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This lesson covers the fundamental building blocks of chemistry — atoms, elements and compounds — as required by the AQA GCSE Combined Science Trilogy specification (8464, Chemistry section 4.1.1). Understanding these concepts is the foundation for everything you will study in chemistry and science more broadly.
An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are incredibly small — a typical atomic radius is about 1 × 10⁻¹⁰ m (0.1 nanometres).
Key facts about atoms:
Exam Tip (AQA 8464): When defining an atom in the exam, say it is "the smallest part of an element that can take part in a chemical reaction." Avoid saying atoms are indivisible — the AQA mark scheme requires acknowledgement that atoms contain sub-atomic particles.
An element is a substance that contains only one type of atom. Elements cannot be broken down into simpler substances by chemical means.
Each element is represented by a one- or two-letter chemical symbol from the periodic table:
| Element | Symbol | Type |
|---|---|---|
| Hydrogen | H | Non-metal |
| Oxygen | O | Non-metal |
| Carbon | C | Non-metal |
| Iron | Fe | Metal |
| Sodium | Na | Metal |
| Chlorine | Cl | Non-metal |
| Gold | Au | Metal |
| Copper | Cu | Metal |
Exam Tip: Case matters! "CO" is carbon monoxide (a compound), while "Co" is cobalt (an element). Always write symbols carefully.
A compound is a substance that contains two or more different elements that are chemically bonded together. The atoms in a compound are held together by chemical bonds — either ionic bonds or covalent bonds. Compounds can only be separated into their elements by chemical reactions, not by physical methods.
| Compound | Formula | Elements Present |
|---|---|---|
| Water | $\text{H}_2\text{O}$ | Hydrogen, Oxygen |
| Carbon dioxide | $\text{CO}_2$ | Carbon, Oxygen |
| Sodium chloride | $\text{NaCl}$ | Sodium, Chlorine |
| Magnesium oxide | $\text{MgO}$ | Magnesium, Oxygen |
| Iron sulfide | $\text{FeS}$ | Iron, Sulfur |
| Calcium carbonate | $\text{CaCO}_3$ | Calcium, Carbon, Oxygen |
Compounds have properties that are completely different from the elements they contain:
This demonstrates that when elements chemically combine, the product has entirely new properties.
A chemical formula shows the number and type of atoms in a molecule or unit of a compound. The small subscript number after an element symbol tells you how many atoms of that element are present:
| Formula | Meaning |
|---|---|
| $\text{H}_2$ | 2 hydrogen atoms bonded together |
| $\text{O}_2$ | 2 oxygen atoms bonded together |
| $\text{H}_2\text{O}$ | 2 hydrogen atoms and 1 oxygen atom |
| $\text{CO}_2$ | 1 carbon atom and 2 oxygen atoms |
| $\text{H}_2\text{SO}_4$ | 2 H, 1 S and 4 O atoms |
| $\text{Ca(OH)}_2$ | 1 Ca, 2 O and 2 H atoms |
When a formula contains brackets, the subscript outside multiplies everything inside:
Exam Tip: A very common mistake is forgetting to multiply — in $\text{Mg(NO}_3\text{)}_2$ there are 6 oxygen atoms (3 × 2), not 3.
graph TD
A["Matter"] --> B["Pure Substances"]
A --> C["Mixtures"]
B --> D["Elements<br/>One type of atom only"]
B --> E["Compounds<br/>Two or more elements chemically bonded"]
C --> F["Not chemically bonded<br/>Can be separated by physical methods"]
style A fill:#2c3e50,color:#fff
style B fill:#2980b9,color:#fff
style C fill:#e67e22,color:#fff
style D fill:#27ae60,color:#fff
style E fill:#8e44ad,color:#fff
style F fill:#d35400,color:#fff
Some elements exist naturally as diatomic molecules — pairs of atoms bonded together:
| Element | Formula |
|---|---|
| Hydrogen | $\text{H}_2$ |
| Nitrogen | $\text{N}_2$ |
| Oxygen | $\text{O}_2$ |
| Fluorine | $\text{F}_2$ |
| Chlorine | $\text{Cl}_2$ |
| Bromine | $\text{Br}_2$ |
| Iodine | $\text{I}_2$ |
Exam Tip: Remember them with: Have No Fear Of Ice Cold Beer. In balanced equations, always write these elements as diatomic molecules — writing "O" instead of "$\text{O}_2$" will cost marks.
Question: Ammonium nitrate has the formula $\text{NH}_4\text{NO}_3$. How many atoms of each element are present in one formula unit?
Answer:
| Mistake | Correction |
|---|---|
| Calling $\text{O}_2$ a compound | $\text{O}_2$ contains only one type of atom — it is an element |
| Writing "co" for cobalt | The correct symbol is "Co" — case matters |
| Saying atoms are "created" or "destroyed" in reactions | Atoms are rearranged in chemical reactions |
| Confusing mixtures and compounds | In a compound, atoms are chemically bonded; in a mixture, substances are just mixed together |
Exam Tip (AQA 8464): A 6-mark question may ask you to explain the differences between elements, compounds and mixtures. Use specific examples with formulae, explain that compounds are chemically bonded while mixtures are not, and note that compounds have different properties from their constituent elements.