Ionisation Energy (Oxford AQA International A Level Chemistry)

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'

Process of Ionisation

• 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 values for ionisation energies are always positive as this is an endothermic process

  • This is because energy is required to break the force of attraction between the electron and the central positive nucleus

Successive Ionisation energies

• The energy required to remove electrons one by one from an element can be measured, starting with the outer electrons

• The first ionisation energy will be the energy required to remove one mole of electrons from one mole of atoms to form one mole of 1+ ions

  • For sodium this is represented as:

Na (g) → Na⁺ (g) + e^-

• The second ionisation energy will be the energy required to remove one mole of electrons from one mole of 1+ ions to form one mole of 2+ ions and so on

  1. For sodium this is represented as:

Na⁺(g) → Na²⁺ (g) + e^-

• The equation for the third ionisation energy of sodium is:

Na²⁺(g) → Na³⁺ (g) + e^-

• These are known as successive ionisation energies

• The amount of energy needed to remove each electron will increase as more electrons are removed

  • The first electron is being removed from an atom

  • The second electron is being removed from a 1+ ion

  • The third electron is being removed from a 2+ ion and so on

• The ionisation energies of sodium in kJ mol^-1 are shown below:

• We can see a distinct set of energy levels here:

  • The first electron is relatively easy to remove

  • The next 8 electrons are hard to remove

  • The last two (those closest to the nucleus) are even harder to remove

• The largest increase between ionisation energies indicates which group an element is located in

  • The largest increase is between the 1st and 2nd ionisation energy

  • This demonstrates sodium is in Group 1 because a much higher amount of energy is needed to remove the second electron as the energy level it is being removed from is closer to the nucleus

Trends in Ionisation energies

• Trends in ionisation energies can be observed across a period and down a group in the Periodic Table

Across Period 3

• The first ionisation energy generally increases for each element going across Period 3 in the Periodic Table

Graph to show ionisation energies of Period 3 elements

• This is because going across Period 3:

  • The nuclear charge (the number of protons in the nucleus) increases

  • This causes the atomic radius of the atoms to decrease, as the outer shell is pulled closer to the nucleus

  • Therefore the distance between the nucleus and the outer electrons decreases

  • The shielding by inner shell electrons remains reasonably constant across the period as electrons are being added to the same shell

  • It becomes harder to remove an electron as you move so more energy is needed

• There are two elements which do not follow the general trend:

  • Aluminium has a lower first I.E than magnesium

  • Sulfur has a lower first I.E than phosphorus

• These deviations provide evidence for the presence of s- and p- orbitals

Why does aluminium have a lower first I.E than magnesium?

• Magnesium has the electronic configuration 1s²2s²2p⁶3s²

  • Magnesium loses its first electron from the 3s orbital

• Aluminium has the electronic configuration 1s²2s²2p⁶3s² 3p¹

  • Aluminium loses its first electron from the 3p orbital

• The 3p orbital is of slightly higher energy than the 3s orbital so less energy is needed to remove the electron, despite the increasing nuclear charge

Why does sulfur have a lower first I.E than phosphorus?

• Phosphorus has the electronic configuration 1s²2s²2p⁶3s² 3p³

  • Each 3p orbital will contain one electron

• Sulfur has the electronic configuration 1s²2s²2p⁶3s² 3p⁴

  • One of the 3p orbitals contains two electrons

• There is repulsion between the two electrons in the 3p orbital of sulfur making it easier to remove the electron, despite the increasing nuclear charge

Down Group 2

• The first ionisation energy of elements in Group 2 generally decreases going down the group

• This is because, going down Group 2:

  • The atomic radius of the atoms increases due to an increasing number of electrons

  • The shielding by inner shell electrons also increases 

  • These factors outweigh the increased nuclear charge, so it becomes easier to remove the outer electron