GCSE Chemistry · AQA 8462 · Topic 4.1.1
A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes
Key points
- An element contains only one type of atom; a compound has two or more elements chemically combined; a mixture has substances that are not chemically joined and can be separated physically.
- Separation methods: filtration (insoluble solid from a liquid), crystallisation (soluble solid from a solution), simple distillation (solvent from a solution), fractional distillation (liquids with different boiling points) and chromatography.
- Rutherford's alpha scattering experiment showed most of the atom's mass is in a small, positive nucleus, replacing the plum pudding model. Bohr added electrons in fixed shells; Chadwick later found the neutron.
- Relative charges: proton +1, neutron 0, electron −1. Relative masses: proton 1, neutron 1, electron very small. Atoms have a radius of about m.
- Relative atomic mass is an average that takes account of the abundance of each isotope, e.g. chlorine is 35.5.
- Electrons fill shells in order: 2, then 8, then 8 (for the first 20 elements), e.g. sodium is 2,8,1.
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This sub-topic gives you the vocabulary and the model of the atom that the rest of the course depends on. You need to know what atoms are made of, how scientists worked this out, and how to separate mixtures of substances.
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