S3 Classification of matter

Guiding question

Official titleClassification of matter

Editorial framingHow do the periodic table and the functional-group system help us predict the properties of matter we have not yet encountered?

The one big idea

Classification turns raw data into predictive power. The periodic table organises elements by electron configuration; functional groups organise organic compounds by reactivity. Both let you predict properties you have never seen.

What this rests on

Our editorial judgement.The dependencies in this section are this site’s own assessment of what a sub-topic rests on. They have notbeen verified by a chemistry teacher and are not part of the official IB guide. By contrast, section 8 (Linking questions) reproduces the IB’s own wording verbatim.

Prerequisites are recorded at the sub-topic level only. The list below aggregates the prerequisites of this topic’s sub-topics.

EditorialS1.2 The nuclear atomAtomic number determines an element's position in the periodic table (linking question S1.2.1). (via S3.1)
EditorialS1.2 The nuclear atomHL onlyMass spectra determine relative atomic mass from isotopic composition (HL S1.2.3); MS fragmentation reappears in organic analysis (HL S3.2.8). (via S3.2)
EditorialS1.3 Electron configurationsElectron configuration is the explanation; the periodic table is the pattern being explained (linking questions S1.3.3, S1.3.4). (via S3.1)
EditorialS1.4 Counting particles by mass: The moleAtoms increase in mass as their position descends in the periodic table; Ar and the mole relate to periodic trends (linking question S1.4.2). (via S3.1)
EditorialS2.2 The covalent modelOrganic structure, formulas and IMF-driven physical trends are covalent-model applications. (via S3.2)
EditorialS3.1 The periodic table: Classification of elementsuncertainA weaker edge linking the 'classification' theme across Structure 3. (via S3.2)

What rests on this

EditorialS2.3 The metallic model
EditorialS3.2 Functional groups: Classification of organic compounds
EditorialR2.2 How fast? The rate of chemical change
EditorialR3.1 Proton transfer reactions
EditorialR3.2 Electron transfer reactions
EditorialR3.4 Electron-pair sharing reactions
EditorialS2.4 From models to materials
EditorialR1.3 Energy from fuels
EditorialR2.1 How much? The amount of chemical change
EditorialR3.2 Electron transfer reactions
EditorialR3.3 Electron sharing reactions
EditorialR3.4 Electron-pair sharing reactions

Sub-topics in this topic

Core concepts that must be mastered

Structure 3 has only two sub-topics, but they carry 31 hours at HL — more than any other Structure topic. Both are classification schemes: the periodic table classifies elements, functional groups classify organic compounds. The shared idea is that organisation reveals patterns, and patterns let you predict.

