Chemical periodicity part 2

Periodic Table

1st ionization energy is the minimum energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous cations.

It is a endothermic process.
Across Periods:
There are more protons in each nucleus so the nuclear charge in each element increases.
Therefore the force of attraction between the nucleus and outer electron is increased.
There is a negligible increase in shielding because each successive electron enters the same energy level.
More energy is needed to remove the outer electron.
1st I.E increases
First ionization energy generally increases going across Period 3.
The first ionization energy drops between magnesium and aluminium before increasing again.
The first ionization energy drops between phosphorus and sulphur before increasing again.


Irregularities:
1st I.E of Magnesium is greater than Aluminium.

Magnesium: 1s2 2s2 2p6 3s2
Aluminium: 1s2 2s2 2p6 3s2 3p1

3p1 electron is at a higher energy level. Thus it is much easier to be removed than 3s2 electrons.

1st I.E of Phosphorus is greater than Sulphur.

Phosphorus: 1s2 2s2 2p6 3s2 3p3
Sulphur: 1s2 2s2 2p6 3s2 3p4

The 3p electrons in phosphorus are all unpaired. In sulphur, there is one group of paired electrons and 2 unpaired ones.
There is some repulsion between paired electrons (interelectronic repulsion) in the same orbital.
Less energy needed to remove one electron from 3p4 than 3p3.

Down the Group:
There will be an additional shell of electrons
There is an increase in screening/shielding effect which outweighs the increase in effective nuclear charge.
Valence electrons is further away from the nucleus.
Less energy needed to remove electrons
1st I.E decreases