The Periodicity Trap: When the Trend Is Not Enough

Most students learn periodicity as a set of trends:

• atomic radius decreases across a period
• first ionisation energy increases across a period
• melting points rise then fall
• electronegativity increases
• metallic character decreases

The problem is that A Level exam questions rarely ask students just to recite the trend. They ask them to explain awkward exceptions, and that is where many students lose marks.

“Periodicity looks like a topic of neat trends — until the examiner asks about the exception.”

Question

The first ionisation energies of the elements sodium to argon generally increase across Period 3.

However, aluminium has a lower first ionisation energy than magnesium, and sulfur has a lower first ionisation energy than phosphorus.

Explain these two decreases in first ionisation energy.

Your answer should refer to the electronic structures of the elements involved.

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The answer

First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.

Across Period 3, first ionisation energy generally increases because the number of protons in the nucleus increases. This means the nuclear charge increases. The electrons being removed are added to the same main energy level, so shielding does not increase very much. As a result, the outer electron is attracted more strongly to the nucleus and more energy is needed to remove it.

However, there are two important drops in the trend.

Why aluminium is lower than magnesium

Magnesium has the electronic structure:

1s² 2s² 2p⁶ 3s²

Aluminium has the electronic structure:

1s² 2s² 2p⁶ 3s² 3p¹

In magnesium, the electron being removed is from the 3s subshell.

In aluminium, the electron being removed is from the 3p subshell.

The 3p electron in aluminium is slightly higher in energy and further from the nucleus than a 3s electron. It is also slightly more shielded. This means it is easier to remove the outer electron from aluminium than from magnesium.

Therefore, aluminium has a lower first ionisation energy than magnesium, despite having more protons.

Why sulfur is lower than phosphorus

Phosphorus has the electronic structure:

1s² 2s² 2p⁶ 3s² 3p³

Sulfur has the electronic structure:

1s² 2s² 2p⁶ 3s² 3p⁴

In phosphorus, the three 3p electrons occupy separate p orbitals. This is relatively stable because the electrons are unpaired and experience less repulsion.

In sulfur, the fourth 3p electron has to pair up with another electron in one of the 3p orbitals. This creates extra electron-electron repulsion within the same orbital.

Because of this repulsion, it is easier to remove one of the paired 3p electrons from sulfur.

Therefore, sulfur has a lower first ionisation energy than phosphorus.

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Why students find this difficult

This is a hard question because students often write:

“Ionisation energy increases because nuclear charge increases.”

That is true for the general trend, but it does not explain the exceptions.

To get the marks, students need to mention:

• the subshell change from 3s to 3p between magnesium and aluminium
• the higher energy of the 3p electron
• the paired electron repulsion in sulfur
• the relative stability of phosphorus with three unpaired 3p electrons

This makes the question excellent for a blog because it shows students that chemistry is not just about memorising trends. It is about explaining the evidence using electronic structure.