Ionisation Energies and electron sub-levels

Ionisation Energy

Definition – The first ionisation energy of an element, X, is defined as the enthalpy change for the reaction:

X(g) →X⁺ + e^-

Ionisation Energy Down a Group

Ionisation energy decreases down the group.

Going from Mg to Be, IE decreases because:

o Be outer electron is in the 3s sub-shell rather than the 2s. This is higher in energy

o The 3s electron is further from the nucleus and shielded by the inner electrons

o So the 3s electron is more easily removed

This is widely regarded as evidence for electrons being ordered in electron energy levels.

A similar decrease occurs in every group in the periodic table.

Ionisation energy across a period

The ionisation energy for the elements neon to potassium vary as follows.

A general increase is seen across the period from sodium to argon.

Why?

From Na to Ar (11 protons to 18 protons), the nuclear charge in each element increases.

The electrons are attracted more strongly to the nucleus – so it takes more energy to remove one from the atom.

Why is there a fall from Mg to Al?

Al has configuration 1s² 2s² 2p⁶ 3s² 3p¹, its outer electron is in a p sublevel

Mg has electronic configuration 1s² 2s² 2p⁶ 3s².

The p level is higher in energy and with Mg the s sub level is full – this gives it a slight stability advantage.

Why is there a fall from P to S?

This can be explained in terms of electron pairing.

As the p sublevel fills up, electrons fill up the vacant sub levels and are unpaired.

3p æææ

This configuration is more energetically stable than S as all the electrons are unpaired. It requires more energy to pair up the electrons in S so it has a lower Ionisation energy.

3p åææ

There is some repulsion between the paired electrons which lessens their attraction to the nucleus.

It becomes easier to remove!

Subsequent ionisation energies and electron sub-levels

The second and third ionisation energies can be represented as follows:

X⁺(g) ? X²⁺(g) + e^-

X²⁺(g) ? X³⁺(g) + e^-

This data can provide a useful guide to the number of electrons in the outer shell of an element.

The big jump after removal of the third electron occurs because the next electron must be removed from an inner shell.

The graph shows that Al has 3 electrons in its outside shell and is therefore in group III.