CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.1.11 Ionisation Energy Trends

Ionisation Energies: Trends

  • Ionisation energies show periodicity
  • As could be expected from their electronic configuration, the group I metals show low IE whereas the noble gases have very high IE’s
  • The first ionisation energy increases across a period and decreases down a group caused by four factors that influence the ionisation energy
    • Size of the nuclear charge: the nuclear charge increases with increasing atomic number, which means that there are greater attractive forces between the nucleus and electrons, so more energy is required to overcome these attractive forces when removing an electron
    • Distance of outer electrons from the nucleus: electrons in shells that are further away from the nucleus are less attracted to the nucleus so the further the outer electron shell is from the nucleus, the lower the ionisation energy
    • Shielding effect of inner electrons: the shielding effect is when the electrons in full inner shells repel electrons in outer shells preventing them to feel the full nuclear charge so the greater the shielding of outer electrons by inner electron shells, the lower the ionisation energy
    • Spin-pair repulsion: electrons in the same atomic orbital in a subshell repel each other more than electrons in different atomic orbitals which makes it easier to remove an electron (which is why the first ionization energy is always the lowest)

Atomic Structure Ionisation Energy across Periods, downloadable AS & A Level Chemistry revision notes

A graph showing the ionisation energies of the elements hydrogen to sodium

Ionisation energy across a period

  • The ionisation energy over a period increases due to the following factors:
    • Across a period the nuclear charge increases
    • The distance between the nucleus and outer electron remains reasonably constant
    • The shielding by inner shell electrons remain reasonably constant
  • There is a rapid decrease in ionisation energy between the last element in one period and the first element in the next period caused by:
    • The increased distance between the nucleus and the outer electrons
    • The increased shielding by inner electrons
    • These two factors outweigh the increased nuclear charge
  • There is a slight decrease in IE1 between beryllium and boron as the fifth electron in boron is in the 2p subshell which is further away from the nucleus than the 2s subshell of beryllium
    • Beryllium has a first ionisation energy of 900 kJ mol-1 as its electron configuration is 1s2 2s2
    • Boron has a first ionisation energy of 800 kJ mol-1 as its electron configuration is 1s2 2s2 2px1
  • There is a slight decrease in IE1 between nitrogen and oxygen and phosphorus due to spin-pair repulsion in the 2px orbital of oxygen
    • Nitrogen has a first ionisation energy of 1400 kJ mol-1 as its electron configuration is 1s2 2s2 2px1 2py1 2pz1
    • Oxygen has a first ionisation energy of 1310 kJ mol-1 as its electron configuration is 1s2 2s2 2px2 2py1 2pz1

Ionisation energy down a group

  • The ionisation energy down a group decreases due to the following factors:
    • Across a period the nuclear charge increases
    • The distance between the nucleus and outer electron increases
    • The shielding by inner shell electrons increases

Ionisation energy trends across a period & going down a group table

Atomic Structure First Ionisation Energy Trends Table, downloadable AS & A Level Chemistry revision notes

Successive ionisation energies of an element

  • The successive ionisation energies of an element increase as removing an electron from a positive ion is more difficult than from a neutral atom
  • As more electrons are removed the attractive forces increase due to decreasing shielding and an increase in the proton to electron ratio
  • The increase in ionisation energy, however, is not constant and is dependent on the atom’s electronic configuration
  • Taking calcium as an example:

Ionisation energies of calcium table

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

Atomic Structure Successive Ionisation Energies of Calcium, downloadable AS & A Level Chemistry revision notes

  • The first electron removed has a low IE1 as it is easily removed from the atom due to the spin-pair repulsion of the electrons in the 4s orbital
  • The second electron is more difficult to remove than the first electron as there is no spin-pair repulsion
  • The third electron is much more difficult to remove than the second one corresponding to the fact that the third electron is in a principal quantum shell which is closer to the nucleus (3p)
  • Removal of the fourth electron is less difficult as the orbital is no longer full and there is less spin-pair repulsion

Exam Tip

It is easy to remove electrons from a full subshell as they undergo spin-pair repulsion.

It gets more difficult to remove electrons from principal quantum shells that get closer to the nucleus as there is less shielding and an increase in attractive forces between the electrons and nuclear charge.

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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