be found at these specific levels, not in between them
    • Each atomic orbital can be occupied by a maximum of two electrons
  • This means that the number of orbitals in each subshell is as follows:
    • s : one orbital (1 x 2 = total of 2 electrons)
    • p : three orbitals ( 3 x 2 = total of 6 electrons)
    • d : five orbitals (5 x 2 = total of 10 electrons)
    • f : seven orbitals (7 x 2 = total of 14 electrons)
  • The orbitals have specific 3-D shapes

s orbital shape

  • The s orbitals are spherical in shape
  • The size of the s orbitals increases with increasing shell number
    • E.g. the s orbital of the third quantum shell (n = 3) is bigger than the s orbital of the first quantum 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

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

Representation of orbitals (the dot represents the nucleus of the atom) showing spherical s orbitals (a), p orbitals containing ‘lobes’ along the x, y and z axis


  • Note that the shape of the d orbitals is not required

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

An overview of the shells, subshells and orbitals in an atom

Ground state

  • The ground state is the most stable electronic configuration of an atom which has the lowest amount of energy
  • This is achieved by filling the subshells of energy with the lowest energy first (1s)
  • The order of the subshells in terms of increasing energy does not follow a regular pattern at n = 3 and higher

Atomic Structure Ground-State, downloadable AS & A Level Chemistry revision notes

The ground state of an atom is achieved by filling the lowest energy subshells first

Electron Arrangement Summary

  • Each shell can be divided further into subshells, labelled s, p, d and f
  • Each subshell can hold a specific number of orbitals:
    • s subshell : 1 orbital
    • p subshell : 3 orbitals
    • d subshell : 5 orbitals
    • f subshell : 7 orbitals
  • Each orbital can hold a maximum number of 2 electrons so the maximum number of electrons in each subshell are as follows:
    • s : 1 x 2 = total of 2 electrons
    • p : 3 x 2 = total of 6 electrons
    • d : 5 x 2 = total of 10 electrons
    • f : 7 x 2 = total of 14 electrons

Summary of the Arrangement of Electrons in Atoms Table

Atomic Structure Electrons in Atoms Table, downloadable AS & A Level Chemistry revision notes

Exam Tip

The three p orbitals are labelled px, py and pz, but you do not need to include this in your electron configurations!


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