Topic 3: Structure 3: Classification of matter

Chemistry makes sense of millions of substances by sorting them into patterns. This theme covers the periodic table and the trends it predicts, and the systematic ways chemists classify and name organic compounds. At HL it extends to coloured transition metal complexes and the spectroscopic tools used to identify unknown molecules.

The periodic table and classification of elements

The periodic table arranges elements in order of increasing atomic number. Vertical columns are groups, horizontal rows are periods. Elements in the same group share the same number of valence (outer-shell) electrons, which is why they behave chemically alike: Group 1 metals all form +1 ions, Group 17 halogens all form -1 ions. The period number tells you how many occupied electron shells an atom has. Broadly, metals sit on the left and centre, non-metals on the upper right, and a diagonal band of metalloids (such as silicon and germanium) lies between them with intermediate properties. Blocks (s, p, d, f) name the sub-shell that the highest-energy electron occupies, so the table also maps directly onto electron configuration.

Periodicity and periodic trends

Periodicity means properties repeat in a regular pattern as you move through the table. Atomic radius decreases across a period because the growing nuclear charge pulls the same shell of electrons inward, but increases down a group as extra shells are added. First ionisation energy (the energy to remove the most loosely held electron) rises across a period and falls down a group, mirroring how tightly the nucleus holds the outer electron. Electronegativity, an atom's pull on a shared bonding pair, increases across and decreases down, peaking at fluorine. Metallic character does the opposite of electronegativity. Down Group 1 reactivity increases (electrons are lost more easily), while down Group 17 reactivity decreases. Oxides also trend from basic on the left, through amphoteric, to acidic on the right.

(HL) Transition elements and coloured complexes

Transition elements are d-block metals that form at least one stable ion with a partially filled d sub-shell. This gives them characteristic behaviour: variable oxidation states (iron as +2 or +3), catalytic activity, and the formation of complex ions in which ligands (electron-pair donors such as water or ammonia) bond to a central metal. Their compounds and solutions are often coloured. Colour arises because ligands split the five d orbitals into two energy levels; an electron can absorb a photon of visible light to jump the gap, and the colour we see is the complement of the light absorbed. The size of the split, and therefore the colour, depends on the metal, its oxidation state, and the identity and number of ligands. Note scandium and zinc are excluded because their common ions are d0 and d10 (full), so they are colourless and not classed as typical transition elements.

Functional groups and classifying organic compounds

Organic compounds are carbon-based, and their reactions are governed by functional groups: specific atoms or bonds that confer characteristic chemistry. The hydrocarbon skeleton is relatively unreactive, so a molecule's behaviour is predicted mostly from its functional groups. Key examples include the carbon-carbon double bond (alkenes), the hydroxyl group -OH (alcohols), the carbonyl C=O (aldehydes and ketones), the carboxyl -COOH (carboxylic acids), the amino group -NH2 (amines), and the halogeno group (halogenoalkanes). Classifying a compound by its functional group lets you anticipate its physical properties and reaction types without memorising every individual molecule.

Homologous series

A homologous series is a family of compounds with the same functional group and general formula, where each successive member differs by a CH2 unit. The alkanes (general formula CnH2n+2) are the simplest example, followed by alkenes (CnH2n) and alcohols (CnH2n+1OH). Members of a series share similar chemical properties because they have the same functional group, while their physical properties change gradually: as chain length and molar mass increase, boiling point rises (more surface area for London dispersion forces) and volatility falls. This predictable gradation is one of the most useful organising ideas in organic chemistry, letting you estimate the properties of an untested member from its neighbours.

IUPAC nomenclature

IUPAC naming gives every organic molecule one unambiguous systematic name. Find the longest continuous carbon chain to set the stem (meth-, eth-, prop-, but-, pent- for one to five carbons), then add a suffix for the principal functional group (-ane, -ene, -ol, -al, -one, -oic acid). Number the chain from whichever end gives the substituents the lowest possible locants, and list branches as prefixes (methyl, ethyl) in alphabetical order with their position numbers. For example, CH3CH(OH)CH3 is propan-2-ol: a three-carbon chain with a hydroxyl group on the second carbon. Multiple identical branches use di-, tri-, tetra-. Careful numbering and locants are where most marks are won or lost.

Structural isomers

Structural isomers are compounds that share the same molecular formula but have different arrangements of atoms, and therefore different structures and often different properties. C4H10 exists as straight-chain butane and branched methylpropane (chain isomerism). C3H8O can be propan-1-ol, propan-2-ol, or methoxyethane, differing in where a group sits (positional isomerism) or in functional group entirely (functional group isomerism). Because structure determines properties, isomers can have markedly different boiling points and reactivity despite identical formulae. Recognising isomers is essential when interpreting spectra of an unknown, since the formula alone never fixes the structure.

(HL) Spectroscopic identification: mass spectrometry and degree of unsaturation

Mass spectrometry ionises a molecule and breaks it into fragments. The molecular ion peak (M+) at the highest mass gives the relative molecular mass, while fragment peaks reveal pieces of the structure; a loss of 15 suggests a methyl group (CH3), a loss of 17 a hydroxyl, and a loss of 29 an ethyl or CHO group. The index of hydrogen deficiency, or degree of unsaturation, counts rings and pi bonds in a molecule and is calculated from the molecular formula. For CnHmNpOq the value is (2n + 2 + p - m) divided by 2; each unit signals one ring or one double bond, and a value of four often hints at a benzene ring. This number is a quick filter that narrows down possible structures before detailed analysis.

(HL) Spectroscopic identification: IR and 1H NMR

Infrared (IR) spectroscopy probes bond vibrations: a substance absorbs IR frequencies that match its bonds, so characteristic absorptions act as fingerprints. A broad band near 3200-3600 wavenumbers indicates O-H, a sharp strong band near 1700 indicates C=O, and these let you confirm which functional groups are present. Proton (1H) NMR examines hydrogen environments. The number of signals equals the number of chemically distinct hydrogen environments; the chemical shift (position) reveals the type of environment; and the relative area under each peak, given by integration, equals the ratio of hydrogens in each environment. Combining mass spectrometry (mass and fragments), IR (functional groups), and 1H NMR (hydrogen environments) usually pins down the structure of an unknown organic compound with confidence.