Periodicity in ionisation energy and physical properties, the reactions and trends of Group 2 (the alkaline earth metals) and Group 7 (the halogens), and the chemical tests that identify them.

What this topic is asking

Edexcel Topic 4 wants you to describe periodicity across a period, explain the trends and reactions of Group 2 (the alkaline earth metals) and Group 7 (the halogens), and carry out and explain the tests used to identify Group 2 and halide ions.

The answer

Periodicity

Across Period 3, first ionisation energy rises overall (increasing nuclear charge with similar shielding) and atomic radius falls. Melting points rise to a maximum at the giant covalent structure (silicon) and then fall sharply at the simple molecular non-metals. The small dips in ionisation energy (aluminium below magnesium, sulfur below phosphorus) are evidence for sub-shell structure and electron pairing.

Group 2 trends

Hydroxide solubility increases down the group (so the solutions become more alkaline), while sulfate solubility decreases down the group ($BaSO_4$ is insoluble, the basis of the test for sulfate ions).

Group 7 trends

Down Group 7 the halogens become less reactive as oxidising agents because the atoms are larger and the incoming electron is more shielded, so electrons are gained less easily. A more reactive halogen displaces a less reactive halide from solution, for example $Cl_2 + 2KBr \rightarrow 2KCl + Br_2$.

Tests for halide ions

The sample must first be acidified with dilute nitric acid (not hydrochloric, which would add chloride ions) to remove carbonate or hydroxide ions that would also precipitate with silver ions.

Examples in context

Example 1. Barium meals in medicine. A patient swallows a suspension of barium sulfate before an X-ray of the gut. Even though barium ions are toxic, $\text{BaSO}_4$ is safe to use because it is essentially insoluble, so almost no $\text{Ba}^{2+}$ enters the bloodstream. Its insolubility is the Group 2 sulfate trend (solubility decreasing down the group) put to medical use, and the same insolubility is why barium chloride solution is the classic test for sulfate ions.

Example 2. Chlorinating drinking water. Chlorine is added to water supplies because it is a strong enough oxidising agent to kill bacteria. Its strength as an oxidising agent (greater than bromine or iodine) follows the Group 7 trend explained above. The displacement reactions of the halogens, used in the laboratory to rank oxidising power, are the same chemistry that makes chlorine effective for disinfection while the less reactive iodine is used only as a milder antiseptic.

Try this

Q1. Explain why calcium is more reactive with water than magnesium. [2 marks]

• Cue. Calcium's outer electrons are further from the nucleus and more shielded, so its ionisation energies are lower and electrons are lost more easily.

Q2. Describe how you would distinguish between solutions of sodium chloride and sodium bromide. [3 marks]

• Cue. Add dilute nitric acid then silver nitrate; chloride gives a white precipitate that dissolves in dilute ammonia, bromide gives a cream precipitate that dissolves only in concentrated ammonia.