The big idea: Chemistry sorts every substance into three kinds. The difference is all about the particles — what they are and whether they are chemically bonded.
- Element — only one type of atom. - Compound — different atoms chemically bonded in a fixed ratio. - Mixture — two or more substances physically together but not bonded.
Element = one kind of atom · Compound = different atoms bonded in a fixed ratio · Mixture = substances physically together, not bonded.
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Bonded or just mixed?: The key question is always: are the atoms chemically bonded?
- Bonded in a fixed ratio → compound (a new substance with its own properties). - Just mixed, in any ratio → mixture (each part keeps its own properties).
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An element cannot be broken down into anything simpler by chemical reactions — it contains only one kind of atom. A compound is two or more elements chemically bonded in a fixed ratio, and it behaves like a brand-new substance.
An element has identical atoms; a compound has different atoms chemically bonded together.
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A compound is not its elements: Sodium (Na) is a soft, reactive metal and chlorine (Cl2) is a poisonous green gas — but the compound sodium chloride (NaCl) is safe table salt. Bonding changes the properties completely.
| Element | Compound | Mixture | |
|---|---|---|---|
| Particles | one type of atom | two or more different atoms bonded | two or more substances, not bonded |
| Composition | fixed | fixed ratio | variable |
| Separated by | cannot be broken down | chemical means only | physical means |
| Properties | its own | different from its elements | those of its components |
| Example | neon (Ne), iron (Fe) | water (H2O), NaCl | air, sea water, brass |
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A mixture is two or more substances physically combined in any ratio. Because nothing is bonded, the parts keep their own properties and can be separated by physical methods.
Homogeneous mixture
- Uniform throughout — you cannot see the separate parts.
- Examples: a salt solution, air.
Heterogeneous mixture
- Not uniform — the parts are visible and unevenly spread.
- Examples: sand and iron filings, oil and water.
Purity and melting point: A pure substance (a single element or compound) has a sharp, fixed melting and boiling point.
A mixture melts and boils over a range of temperatures — and impurities lower a melting point. This is how a sharp melting point is used as a test of purity.
How this is tested: S1.1 appears as a quick Paper 1A classification MCQ ('which is a compound / mixture?') and a short Paper 2 describe or distinguish question.
The classic Paper 2 ask is 'describe how an element differs from a compound' — for both marks you must mention the particle difference and that a compound is chemically bonded (in a fixed ratio).
Two easy marks: Don't just say 'a compound has more than one element'. Say it is chemically bonded and in a fixed ratio — that's the marking point students miss.
IB-style question — element vs compound
Describe two ways in which a compound differs from an element. [2]
How to score both marks
- Mark 1 — the particles. An element contains only one type of atom, whereas a compound contains two or more different types of atom.
- Mark 2 — the bonding/ratio. In a compound the atoms are chemically bonded together in a fixed ratio; this also means a compound can only be separated by chemical means, not physical ones.
Final answer
(1) an element has one type of atom, a compound has two or more different atoms; (2) a compound's atoms are chemically bonded in a fixed ratio (so its properties differ from the elements).