The periodic table — classification of elements(3.1.1–3.1.6)The periodic table consists of periods, groups and blocks (s, p, d, f) (3.1.1). Period number equals the outer occupied energy level; group members share valence electron count, which is why you can deduce electron configuration up to Z = 36 from position and vice versa (3.1.2). You should know the group names — alkali metals, halogens, transition elements, noble gases — and identify metals, metalloids and non-metals. Periodicity — trends across a period and down a group — is the predictive heart of the table (3.1.3). Atomic radius decreases across a period (more protons pull the same shell tighter) and increases down a group (extra shells). Ionic radius follows the same patterns, with cations smaller and anions larger than their parent atoms. Ionisation energy generally increases across a period (stronger nuclear charge) and decreases down a group (outer electrons further away and more shielded). Electron affinity and electronegativity track similar trends. Down-group trends show increasing metallic character in group 1 and decreasing non-metallic character in group 17, demonstrated by reactions of group 1 metals with water and group 17 elements with halide ions (3.1.4). Metallic and non-metallic properties are themselves a continuum: basic metal oxides give way to amphoteric oxides and then to acidic non-metal oxides across a period (3.1.5). You must be able to write equations for reactions of group 1 and 2 metal oxides with water, and for non-metal oxides like CO₂ and SO₂ (including acid rain and ocean acidification). Oxidation state — the charge an atom would have if the compound were ionic — is the bookkeeping tool that connects Structure 3 to redox in Reactivity 3.2 (3.1.6). The sign-then-numeral notation (+2, −1) is used throughout; hydrogen is −1 in metal hydrides and oxygen is −1 in peroxides.
Transition elements and ionisation-energy discontinuities (HL)(3.1.7–3.1.10)At HL, discontinuities in first ionisation energy across a period provide evidence for the existence of energy sublevels (3.1.7) — the explanation must be based on the energy of the electron removed, not on "special stability" of filled or half-filled sublevels. Transition elements have incomplete d-sublevels giving variable oxidation states, high melting points, magnetic properties, catalytic properties, coloured compounds and complex-ion formation with ligands (3.1.8). Variable oxidation states arise because successive ionisation energies are close in value (3.1.9). Complex colour arises when light is absorbed as electrons are promoted between split d-sublevels, and the colour observed is complementary to the colour absorbed — you apply the colour wheel and c = λf to deduce wavelengths (3.1.10).
Functional groups — classification of organic compounds(3.2.1–3.2.7)Organic compounds can be represented by empirical, molecular, structural (condensed and full), stereochemical and skeletal formulas (3.2.1). You must interconvert molecular, skeletal and structural formulas fluently. A functional group gives characteristic physical and chemical properties, and compounds are classified by it — halogeno, hydroxyl, carbonyl, carboxyl, alkoxy, amino, amido, ester, phenyl (3.2.2). A homologous series differs by a common structural unit, typically CH₂, and shares a general formula (3.2.3): alkanes, alkenes, alkynes, halogenoalkanes, alcohols, aldehydes, ketones, carboxylic acids, ethers, amines, amides, esters. Successive members show trends in melting and boiling points from increasing London forces (3.2.4). IUPAC nomenclature names compounds systematically for up to six carbons in the parent chain, covering straight- and branched-chain isomers (3.2.5). Structural isomers — same molecular formula, different connectivity — include branched, straight-chain, position and functional-group isomers, including primary, secondary and tertiary alcohols, halogenoalkanes and amines (3.2.6). At HL, stereoisomerism enters: cis–trans isomerism in non-cyclic alkenes and C3/C4 cycloalkanes, and chirality — a tetrahedral carbon with four different groups giving non-superimposable mirror images called enantiomers with optical activity (3.2.7). A racemic mixture contains equal amounts of both enantiomers. Wedge–dash representations show the 3-D arrangement. The E–Z system is not assessed.
The HL analytical toolkit(3.2.8–3.2.12)The analytical toolkit lets you identify unknown compounds. Mass spectrometry causes fragmentation, and you can deduce structural features from fragmentation patterns and the molecular ion peak (3.2.8). IR spectroscopy identifies bond types from characteristic wavenumbers (cm⁻¹) in the functional group region, using a table from the data booklet (3.2.9) — greenhouse gases absorb IR, which is why CO₂ and CH₄ trap heat. ¹H NMR reveals different chemical environments of hydrogen atoms through the number of signals, chemical shifts and relative areas (integration traces) (3.2.10), with splitting into singlets, doublets, triplets and quartets providing further structural detail (3.2.11). Combining data from all techniques — MS for molar mass, IR for functional groups, NMR for the hydrogen environment — lets you determine a full molecular structure (3.2.12).
How this topic connectsStructure 3 is the bridge from structure to reactivity. The periodic table (3.1) encodes the electron configurations of Structure 1.3 and predicts the bonding types of Structure 2. The functional-group system (3.2) names the reactive sites that every mechanism in Reactivity 3 targets. Oxidation state (3.1.6) is the bookkeeping tool for redox (Reactivity 3.2), and the HL analytical toolkit — MS, IR, NMR — is how you identify the products of those reactions. The linking questions in §8 make these connections explicit: they are the IB's own edges between classification and reactivity.

Quantitative non-negotiables

Deduce electron configuration up to Z = 36 from position in the periodic table and vice versa.
Explain periodic trends in atomic radius, ionic radius, ionisation energy, electron affinity and electronegativity.
Assign oxidation states in ions and compounds (sign-then-Arabic-numeral, e.g. +2, −1).
Apply IUPAC nomenclature to saturated or mono-unsaturated compounds with up to six carbons in the parent chain.
Interconvert molecular, structural, condensed and skeletal formulas.

