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Specific Impacts on Ionisation Energies

Chemistry: AQA A Level, topic 1.1.9

Impact of Sub-Shell Structure on Ionisation Energy

The impact of the sub-shell structure is enough to sometimes outweigh and contrast general trends in ionisation energy.

Example of sub-shell impact

Example of sub-shell impact

  • An example of the impact of the sub-shell structure is the difference in ionisation energies between magnesium and aluminium within Period 3.
  • They are in the same period and so we would expect aluminium's ionisation energy to be greater due to a greater nuclear charge.
Contrast to the trend

Contrast to the trend

  • But the ionisation energies are:
  • Mg = 738 kJ/mol.
  • Al = 578 kJ/mol.
  • These values do not align with the general trend we predicted.
  • We need to look at the sub-shell structure of these atoms to understand what is going on.
Mg vs Al sub-shell structure

Mg vs Al sub-shell structure

  • Mg = 1s22s22p63s2
  • So the first ionisation removes an electron from the 3s orbital.
  • Al = 1s22s22p63s23p1
  • So the first ionisation removes an electron from the 3p orbital.
Sub-shell energies

Sub-shell energies

  • The 3p orbital is further from the nucleus on average than the 3s orbital.
  • So this orbital has a higher energy and its electrons have a lower electrostatic attraction to the positive nucleus.
  • It is also slightly shielded by the 3s2 electrons.
  • So it's easier to ionise an electron in the 3p orbital than in the 3s orbital.
    • This explains why it is easier to ionise Al than Mg.
    • This effect outweighs the general trend we looked at first.
Evidence

Evidence

  • From these arguments, we can use the relative first ionisation energy values of Mg and Al as evidence for sub-shell structures.

Impact of Electron Repulsion on Ionisation Energy

The impact of electron repulsion can be seen in the ionisation energies of Group 5 and Group 6.

Phosphorus and sulfur

Phosphorus and sulfur

  • Both P and S have their outer electron in a 3p orbital.
  • P = 1s22s22p63s23p3
  • S = 1s22s22p63s23p4
  • The shielding for each atom's outer electron is identical.
  • S has a higher nuclear charge and so you might expect S to have a higher ionisation energy. This is NOT the case.
Singly vs doubly occupied orbitals

Singly vs doubly occupied orbitals

  • The P electron is removed from a singly occupied orbital but the S electron is removed from a doubly occupied orbital.
  • S has a lower ionisation energy because electrons in the same orbital repel each other.
  • This means that an electron is easier to remove from a doubly occupied orbital.
  • This gives more evidence for the electronic structure model we are using.

Test yourself on Specific Impacts on Ionisation Energies

  • How is ionisation energy expected to change as you go across Period 3?

  • Which of the elements has the highest ionisation energy?

  • Why is it easier to ionise a 3p electron than a 3s electron?

  • Why is sulfur expected to have a higher ionisation energy than phosphorus?

  • Which of the following is identical in phosphorus and sulfur?

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