12.1.2 Ionisation Energy Trends

• 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

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 2p_x¹

• 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 2p_x¹ 2p_y¹ 2p_z¹
  • Oxygen has a first ionisation energy of 1310 kJ mol^-1 as its electron configuration is 1s² 2s² 2p_x² 2p_y¹ 2p_z¹
  • In oxygen, there are 2 electrons in the 2p_x 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

Successive ionisation energies of an element

• The successive ionisation energies of an element increase
• This is because once you have removed the outer electron from an atom, you have formed a positive ion
• 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

Successive ionisation energies for the element calcium

• 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 more difficult as the orbital is no longer full, and there is less spin-pair repulsion
• The graph shows there is a large increase in successive ionisation energy as the electrons are being removed from an increasingly positive ion
• The big jumps on the graph show the change of shell and the small jumps are the change of subshell

Answer:

The largest jump is between IE₂ and IE₃ which will correspond to a change in energy level. Therefore, the unknown element must be in group 2.

Answer:

• • The largest jump in ionisation energy is between IE₆ and IE₇ meaning that the 7th electron is being removed from an energy level closer to the nucleus
  • Therefore, element X must be group 16 (6)
  • If element X is in group 16 (6) and in period 2, it must be oxygen