Quick questions on The periodic table - WJEC A-Level Chemistry

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).

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.

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.

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.

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

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

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

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

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