Common failure modes

M-04 — Ionisation energy increases smoothly across a period.
Confidence: verified
Why it’s wrong: There are dips at group 2→13 and group 15→16.
Correction: The group 13 dip is because the outer electron is in a higher-energy p orbital; the group 16 dip is because of repulsion between the first pair of paired p electrons.
M-07 — The atom gets bigger across a period because you're adding electrons.
Confidence: verified
Why it’s wrong: Atomic radius decreases across a period.
Correction: Electrons are added to the same main energy level while nuclear charge increases, so effective nuclear charge rises and the electron cloud is pulled in.
M-62 — Oxidation state is the same as ionic charge.
Confidence: verified
Why it’s wrong: They coincide for monatomic ions but not otherwise.
Correction: Oxidation state is a formal bookkeeping value assigned by electronegativity even in purely covalent species (S in SO₄²⁻ is +6, though there's no S⁶⁺ ion).
M-71 — Any carbon with four different-looking groups is chiral.
Confidence: verified
Why it’s wrong: The four groups must be genuinely different as whole substituents, not just different at the first atom.
Correction: Check the entire branch.
M-72 — Structural isomers have the same properties.
Confidence: verified
Why it’s wrong: Different connectivity gives different physical and often different chemical properties.
Correction: E.g. branched alkanes have lower boiling points because branching reduces surface contact and thus London forces.
M-76 — The molecular ion peak in a mass spectrum is the tallest peak.
Confidence: verified
Why it’s wrong: The tallest is the base peak (most stable fragment).
Correction: The molecular ion is the highest-m/z peak (ignoring isotope peaks).
M-77 — In ¹H NMR, the number of peaks equals the number of hydrogens.
Confidence: verified
Why it’s wrong: The number of signals equals the number of distinct chemical environments.
Correction: The integration (relative area) gives the ratio of hydrogens in each environment; the splitting (n+1) gives the number of hydrogens on adjacent carbons.

What "HL standard" actually looks like

HL adds ionisation-energy discontinuities as evidence for sublevels, transition-element properties (variable oxidation states, coloured complexes, d-orbital splitting), and the full analytical toolkit: stereoisomerism (cis–trans, chirality), mass-spectrometry fragmentation, IR spectroscopy and ¹H NMR (chemical shift, integration, splitting patterns).

See the command terms reference →

Linking questions

Official IB.The questions in this section are the IB’s own linking questions, reproduced verbatim from the guide. They are not this site’s editorial judgement — see section 3 for that distinction.

Linking questions are recorded at the sub-topic level only. The list below aggregates the linking questions of this topic’s sub-topics.

