Atomic Structure and the Periodic Table
Elements, Compounds and Mixtures
AQA GCSE Chemistry
Elements, compounds and mixtures
Reading a particle diagram
| What the box shows | Substance |
|---|---|
| one kind of atom only, as single atoms or as molecules | element |
| identical particles, each made of two or more kinds of atom bonded together | compound |
| particles of two different elements, not bonded to each other | mixture of elements |
| particles of two different compounds, not bonded to each other | mixture of compounds |
| particles of an element and particles of a compound, not bonded to each other | mixture of an element and a compound |
A box holding one element only is a pure element; one compound only is a pure compound.
Reading a description
| Description | Substance |
|---|---|
| only one type of atom, does not conduct electricity | non-metallic element |
| two or more types of atom chemically combined in fixed proportions | compound |
| an alloy | mixture |
What a mixture is
A compound always has the same percentage of each element; a mixture contains a variable percentage of each substance in it.
Symbols, formulae and names
- A name means the name: carbon, not the symbol C.
- A subscript counts only the atom before it. No subscript means one atom.
- Oxygen and fluorine are made of molecules of two atoms: O2, F2.
| Substance | Formula |
|---|---|
| water | H2O |
| hydrogen peroxide | H2O2 |
| copper sulfate | CuSO4 |
Naming a compound from its formula
- Metal first, then the non-metal with its ending changed to -ide.
- OH is hydroxide.
| Formula | Name |
|---|---|
| MgBr2 | magnesium bromide |
| ZnBr2 | zinc bromide |
| NaOH | sodium hydroxide |
Chemical reactions and word equations
- A chemical reaction makes a new substance. Signs of a reaction: a flame, a colour change, a temperature change or bubbles.
- A word equation is the names of the reactants, an arrow, then the names of the products. Names only, never formulae.
- Write only substances. Never write energy as a product.
| Reactants | Word equation |
|---|---|
| potassium and sulfur | potassium + sulfur → potassium sulfide |
| hydrogen and oxygen | hydrogen + oxygen → water |
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