Ionisation Energy (AQA A Level Chemistry): Revision Note

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 100 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 is:

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

Trends in Ionisation Energies

• Ionisation energy shows periodicity - a recurring pattern in the Periodic Table

• The Group 1 metals have a relatively low first ionisation energy, whereas the noble gases have very high ionisation energies

• The size of the first ionisation energy is affected by four factors:

  • The size of the nuclear charge

  • The distance between the outer electrons and the nucleus

  • The shielding effect of the inner electrons

  • Spin-pair repulsion

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

Ionisation energy across a period

• The ionisation energy across a period generally increases due to the following factors:

  • Across a period, the effective nuclear charge increases

  • This causes the atomic radius of the atoms to decrease, as the outer shell electrons are pulled closer to the nucleus, so the distance between the nucleus and the outer electrons decreases

  • The shielding by inner shell electrons remains fairly 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 2p¹

• There is a slight decrease in IE₁ between nitrogen and oxygen due to spin-pair repulsion in the 2p_x orbital of oxygen

  • Nitrogen has a first ionisation energy of 1400 kJ mol^-1 as its electron configuration is 1s² 2s² 2p³

  • Oxygen has a first ionisation energy of 1310 kJ mol^-1 as its electron configuration is 1s² 2s² 2p⁴

  • In oxygen, there are 2 electrons in a 2p 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 an 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 add more shells of electrons, making the atoms bigger

  • So, the distance between the nucleus and the 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, so 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