Dips in the trend Ionisation energy

Question 1

What are the deviants in period 2

Answer 1

beryllium to boron

nitrogen to oxygen

Question 2

Why is there a dip from beryllium to boron

Answer 2

There is a slight decrease in IE1 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

2p orbitals are higher in energy so it is easier to remove an electron.

Question 3

Why is there a dip from nitrogen to oxygen

Answer 3

There is a slight decrease in IE1 between nitrogen and oxygen due to spin-pair repulsion in the 2px orbital of oxygen

Nitrogen has a first ionisation energy of 1400 kJ mol-1 as its electron configuration is 1s2 2s2 2px1 2py1 2pz1

Oxygen has a first ionisation energy of 1310 kJ mol-1 as its electron configuration is 1s2 2s2 2px2 2py1 2pz1

In oxygen, there are 2 electrons in the 2px orbital, so the repulsion between those electrons makes it slightly easier for one of those electrons to be removed

Question 4

What are the anomalies of period 3

Answer 4

Magnesium and aluminium

phosphorus and sulphur

Question 5

Why is there a anomaly between magnesium and aluminium

Answer 5

The first is between Mg and Al, because the outer electron of Mg is in the orbital 3s, whereas that of Al is in 3p.

The 3p electron has more energy than the 3s electron, so the ionization energy of Al is actually less than that of Mg. This makes sense because the 3p electron requires less energy to be removed from the atom.

Remember that if an electron has more energy, it needs less external energy to be removed from the atom because having more energy makes it more unstable.

Question 6

The second anomaly is between P and S why does this happen

Answer 6

Although both have outer electrons in the 3p orbital, that of P is unpaired (3p3) but that of S is paired (3p4).

3p orbitals can fit up to 6 electrons, but if there are 3 or less electrons, they are unpaired and take up a “space” just for themselves.

Since the fourth electron in 3p4 is paired, it will experience a repulsive force from the third electron.

This makes it easier to remove, and therefore the ionisation energy of S is less than that of P.