The periodic table: periodicity, groups and transition elements

Periodicity (Period 3)

• Atomic radius decreases across the period (increasing nuclear charge, same shielding).
• Melting point rises to silicon (giant covalent), then falls for simple molecular P₄, S₈, Cl₂ and Ar.
• Oxides change from basic (Na₂O, MgO) through amphoteric (Al₂O₃) to acidic (SiO₂, P₄O₁₀, SO₂, SO₃).

Group 2

Reactivity increases down the group. Hydroxides become more soluble and sulfates less soluble down the group; carbonates and nitrates become more thermally stable down the group (larger cations polarise the anion less).

Group 17 (halogens)

Oxidising power decreases down the group. Halide ions: reducing power increases down the group (I⁻ reduces sulfuric acid to H₂S). Tests: silver nitrate gives AgCl (white, soluble in dilute NH₃), AgBr (cream, soluble in concentrated NH₃), AgI (yellow, insoluble). Chlorine disproportionates in NaOH (bleach) and is used to treat water.

Transition elements

d-block elements forming at least one stable ion with a partly filled d sub-shell.

Properties: variable oxidation states, coloured ions (d–d transitions when ligands split the d orbitals), catalytic activity, and complex ion formation.

• Ligands donate lone pairs (dative bonds): H₂O, NH₃, Cl⁻, CN⁻, EDTA⁴⁻ (hexadentate).
• Shapes: octahedral ([Cu(H₂O)₆]²⁺), tetrahedral ([CuCl₄]²⁻), square planar (cisplatin).
• Ligand exchange changes colour: [Cu(H₂O)₆]²⁺ (blue) → [Cu(NH₃)₄(H₂O)₂]²⁺ (deep blue) with excess ammonia.

Check yourself

1. Why does atomic radius decrease across Period 3?
2. Describe the test for bromide ions.
3. Give three characteristic properties of transition elements.