Official IB
How does the atomic number relate to the position of an element in the periodic table?
Structure 1.2.1 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
How does the fragmentation pattern of a compound in the mass spectrometer help in the determination of its structure?
Structure 1.2.3 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
How does an element's highest main energy level relate to its period number in the periodic table?
Structure 1.3.3 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
What is the relationship between energy sublevels and the block nature of the periodic table?
Structure 1.3.4 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
How does the trend in IE values across a period and down a group explain the trends in properties of metals and non-metals?
Structure 1.3.6 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
How do patterns of successive IEs of transition elements help to explain the variable oxidation states of these elements?
Structure 1.3.7 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
Atoms increase in mass as their position descends in the periodic table. What properties might be related to this trend?
Structure 1.4.2 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
What is the importance of approximation in the determination of an empirical formula?
Structure 1.4.4 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
How does the position of an element in the periodic table relate to the charge of its ion(s)?
Structure 2.1.1 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
How can lattice enthalpies and the bonding continuum explain the trend in melting points of metal chlorides across period 3?
Structure 2.1.3 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
What features of a molecule make it "infrared (IR) active"?
Structure 2.2.6 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
Why are silicon–silicon bonds generally weaker than carbon–carbon bonds?
Structure 2.2.7 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
To what extent does a functional group determine the nature of the intermolecular forces?
Structure 2.2.9 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
How does the ability of some atoms to expand their octet relate to their position in the periodic table?
Structure 2.2.13 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
What are the different assumptions made in the calculation of formal charge and of oxidation states for atoms in a species?
Structure 2.2.14 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
What experimental data demonstrate the physical properties of metals, and trends in these properties, in the periodic table?
Structure 2.3.1 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IBwording unverified against the guide
Why is the trend in melting points of metals across a period less evident across the d-block?
Structure 2.3.3 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
How do the trends in properties of period 3 oxides reflect the trend in their bonding?
Structure 2.4.1 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
What are the structural features of some plastics that make them biodegradable?
Structure 2.4.4 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
What functional groups in molecules can enable them to act as monomers for addition reactions?
Structure 2.4.5 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
What functional groups in molecules can enable them to act as monomers for condensation reactions?
Structure 2.4.6 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
How has the organization of elements in the periodic table facilitated the discovery of new elements?
Structure 3.1.2 → S1.2 The nuclear atom
(via S3.1)
Official IB
How do differences in bonding explain the differences in the properties of metal and non-metal oxides?
Structure 3.1.5 → S2.1 The ionic model
(via S3.1)
Official IB
How can oxidation states be used to analyse redox reactions?
Structure 3.1.6 → R3.2 Electron transfer reactions
(via S3.1)
Official IB
What are the arguments for and against including scandium as a transition element?
Structure 3.1.8 → S2.3 The metallic model
(via S3.1)
Official IB
What is the nature of the reaction between transition element ions and ligands in forming complex ions?
Structure 3.1.10 → R3.4 Electron-pair sharing reactions
(via S3.1)
Official IB
What is unique about carbon that enables it to form more compounds than the sum of all the other elements' compounds?
Structure 3.2.1 → S2.2 The covalent model
(via S3.2)
Official IB
What are the advantages and disadvantages of different depictions of an organic compound (e.g. structural formula, stereochemical formula, skeletal formula, 3D models)?
Structure 3.2.1 → S2.2 The covalent model
(via S3.2)
Official IB
What is the nature of the reaction that occurs when two amino acids form a dipeptide?
Structure 3.2.2 → S2.4 From models to materials
(via S3.2)
Official IB
How can functional group reactivity be used to determine a reaction pathway between compounds, e.g. converting ethene into ethanoic acid?
Structure 3.2.2 → R3.2 Electron transfer reactions
(via S3.2)
Official IB
What is the influence of the carbon chain length, branching and the nature of the functional groups on intermolecular forces?
Structure 3.2.4 → S2.2 The covalent model
(via S3.2)
Official IB
How does the fact that there are only 3 isomers of dibromobenzene support the current model of benzene's structure?
Structure 3.2.6 → S2.2 The covalent model
(via S3.2)
Official IB
What features of a molecule determine whether it is IR active or not?
Structure 3.2.9 → S2.2 The covalent model
(via S3.2)
Official IB
What properties of a greenhouse gas determine its "global warming potential"?
Structure 3.2.9 → R1.3 Energy from fuels
(via S3.2)
Official IB
Why do larger hydrocarbons have a greater tendency to undergo incomplete combustion?
Reactivity 1.3.3 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
Why is carbon dioxide described as a greenhouse gas?
Reactivity 1.3.3 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
What are the features of transition elements that make them useful as catalysts?
Reactivity 2.2.5 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
What is the periodic trend in the acid–base properties of metal and non-metal oxides?
Reactivity 3.1.3 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
Why does the release of oxides of nitrogen and sulfur into the atmosphere cause acid rain?
Reactivity 3.1.3 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
What are the advantages and limitations of using oxidation states to track redox changes?
Reactivity 3.2.1 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
Why does metal reactivity increase, and non-metal reactivity decrease, down the main groups of the periodic table?
Reactivity 3.2.3 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
How does the nature of the functional group in a molecule affect its physical properties, such as boiling point?
Reactivity 3.2.9 → S3.2 Functional groups: Classification of organic compounds
(via S3.2)
Official IB
Why is there a colour change when an alcohol is oxidized by a transition element compound?
Reactivity 3.2.9 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IBwording unverified against the guide
How can oxidation states be used to show that the following molecules are given in increasing order of oxidation: CH₄, CH₃OH, HCHO, HCOOH, CO₂?
Reactivity 3.2.10 → S3.1 The periodic table: Classification of elements
(via S3.1)
Official IB
Why is the iodide ion a better leaving group than the chloride ion?
Reactivity 3.4.10 → S3.1 The periodic table: Classification of elements
(via S3.1)

3 further official linking questions target Tool, Inquiry or Nature of Science strands and are not drawn as edges on the map:

Editorial The decision to surface these off-graph questions here is our editorial judgement — the IB does not prescribe where they should appear.

Official IB
Why are simulations often used in exploring the trends in chemical reactivity of group 1 and group 17 elements?
Structure 3.1.4 → Inquiry
(via S3.1)
Official IB
How can colorimetry or spectrophotometry be used to calculate the concentration of a solution of coloured ions?
Structure 3.1.10 → Tools 1
(via S3.1)
Official IB
How useful are 3D models (real or virtual) to visualize the invisible?
Structure 3.2.3 → Nature of Science
(via S3.2)

Bridge: GCSE → IB HL

Derived. These are the bridge items tagged to this topic— places where the GCSE model gets redefined, genuinely new territory, or carry-over strengths. The tagging is this site’s editorial judgement.

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Notes

Open questions

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