Structure 3 - Classification of matter
Structure 3 organizes matter and reveals patterns. The exam tests classification of substances and mixtures, separation and purity, periodic table position and trends, and organic families and isomerism.
Classifying and separating
- Elements contain one type of atom; compounds contain chemically combined elements; mixtures contain substances not chemically combined.
- Homogeneous mixtures have uniform composition; heterogeneous mixtures do not.
- Filtration separates insoluble solid from liquid; distillation separates by boiling point; chromatography separates by differing attraction to stationary and mobile phases, using Rf = distance moved by spot / distance moved by solvent.
- A pure substance has a sharp melting point; impurities lower and broaden it.
- Empirical formula is the simplest ratio; molecular formula is the actual number of atoms.
Periodic patterns
- Atomic radius decreases across a period because nuclear charge rises while shielding stays similar; it increases down a group as shells are added.
- First ionisation energy increases across a period and decreases down a group, for the same reasons.
- Electronegativity follows the same trend. Metallic character decreases across a period.
Organic families
- Homologous series share a general formula and functional group; naming uses the longest chain and suffix.
- Isomers have the same molecular formula but different structures: chain, position, functional group and cis–trans.
Worked example
Explain why Mg has a higher first ionisation energy than Al. Mg is 1s²2s²2p⁶3s²; Al is 1s²2s²2p⁶3s²3p¹. Al's outer electron is in a higher-energy 3p orbital and is shielded by the 3s² pair, so it is removed more easily.
Traps
- Confusing empirical and molecular formulae.
- Quoting ionisation energy without specifying gaseous atoms.
- Reversing periodic trends.