Atoms, Elements and CompoundsAQA GCSE Chemistry: Revision notes
Section 1
What is an atom, and how do we represent it?
An atom is the smallest part of an element that can exist. Every substance in the universe is made up of atoms.
Each element has its own chemical symbol — a one or two-letter code (e.g. Na for sodium, Cl for chlorine, O for oxygen). You need to know the names and symbols of the first 20 elements, plus the elements in Groups 1 and 7, and the other elements named in this specification.
Atoms of each element are represented by their chemical symbol in formulae and equations. Compounds can be named directly from a given formula, e.g. NaCl is sodium chloride, CO2 is carbon dioxide.
Capital letter first, then a lower-case second letter if there is one — Co is cobalt, but CO is carbon monoxide.
Section 2
How are elements, compounds and mixtures different?
- An element contains only one type of atom.
- A compound forms when two or more elements are chemically combined — new bonds form, and the properties are completely different from the original elements.
- A mixture contains two or more elements or compounds that are not chemically combined — each substance keeps its own properties.
Compounds can only be separated back into their elements by a chemical reaction — never by physical means.
A mixture of iron and sulfur can be separated with a magnet; once heated to form iron sulfide (a compound), a magnet no longer works — that is the key distinguishing test.
Section 3
Naming compounds from formulae
A chemical formula shows the elements present and the ratio of atoms of each. For example:
| Formula | Name |
|---|---|
| H2O | Water |
| CO2 | Carbon dioxide |
| NaCl | Sodium chloride |
| MgO | Magnesium oxide |
You should also be able to write word equations and balanced symbol equations for reactions in the specification, using the correct symbols and formulae for every element and compound involved.
Word equation: magnesium + oxygen → magnesium oxide. Symbol equation: 2Mg + O2 → 2MgO.
Section 4
How can mixtures be separated?
Because the substances in a mixture are not chemically bonded, they can be separated using physical methods based on their different physical properties:
- Filtration — separates an insoluble solid from a liquid
- Crystallisation — separates a soluble solid from a solution by evaporating the solvent
- Simple distillation — separates a solvent from a solution
- Fractional distillation — separates a mixture of liquids with different boiling points
- Chromatography — separates substances based on how they distribute between a stationary phase and a mobile phase
Must Know
- An atom is the smallest part of an element that can exist
- Elements contain one type of atom; compounds are elements chemically combined; mixtures are not chemically combined
- Compounds can only be separated into elements by a chemical reaction
- Learn the names and symbols of the first 20 elements plus Groups 1 and 7
- Choose the right separation technique: filtration, crystallisation, simple distillation, fractional distillation or chromatography
That's the notes covered.
Carry on to the next subtopic.