Periodicity & the groups

Period 3 trends, Group 2, Group 7

Period 3 trends

Across Period 3 the atomic radius decreases: there are more protons pulling on electrons in the same shell. Melting points climb from Na to Al as metallic bonding strengthens, peak at Si (giant covalent), then collapse for the simple molecular substances P₄, S₈, Cl₂ and Ar — ranked S₈ > P₄ > Cl₂ by electron count.

Group 2 (alkaline earths)

Down the group, reactivity with water increases. Hydroxide solubility also increases — Mg(OH)₂ is insoluble, Ba(OH)₂ soluble — but sulfate solubility decreases. BaSO₄ is insoluble, which is the basis of the sulfate test with acidified BaCl₂.

Group 7 (halogens)

Oxidising power decreases down the group: Cl₂ displaces both Br⁻ and I⁻, while Br₂ displaces only I⁻. Running the other way, the reducing power of the halide ions increases down the group — with conc. H₂SO₄, Cl⁻ gives HCl only, Br⁻ also gives SO₂, and I⁻ gives H₂S, S and I₂.

The halide test uses acidified silver nitrate. AgCl is white and dissolves in dilute NH₃; AgBr is cream and dissolves in conc. NH₃; AgI is yellow and insoluble in NH₃.

3.2.1Period 3 metals with water and oxygen

Sodium reacts vigorously with cold water, floating and fizzing to give NaOH(aq) + H₂ (solution pH ~13–14). Magnesium with cold water is extremely slow, giving Mg(OH)₂ — but with steam it is fast, burning with a bright white flame to give MgO + H₂.

The period 3 elements Na to S burn in O₂ to their oxides — Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂ — with S needing ignition; Cl₂ and Ar do not react with oxygen directly.

3.2.2Group 2 — the applications examiners name

Mg(OH)₂ is the antacid "milk of magnesia"; Ca(OH)₂ neutralises acidic soil; CaO/CaCO₃ removes SO₂ from flue gases by wet scrubbing. BaSO₄ is the "barium meal" X-ray contrast agent, safe because it is virtually insoluble, and acidified BaCl₂ is the test for sulfate ions, giving a white precipitate.

One more to bank: Mg reduces TiCl₄ to titanium in the extraction of Ti (TiCl₄ + 2Mg → Ti + 2MgCl₂).