C2.10654 · Section C2: Atoms, elements and compounds

Elements, compounds and mixtures

Key learnings

Element — only one type of atom
An element contains only one type of atom — every atom in a pure sample has the same proton (atomic) number — so it cannot be split into anything chemically simpler. Just over 100 elements appear on the Periodic Table, each with a one- or two-letter symbol. Some exist as single atoms (the noble gases, e.g. argon $\text{Ar}$ ); some as molecules of two identical atoms ( $\text{O}_2$ , $\text{N}_2$ ). Both are still elements — only one type of atom is present.
Compound — chemically combined, fixed ratio
A compound is two or more elements chemically combined in a fixed proportion. Both clauses are markable: new chemical bonds form (not merely placed together), and every sample has the same composition. Because a genuinely new substance exists, its properties differ from its elements — reactive sodium plus toxic chlorine give edible sodium chloride, $\text{NaCl}$ . Only a chemical reaction (electrolysis, strong heating) recovers the elements.
Mixture — not combined, any proportion
A mixture is two or more substances — elements, compounds or both — that are not chemically combined. So the ratio is variable, each component keeps its own properties, and a physical method separates it. In an iron/sulfur mixture the iron is still magnetic and still gives hydrogen with dilute acid, while the sulfur stays unreacted. Examples: air; sea water (water plus dissolved salts); crude oil (a mixture of many hydrocarbon compounds).

Common mistakes

Drawn from real examiner reports.

Compound ≠ elements simply joined

Writing that a compound is elements "joined", "put together" or "mixed" scores nothing — that description fits a mixture equally well. The word chemically (or an equivalent, such as "by chemical bonds" or "a chemical reaction has happened") must appear. It is the most heavily penalised omission in this definition.

Jun 2022 P31 Q11b; Jun 2023 P11 Q15

A compound is not "a mixture of elements"

Calling a compound "a mixture of elements" is a common wrong-form answer that contradicts itself: a mixture is by definition not chemically combined, so the phrase describes the opposite of a compound. If the substance has one fixed formula and properties unlike its elements, call it a compound and say the elements are chemically combined.

Jun 2022 P31 Q11b; Jun 2023 P11 Q15

Fixed proportion vs any proportion

The second markable clause of the compound definition is routinely dropped. A compound has a fixed ratio of its elements — every sample of $\text{FeS}$ is one iron to one sulfur. A mixture has no fixed ratio; the experimenter chooses how much of each to weigh out. Omitting "fixed proportion" loses the mark that separates the two.

Mixtures can contain compounds too

Because the standard example is iron filings plus sulfur powder — two elements — candidates assume a mixture must be elements only. A mixture is any substances not chemically combined with each other, and those substances may be elements, compounds or both. Air is $\text{N}_2$ and $\text{O}_2$ plus $\text{CO}_2$ and water vapour; sea water is water plus dissolved salts.

Separation methods the wrong way round

A mixture is not chemically combined, so a physical method separates it — a magnet, filtration, evaporation, distillation. A compound is chemically combined, so only a chemical reaction (electrolysis, or strong heating) recovers its elements. Answers reverse this, or offer "use a magnet" for a compound. Ask first: has a chemical reaction happened?

Alloys such as brass are mixtures

An alloy is a mixture of a metal with other elements or metals — not a compound, and not "metallic bonding". Candidates classify brass as a compound because it looks like one uniform substance, but copper and zinc are not chemically combined and the proportions can be varied to change the alloy's properties.

Jun 2022 P31 Q5ci; Jun 2023 P31 Q1di, P32 Q1cii

Oxygen gas is an element, not a compound

A molecule made of two joined atoms is not automatically a compound. Oxygen gas, $\text{O}_2$ , and nitrogen gas, $\text{N}_2$ , each contain only one type of atom, so they are elements even though their particles are molecules rather than single atoms. A compound needs two or more different elements chemically combined, as in $\text{CO}_2$ .

Exam tips

"Describe the differences" means compare
Work through four contrasts for each pair: particles present, chemically combined or not, fixed or variable proportion, and separation method. Link the halves with "whereas" or "but" — two isolated definitions written side by side leave the difference itself unmarkable.
Match the number of points to the marks
A two-mark definition wants two separate clauses, not one idea restated twice. A compound needs "chemically combined" and "fixed proportion"; a mixture needs "not chemically combined" and "no fixed proportion". Count your distinct points against the tariff.
Answer the command word actually set
"State" wants the classification alone (element, compound or mixture). "Describe" wants the features. "Explain" wants the reason — usually that the substances are, or are not, chemically combined. A scientifically true answer to a different question scores nothing.
Particle-diagram rules examiners check
An element: all particles identical. A compound: every particle the same joined group of different circles, with no free single atoms left over. A mixture: unjoined different particles in no fixed ratio. Keep each element's circles the same size in every box of the diagram.

Revision notes