Ionisation Energy Trends Across a Period (HL) (HL IB Chemistry)

• The trends in ionisation energy across a period and down a group have been discussed in our revision note on Periodicity 
  • Trends in ionisation energy across a period provide evidence for the existence of energy sublevels

Graph showing Ionisation Energies From H to Ne

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

• The ionization energy across 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 remains the same
• 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

Exceptions to the general trend in ionisation energy

• There are discontinuities in the general trend which are caused by the following:
  • There is a slight decrease in 1st I.E. 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 801 kJ mol^-1 as its electron configuration is 1s² 2s² 2p¹
  • There is a slight decrease in 1st I.E. between nitrogen and oxygen due to spin-pair repulsion in the 2p subshell of oxygen
    • Nitrogen has a first ionisation energy of 1402 kJ mol^-1 as its electron configuration is 1s² 2s² 2p³
    • Oxygen has a first ionisation energy of 1314 kJ mol^-1 as its electron configuration is 1s² 2s² 2p⁴

Summary of 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
Shell number is the same  The distance of the outer electron to the nucleus is the same	Increase in shells Distance of outer electron to nucleus increases The shielding effect increases, therefore, the attraction of valence electrons to the nucleus decreases
Shielding remains reasonably constant	Increased shielding
Deceased atomic/ionic radius	Increases atomic/ionic radius
The outer electron is held more tightly to the nucleus so it gets harder to remove it	The outer electron is held more loosely to the nucleus so it gets easier to remove it