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Electronic Structure (CIE AS Chemistry)

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Basic Electronic Structure

Shells

  • The arrangement of electrons in an atom is called the electronic 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
    • The higher the principal quantum number, the higher the energy of the shell

  • 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

Principal quantum shells

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

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

Sub-shells

  • The principal quantum shells are split into sub-shells 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 sub-shells increases in the order s < p < d

  • The order of sub-shells appear to overlap for the higher principal quantum shells as seen in the diagram below:

Energy levels of the quantum shells

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

At principal quantum shell n = 3, the energy levels of the sub-shells start to overlap

Orbitals

  • Sub-shells 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 sub-shell 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

Shapes of the electron orbitals

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

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

Examiner Tip

Note that the shape of the d orbitals is not required at AS Level

An overview of the shells, sub-shells and orbitals in an atom

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

s and p orbitals have different shapes

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 sub-shells with the lowest energy first (1s)
  • The order of the sub-shells in terms of increasing energy does not follow a regular pattern at n = 3 and higher

Summary of filling sub-shells 

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

The ground state of an atom is achieved by filling the lowest energy sub-shells first

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.