Elements & Compounds
Understand the difference between elements and compounds, learn to read chemical symbols and formulas, and explore everyday examples from around Australia.
Pure Substances: Elements and Compounds
All matter can be classified as either a pure substance or a mixture. Pure substances have a definite, fixed composition. They include elements and compounds.
Element
One type of atom only
Element
One type of atom only
Compound
Two or more elements chemically bonded
Australian Curriculum Connection
This lesson aligns with AC9S7U03: "All matter is made of atoms; elements are organised on the periodic table; compounds are formed when atoms of different elements join together."
Elements
An element is a pure substance made of only one type of atom. It cannot be broken down into simpler substances by ordinary chemical methods. Elements are represented by chemical symbols — one or two letters, with the first always capitalised.
| Element Name | Symbol | State (room temp) | Australian Relevance |
|---|---|---|---|
| Oxygen | O | Gas | Essential for breathing; makes up 21% of air |
| Iron | Fe | Solid (metal) | Mined in Pilbara, WA — Australia's top export |
| Calcium | Ca | Solid (metal) | In bones, teeth, coral (Great Barrier Reef) |
| Nitrogen | N | Gas | Makes up 78% of Earth's atmosphere |
| Carbon | C | Solid | In all living things; also as coal, diamonds, graphite |
Note: Some symbols come from Latin names — Fe (ferrum = iron), Au (aurum = gold), Na (natrium = sodium). That is why they do not match the English name!
Compounds
A compound is a pure substance made when atoms of two or more different elements are chemically bonded together in fixed ratios. Compounds have different properties from the elements they contain. They are represented by chemical formulas.
Water — H₂O
2 hydrogen atoms bonded to 1 oxygen atom.
Hydrogen is a flammable gas. Oxygen supports burning. Yet water puts fires out — a completely different property!
Carbon Dioxide — CO₂
1 carbon atom bonded to 2 oxygen atoms.
Produced by burning and respiration. Used by plants in photosynthesis. Greenhouse gas causing warming of Australia.
Salt (Sodium Chloride) — NaCl
1 sodium atom bonded to 1 chlorine atom.
Sodium is an explosive metal; chlorine is a toxic gas. Together, they form safe table salt and seawater.
Glucose — C₆H₁₂O₆
6 carbon + 12 hydrogen + 6 oxygen atoms.
Made by plants during photosynthesis. Used by all living things for energy in cellular respiration.
Reading a Chemical Formula
6 carbon atoms
12 hydrogen atoms
6 oxygen atoms
Key Vocabulary
Element
A pure substance made of only one type of atom; cannot be broken down chemically. Represented by a symbol (e.g. O, Fe, C).
Compound
A pure substance formed when two or more different elements are chemically bonded together in fixed ratios (e.g. H₂O, NaCl).
Chemical Formula
A shorthand way to show the types and numbers of atoms in a compound (e.g. H₂O means 2 hydrogen and 1 oxygen).
Mixture
A combination of two or more substances that are NOT chemically bonded — they can be separated by physical means (e.g. air, seawater, soil).
Worked Examples
How many atoms of each element are in a molecule of sulfuric acid (H₂SO₄)?
Read each symbol and subscript:
H₂ = 2 hydrogen atoms
S = 1 sulfur atom (no subscript = 1)
O₄ = 4 oxygen atoms
Answer: H₂SO₄ contains 2 hydrogen, 1 sulfur, and 4 oxygen atoms. Total = 7 atoms per molecule.
Is seawater an element, compound, or mixture? Explain.
Step 1: Seawater contains water (H₂O), dissolved salts (NaCl, MgCl₂), dissolved gases (O₂, CO₂), and minerals — multiple substances.
Step 2: These substances are not chemically bonded — they can be separated by evaporation (leaving salt crystals).
Answer: Seawater is a mixture. It contains many substances that are dissolved but not chemically bonded together.
Explain why water (H₂O) has different properties from hydrogen (H₂) and oxygen (O₂).
Hydrogen is a highly flammable gas; oxygen supports and intensifies combustion.
Water is a liquid at room temperature, is not flammable, and is used to put out many fires.
Reason: When elements combine chemically to form a compound, the atoms bond in a new arrangement. The new compound has entirely different properties from the original elements because the atoms interact with each other differently.
Knowledge Check
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Key Concepts Summary
- ✓ An element is a pure substance made of one type of atom; represented by a chemical symbol (e.g. O, Fe, Au).
- ✓ A compound is formed when two or more elements chemically bond; represented by a chemical formula (e.g. H₂O, NaCl).
- ✓ Compounds have different properties to the elements they are made from.
- ✓ The subscript number in a formula tells you how many atoms of that element are present (e.g. H₂ = 2 hydrogen).
- ✓ Mixtures (seawater, air, soil) are NOT compounds — the substances are not chemically bonded and can be separated.