Ionisation Energy (AQA A Level Chemistry)

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What is Ionisation Energy?

  • The Ionisation Energy (IE) of an element is the amount of energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous ions
  • Ionisation energies are measured under standard conditions which are 298 K and 101 kPa
  • The units of IE are kilojoules per mole (kJ mol-1)
  • The first ionisation energy (IE1) is the energy required to remove one mole of electrons from one mole of atoms of an element to form one mole of 1+ ions
    • E.g. the first ionisation energy of gaseous calcium:

Ca(g) → Ca+ (g) + e-          IE1 = +590 kJ mol-1

Trends in Ionisation Energies

  • Ionisation energies show periodicity - a trend across a period of the Periodic Table
  • As could be expected from their electron configuration, the group 1 metals have a relatively low ionisation energy, whereas the noble gases have very high ionisation energies
  • The size of the first ionisation energy is affected by four factors:
    • Size of the nuclear charge
    • Distance of outer electrons from the nucleus
    • Shielding effect of inner electrons
    • Spin-pair repulsion

  • First ionisation energy increases across a period and decreases down a group

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 across a period generally increases due to the following factors:
    • Across a period the nuclear charge increases
    • This causes the atomic radius of the atoms to decrease, as the outer shell is pulled closer to the nucleus, so the distance between the nucleus and the outer electrons decreases
    • The shielding by inner shell electrons remain reasonably constant as electrons are being added to the same shell
    • It becomes harder to remove an electron as you move across a period; more energy is needed
    • So, the ionisation energy increases

Dips in the trend

  • 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 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
    • In oxygen, there are 2 electrons in the 2px orbital, so the repulsion between those electrons makes it slightly easier for one of those electrons to be removed

From one period to the next

  • There is a large decrease in ionisation energy between the last element in one period, and the first element in the next period
  • This is because:
    • There is increased distance between the nucleus and the outer electrons as you have added a new shell
    • There is increased shielding by inner electrons because of the added shell
    • These two factors outweigh the increased nuclear charge

Ionisation energy down a group

  • The ionisation energy down a group decreases due to the following factors:
    • The number of protons in the atom is increased, so the nuclear charge increases
    • But, the atomic radius of the atoms increases as you are adding more shells of electrons, making the atoms bigger
    • So, the distance between the nucleus and outer electron increases as you descend the group
    • The shielding by inner shell electrons increases as there are more shells of electrons
    • These factors outweigh the increased nuclear charge, meaning it becomes easier to remove the outer electron as you descend a group
    • So, the ionisation energy decreases

Ionisation Energy Trends across a Period & going down a Group Table

Across a period: Ionisation energy increases Down a group: Ionisation energy decreases
Increase in nuclear charge Increase in nuclear charge
Same number of shells More shells
Distance from outer electron to the nucleus decreases Distance from outer electron to the nucleus increases
Shielding remains reasonably constant  Shielding increases, cause a weaker force of attraction between the outer electron and the nucleus
Decreased atomic / ionic radius Increased atomic / ionic radius
The outer electron is held more tightly to the nucleus so it requires more energy to remove The outer electron is held less tightly to the nucleus so it requires lessenergy to remove

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Francesca

Author: Francesca

Expertise: Head of Science

Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.