First ionisation energies of Period 3 elements

What is here?

First ionisation energy generally increases going across Period 3. However, it needs more detailed consideration than the trend in Group 2 because:

• The first ionisation energy drops between magnesium and aluminium before increasing again.
• The first ionisation energy drops between phosphorus and sulphur before increasing again.

To explain these dips, you need to think about the electronic configurations of these elements.

You can:

• view a table of first ionisation energies
• view a graph of first ionisation energies
• learn about why the first ionisation energy increases going across Period 3

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Table of physical data

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Graph of physical data

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Explanation of this trend

General increase across the period

The first ionisation energy is the enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge. It is an endothermic process, i.e. is positive.
A general equation for this enthalpy change is:

Going across Period 3:

• 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, and ...
• there is a negligible increase in shielding because each successive electron enters the same energy level ...
• so more energy is needed to remove the outer electron.

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Magnesium to aluminium

Look at their electronic configurations:

Magnesium: 1s² 2s² 2p⁶ 3s² ... and ... aluminium: 1s² 2s² 2p⁶ 3s² 3p¹

The outer electron in aluminium is in a p sub-level. This is higher in energy than the outer electron in magnesium, which is in an s sub-level, so less energy is needed to remove it.

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Phosphorus to sulphur

Look at their electronic configurations:

Phosphorus: 1s² 2s² 2p⁶ 3s² 3p³ ... and ... sulphur: 1s² 2s² 2p⁶ 3s² 3p⁴

It's not immediately obvious what's going on until we look at the arrangements of the electrons:

The 3p electrons in phosphorus are all unpaired. In sulphur, two of the 3p electrons are paired. There is some repulsion between paired electrons in the same sub-level. This reduces the force of their attraction to the nucleus, so less energy is needed to remove one of these paired electrons than is needed to remove an unpaired electron from phosphorus.

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