EvidenceFor Sub-Shells From Ionisation Energies (AHL)

Evidence for the existence of sub-shells is given by studies in the trends of ionisation energies within the periodic table.

The first ionisation energy is defined as the energy absorbed when 1 mole of electrons is removed from 1 mole of atoms in its gaseous state, giving 1 mole of positive ions.

The size of the first ionisation energy depends on a number of factors;

  1. Nuclear charge:Nucleus is positively charged due to the protons. Greater the number of protons, the greater the size of this charge. Atoms with more protons have a stronger attractive force towards the electrons, causing an increase in ionisation energy.
  1. Distance:The greater the distance between the nucleus and the outer most electrons, the less the attractive force between them, causing a decrease in ionisation energy.
  1. Shielding:A repelling force from the filled inner orbitals makes the electrons in the outer orbitals easier to remove, causing a decrease in ionisation energy. A paired e- is also easier to remove than an unpaired e- due to this repelling force.
First Ionisation Energies In The Second Period (Li to Ne) (AHL)

Plot a graph of first ionisation energy (y axis) against proton number (x axis) for the second period elements.

Label each point with the symbol of the element.

Label each point with the outer electron structure of the element.

Li ( 2 s 1 ) to Be ( 2 s 2 )=Increase caused by increasing nuclear

charge. No change in distance as same sub-shell.

Be to B ( 2 s 2 , 2 p 1 ) =Decreasecausedby new sub-shell. Greater distance and increase in shielding outweighs the increase in nuclear charge.

B to C to N ( 2 s2 , 2 p 3 ) = Increase caused by increase in nuclear charge and electrons are added to same sub-shell.

N to O ( 2 p 4 ) = Decrease. In 2 p 3 all e- are unpaired, but in 2 p 4 the paired electrons shield the removal of one of the unpaired electrons.

O to F to Ne ( 2 s 2 , 2 p 6 ) = Increase caused by increasing nuclear charge.

Successive Ionisation Energies (AHL)
  • After removing the first electron, possible to continue removing electrons giving 2nd, 3rd, etc ionisation energies.
  • E.g.MgMg++e-

Mg+Mg2++e-

Mg11+Mg12++e-

  • Energy required to remove 1st electron is not 12 times that to remove the 12th.
  • This is because increased number of protons compared to electrons. Increased effective nuclear charge.