Periodicity & the groups

Period 3 trends, Group 2, Group 7

Period 3 trends

• Atomic radius decreases (more protons, same shell). Melting points climb Na→Al (stronger metallic bonding), peak at Si (giant covalent), collapse for P₄, S₈, Cl₂, Ar (simple molecular — S₈ > P₄ > Cl₂ by electron count).

Group 2 (alkaline earths)

• Down the group: reactivity with water increases; hydroxide solubility increases (Mg(OH)₂ insoluble → Ba(OH)₂ soluble); sulfate solubility decreases (BaSO₄ insoluble — basis of the sulfate test with acidified BaCl₂).

Group 7 (halogens)

• Oxidising power decreases down the group: Cl₂ displaces Br⁻ and I⁻; Br₂ displaces only I⁻.
• Halide test: acidified silver nitrate → AgCl white (dissolves in dilute NH₃), AgBr cream (dissolves in conc. NH₃), AgI yellow (insoluble in NH₃).
• Reducing power of halides increases down the group — with conc. H₂SO₄, Cl⁻ gives HCl only; Br⁻ also gives SO₂; I⁻ gives H₂S, S and I₂.

3.2.1Period 3 metals with water and oxygen

• Na + cold water: vigorous, floats and fizzes → NaOH(aq) + H₂ (solution pH ~13–14).
• Mg + cold water: extremely slow → Mg(OH)₂; Mg + steam: fast, bright white flame → MgO + H₂.
• All period 3 elements (except Ar, and S needing ignition) burn in O₂ to their oxides: Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂.

3.2.2Group 2 — the applications examiners name

• Mg(OH)₂ — antacid ("milk of magnesia"); Ca(OH)₂ — neutralising acidic soil; CaO/CaCO₃ — removing SO₂ from flue gases (wet scrubbing).
• BaSO₄ — "barium meal" X-ray contrast (safe because virtually insoluble); acidified BaCl₂ — the test for sulfate ions (white precipitate).
• Mg reduces TiCl₄ to titanium in the extraction of Ti (TiCl₄ + 2Mg → Ti + 2MgCl₂).