CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.1.12 Ionisation Energies & Electronic Configurations

Ionisation Energies: Explained

  • Energy is required to remove an outer shell electron as this involves breaking the attractive forces between the electron and the positively charged nucleus
  • There are several factors which affect the magnitude of the ionisation energy:
  • Nuclear charge
    • Positive nuclear charge increases with increasing number of protons
    • The greater the positive charge, the greater the attractive forces between the outer electron(s) and the nucleus
    • More energy is required to overcome these forces so ionisation energy increases with increasing nuclear charge
  • Shielding
    • Electrons repel each other and electrons occupying the inner shells repel electrons located in shells further outside the nucleus and prevent them from feeling the full effect of the nuclear charge
    • The greater the shielding effect is, the weaker the attractive forces between the positive nucleus and the negatively charged electrons
    • Less energy is required to overcome the weakened attractive forces so ionisation energy decreases with increasing shielding effects

Atomic Structure Ionisation & Shielding, downloadable AS & A Level Chemistry revision notes

Shielding makes it easier to remove the outermost electrons

  • Atomic/ionic radius
    • The larger the radius, the greater the distance between the nucleus and the outer shell electron(s)
    • Increasing distance weakens the strength of the attractive forces
    • Larger atoms/ions also result in greater shielding due to the presence of more inner electrons
    • Less energy is required to remove the outer shell electron(s) so ionisation energy decreases with increasing atomic/ionic radius
  • Spin-pair repulsion
    • Spin pair repulsion occurs when the electron being removed is spin paired with another electron in the same orbital
    • The proximity of the like charges of electrons in the orbital results in repulsion
    • Less energy is required to remove one of the electrons so ionisation energy decreases when there is spin-pair repulsion

Atomic Structure Factors Affecting Ionisation Energies, downloadable AS & A Level Chemistry revision notes

Summary of factors affecting ionisation energies of atoms

Ionisation Energies: Electronic Configuration

  • Successive ionisation data can be used to:
    • Predict or confirm the simple electronic configuration of elements
    • Confirm the number of electrons in the outer shell of an element
    • Deduce the Group an element belongs to in the Periodic Table
  • By analyzing where the large jumps appear and the number of electrons removed when these large jumps occur, the electron configuration of an atom can be determined
  • Na, Mg and Al will be used as examples to deduce the electronic configuration and positions of elements in the Periodic Table using their successive ionisation energies

Successive ionisation energies table

Atomic Structure First Four Ionisation Energies of Sodium, Magnesium & Aluminium Table, downloadable AS & A Level Chemistry revision notes

Sodium

  • For sodium, there is a huge jump from the first to the second ionisation energy, indicating that it is much easier to remove the first electron than the second
  • Therefore, the first electron to be removed must be the last electron in the valence shell thus Na belongs to group I
  • The large jump corresponds to moving from the 3s to the full 2p subshell
    Na       1s2 2s2 2p6 3s1

Magnesium

  • There is a huge increase from the second to the third ionisation energy, indicating that it is far easier to remove the first two electrons than the third
  • Therefore the valence shell must contain only two electrons indicating that magnesium belongs to group II
  • The large jump corresponds to moving from the 3s to the full 2p subshell
    Mg       1s2 2s2 2p6 3s2

Aluminium

  • There is a huge increase from the third to the fourth ionisation energy, indicating that it is far easier to remove the first three electrons than the fourth
  • The 3p electron and 3s electrons are relatively easy to remove compared with the 2p electrons which are located closer to the nucleus and experience greater nuclear charge
  • This is due to weakened shielding effects through the loss of three electrons
  • The large jump corresponds to moving from the third shell to the second shell
    Al         1s2 2s2 2p6 3s2 3p1

Exam Tip

Find the large jumps by subtracting the successive ionisation energies from each other to identify when an electron has been removed from a different subshell.

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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