Periodicity examines patterns across periods and down groups — linking electron configuration to physical and chemical properties of elements and their compounds.
Across Period 3: atomic radius decreases (more protons, same shells). First ionisation energy generally increases (exceptions at Al and S). Electronegativity increases. Metallic character decreases (Na, Mg, Al are metals; Si metalloid; P, S, Cl non-metals). Melting points: Na/Mg/Al increase (metallic bonding strengthens), Si very high (giant covalent), P₄/S₈/Cl₂ low (simple molecular, depends on molecular size).
Group 2 (alkaline earth metals): reactivity increases down the group (easier to lose 2 electrons). React with water: Mg slowly, Ca vigorously. Hydroxides become more soluble down the group. Sulfates become less soluble. Group 17 (halogens): oxidising power decreases down group (harder to gain electron). Reactivity decreases: F₂ > Cl₂ > Br₂ > I₂. Halide tests: add AgNO₃ → white AgCl, cream AgBr, yellow AgI. Acid reactions: HCl → no redox. HBr → some oxidised by conc. H₂SO₄. HI → strongly reduced.
D-block elements with partially filled d orbitals in at least one oxidation state. Properties: variable oxidation states, coloured compounds (d-d transitions), catalytic activity, complex ion formation. Complex ions: central metal ion surrounded by ligands (donate lone pairs → coordinate/dative bonds). Coordination number: number of coordinate bonds. Shapes: octahedral (6), tetrahedral (4), square planar (4), linear (2). Colour: ligand field splitting of d orbitals → absorption of specific wavelength → complementary colour seen.
Transition metals are effective catalysts because: (1) They have variable oxidation states, allowing them to donate and accept electrons during reactions, facilitating electron transfer. (2) They provide d orbitals that can form temporary bonds with reactant molecules, lowering activation energy. (3) In heterogeneous catalysis, they have surface sites where reactants can adsorb, orient correctly, and react (e.g., iron in Haber process, V₂O₅ in Contact process). (4) In homogeneous catalysis, they can change oxidation state to act as intermediaries (e.g., Fe²⁺/Fe³⁺ in the reaction between S₂O₈²⁻ and I⁻).
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