3.3    Ionisation Energy

What is ionisation energy?

 

• The 1st ionisation energy, ΔHi1 is the energy needed to remove one electron from each atom in one mole of the atoms of the element in the gaseous state to form one mole of gaseous 1+ ions

 

• The general unit for ionisation energy is kJ mol⁻¹.

 

Ca(g) → Ca⁺(g) + e⁻      ; ΔHi1 = +590 kJ mol⁻¹

 

• If a second electron is removed from the gaseous 1+ ions, it is the 2nd ionisation energy, ΔHi2.

 

Ca⁺(g) → Ca²⁺(g) + e⁻    ; ΔHi2 = +1150 kJ mol⁻¹

 

• The 2nd ionisation energy, ΔHi2 is the energy needed to remove one electron from each gaseous 1+ ion in one mole of the ions to form one mole of gaseous 2+ ion.

 

• The continuous removal of electrons until the nucleus is left only will result in successive ionisation energies

 

Factors affecting the ionisation energy

1)  Charge on the nucleus (Number of proton)

– The greater the number of proton in the nucleus, the greater the amount of positive charge.

• The greater the positive charge, the greater the attractive force between the nucleus and outer electrons
• More energy is needed to overcome the attractive So, the ionisation energy is higher.

– The greater the nuclear charge, the higher the ionisation energy.

2) Distance between nucleus and outer electrons (Size of atom/ion)

• The larger the size of the atom, the greater the distance between the nucleus and the outer electrons
• The greater the distance between the nucleus and the outer electrons, the weaker the attractive force between nucleus and outer  electrons.

• Furthermore, the outer electrons experience greater shielding effect by the inner electrons
• Less energy is required to overcome the attractive So, the ionisation energy is lower.

– The greater the distance between nucleus and outer electrons, the lower the ionisation energy.

 

3) Shielding effect by the inner electrons

• All electrons are negatively-charged, so they repel each Electrons in full inner shells will repel the outer electrons and so prevent the full nuclear charge being felt by the outer electrons. This is called shielding or screening.
• The greater the shielding effect, the weaker the attractive force between the nucleus and outer electrons
• Less energy is required to overcome the attractive So, the ionisation energy os lower.

– The greater the shielding effect, the lower the ionisation energy.

 

Pattern of ionisation down a Group

• The first ionisation energy decreases down a Group

• This is because the atomic size increases and hence the distance between the nucleus and outer electrons increases. The outer electrons also experience a greater shielding effect

 

• These two factors outweigh the increasing nuclear charge

 

• The above factors causes the attractive force between nucleus and outer electrons to decrease, less energy is required to overcome the weaker attractive Hence, the ionisation energy is lower.

 

Pattern of ionisation energy across a Period

• The general trend of ionisation energy across a Period is increasing.

 

• This is because, across a Period, the number of proton in the nucleus increases by one therefore the nuclear charge increases

 

• However, the distance between the nucleus and outer electrons decreases across a Period and the outer electrons experience the same amount of shielding

 

• The above factors causes the attractive force between nucleus and outer electrons to increase, more energy is required to overcome the stronger attractive Hence, the ionisation energy is higher.

The drop between (Be-B) and (Mg-Al)

 

• There is a slight decrease in first ionisation energy between beryllium-born and magnesium-aluminium.

 

• This is because the fifth electron in boron is located in the 2p sub-shell, which is slightly further away from the nucleus. The outer electron in boron is shielded by the 1s² as well as 2s² electrons

 

Be : 1s²2s²            B : 1s²2s²2p¹

 

• The decrease in first ionisation energy between magnesium and aluminium has the same reason, except that everything is happening at the third energy lewel

 

Mg : 1s²2s²2p⁶3s²        Al : 1s²2s²2p⁶3s²3p¹

The drop between (N-O) and (P-S)

 

• There is a slight decrease in first ionisation energy between nitrogen-oxygen and phosphorus-sulphur.

 

• This is because the electron being removed in oxygen is from the orbital which contains a pair of electrons. The extra repulsion between the pair of electrons results in less energy needed to remove the electron. This is called spin-pair repulsion.

 

N : 1s²2s²2px¹2py¹2pz¹         O : 1s²2s²2px²2py¹2pz¹

 

• The decrease in first ionisation energy between phosphorus and sulphur has the same reason, except that everything is happening at the third energy level

 

P : 1s²2s²2p⁶3s²3px¹3py¹3pz¹       S : 1s²2s²2p⁶3s²3px²3py¹3pz¹

 

Successive ionisation energy

• The following data can be obtained from successive ionisation energy:
  1. Total number of electrons in an atom

– is equal to the number of separate ionisation energies possessed by the atom.

 

• Number of principal quantum shells occupied and the number of electrons in each

-by looking the big difference between two successive ionisation energies.

 

• Number of sub-shells occupied and the number of electrons in each

 

• Successive ionisation energies get larger because removing an electron from a positive ion with increasing positive charge is going to be more difficult due to the increasing attractive force

• There is a relatively big increase in ionisation energy between the first and second electron being This suggests that the second electron being removed is from a principal quantum shell closer to the nucleus.

 

• The big jump occurs three times, so there are four principal quantum shells occupied by this atom

• After the big jumps, there is a steady increase in ionisation energy, this suggests that the electrons being removed come from the same principal quantum shell

 

• The electronic configuration for this atom can be written as 2,8,8,1.

 

• Two more examples:

Looking at the pattern in more detail

• The electronic configuration of chlorine is 1s²2s²2p⁶3s²3px²3py²3pz¹.

 

• Between the second and third ionisation energy, there is a slight increase in difference in ionisation This is because the first two electrons being removed come from the orbitals which contain a paired electrons. The extra repulsion between the electrons result in the ionisation energy being lower.

 

• There is also a slight increase in difference in ionisation energy between the fifth and sixth electron being This is because the sixth electron being removed comes from the 3s sub-shell, which is slightly closer to the nucleus.

 

• The drastic increase in ionisation energy between the seventh and eighth electrons suggests that the eighth electron comes from a principal quantum shell closer to the nucleus