CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.1.6 Electronic Structure

Electron Shells: Basics

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 lesser 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

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

Subshells

  • 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

Orbitals

  • The 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
    • 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 3D shapes

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 at AS Level

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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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