Ionisation Energy and Period 3 Trends


Definition of First Ionisation Energy
The energy required to remove one electron from each atom in a mole of gaseous atoms.

Example of first ionisation energy: Example of second ionisation energy:
Mg (g) Mg+ (g) + e-
Mg+ (g) Mg2+ (g) + e-


Trends in Ionisation energy

Successive ionisation energy for an element
Successive ionisation energy provides evidence that electrons are arranged in shells. As the number of electrons removed increases more ionisation energy is required because the shells become increasingly closer to the influence of the nucleus.

Notice the large difference in ionisation energy between the 1st and 2nd electron. This is because electrons are arranged in shells.

The first electron removed is the outer electron in the n=3 shell. The second electron is in the n=2 shell. The n=2 shell is lower in energy as it is closer to the influence of the nucleus. This means there is decreased shielding from inner electrons and stronger nuclear attraction.

Trends in first ionisation energy across the period


General increase across the period because:
· The nuclear charge increases;
· Electrons are removed from the same energy level;
· Therefore there’s stronger nuclear attraction of the outer electron so more energy is needed to remove the outer electron.

1. Decrease in ionisation energy between Be and B because:
· When you look at the elements subshells Be is 1s12s2 and B 1s22s22p1
· The s subshell is lower in energy (closer to the nucleus) than the p subshell;
· Therefore less energy is required to remove the outer electron in Boron despite an increased nuclear charge.

2. Decrease in ionisation energy between N and O because:

· When you look at the orbitals the arrangement is:
· The extra repulsion from the paired electron in the outer subshell of Oxygen lowers the energy required to remove the outer electron despite an increased nuclear charge.

3. Large decrease in ionisation energy between Ne and Na because:
· The outer electron in Ne is in the n=2 subshell but the outer electron in Na is in the n=3 subshell;
· There is more shielding of inner electrons in the Na atom;
· Therefore less energy is needed to remove the outer electron despite an increased nuclear charge.

Trends in first ionisation energy down the group
General decrease because:
· The outer electrons are in different energy shells (e.g. n=2, n=3)
· Therefore there is increased shielding from inner electrons and weaker attraction to outer electrons;
· This means less energy is needed to remove the outer electron despite and increased nuclear charge.

 


Period 3 Trends

Atomic radius
As you move across the period the atomic radius decreases because the nuclear charge increases, however the outer electrons are in the same shell (n=3) and therefore shielding from inner electrons is the same so the outer electrons are attracted closer to the nucleus.

Na 2,8,1
Mg 2,8,2
Al 2,8,3
Si 2,8,4
P 2,8,5
S 2,8,6
Cl 2,8,7
Ar 2,8,8

Conductivity
Between Na and Al there is increased electrical conductivity as there are more delocalised electrons per cation and therefore there are more electrons to carry the charge.

Melting and boiling points
There is an increase in melting and boiling points from Na to Al as there are a greater number of delocalised electrons per atom therefore the strength of the metallic bonding increases.

Silicon has a high melting and boiling point because it forms 4 covalent bonds which need a large amount of energy to break.

Phosphorus, Sulphur and Chlorine have low meting and boiling points because although the bonds between atoms is strong the bonds between molecules is weak. Sulphur has a higher melting/boiling point than Prosperous or Chlorine because a Phosphorous molecule is P4, and a chlorine molecule is Cl2, however Sulphur exists as S8 molecule. This means there are more electrons in a Sulphur molecule so the strength of the Van der Waals forces increases.

Why is the boiling point of Al above Si?
This is because once Silicon has melted the strong covalent structure has broken down and therefore it has weaker bonds than molten Aluminium so has a lower boiling point than ‘expected’.