Periodicity of atomic radius, ionisation energy, melting point and electronegativity across Period 3, and trends in Group 2 and Group 7 chemistry.

What this dot point is asking

WJEC wants you to describe and explain periodic trends in atomic radius, ionisation energy, melting point and electronegativity across Period 3, and the characteristic reactivity trends in Group 2 (the alkaline earth metals) and Group 7 (the halogens).

The answer

Periodicity across Period 3

The ionisation energy trend has two dips: aluminium (outer electron in a higher-energy $3p$ orbital) and sulfur (paired $3p$ electrons repel, easing removal).

Group 2: the alkaline earth metals

Their hydroxides become more soluble down the group, while their sulfates become less soluble (barium sulfate is the insoluble white precipitate used to test for sulfate ions).

Group 7: the halogens

Down Group 7 ($\text{F}_2$ to $\text{I}_2$), the elements get less reactive as oxidising agents because the larger atoms gain an electron less readily. Boiling points rise down the group as molecules have more electrons and stronger London forces. A more reactive halogen displaces a less reactive one from solution.

The two dips in the Period 3 ionisation trend

Although first ionisation energy rises across Period 3, there are two dips that examiners love to test. The first, at aluminium, occurs because aluminium's outer electron is in a $3p$ orbital, slightly higher in energy than magnesium's $3s$ electron, so it is easier to remove. The second, at sulfur, occurs because sulfur's $3p$ sub-shell has a pair of electrons in one orbital; the repulsion between this pair makes one electron easier to remove than the corresponding electron in phosphorus, whose $3p$ electrons are unpaired. Explaining these dips in terms of sub-shell energy and electron pairing is a frequent extended-answer task.

Thermal stability and solubility trends in Group 2

Down Group 2, the carbonates and nitrates become more thermally stable, because the larger cations polarise the anion less, so it decomposes less readily. The hydroxides become more soluble down the group, while the sulfates become less soluble (barium sulfate is essentially insoluble, the basis of the sulfate test with acidified barium chloride). Linking these trends to ion size gives a coherent picture of how Group 2 chemistry changes from magnesium to barium.

Examples in context

Barium sulfate as a contrast medium. Because $\text{BaSO}_4$ is insoluble and opaque to X-rays, patients drink a barium meal for digestive imaging, a direct use of the Group 2 solubility trend. Chlorination of water. Chlorine's strength as an oxidising agent (high in Group 7) makes it effective at killing pathogens in drinking water, a public-health application of halogen reactivity.

Try this

Q1. State the trend in atomic radius across Period 3. [1 mark]

• Cue. It decreases from sodium to argon.

Q2. Explain why iodine has a higher boiling point than chlorine. [2 marks]

• Cue. Iodine molecules have more electrons, so stronger London forces require more energy to overcome.

Q3. Write the equation for the reaction of barium with water. [1 mark]

• Cue. $\text{Ba} + 2\text{H}_2\text{O} \rightarrow \text{Ba}(\text{OH})_2 + \text{H}_2$.

Q4. Explain why sulfur has a lower first ionisation energy than phosphorus. [2 marks]

• Cue. Sulfur's $3p$ sub-shell has a paired electron whose repulsion makes it easier to remove; phosphorus's $3p$ electrons are all unpaired.

Q5. State the trend in solubility of the Group 2 sulfates down the group. [1 mark]

• Cue. Solubility decreases down the group (barium sulfate is essentially insoluble).