GCSE Physics · AQA 8463 · Topic 4.4.1
Atoms and isotopes
Key points
- Atoms are about m across; the nucleus is less than 1/10 000 of that radius but holds almost all the mass.
- Electrons sit at fixed distances (energy levels). Absorbing electromagnetic radiation can move an electron to a higher level; emitting it moves an electron lower.
- Atoms have equal numbers of protons and electrons, so they have no overall charge. Losing electrons makes a positive ion.
- Atomic number = number of protons; mass number = protons + neutrons. Isotopes have the same atomic number but different mass numbers.
- Model history: plum pudding (ball of positive charge with electrons in it) → alpha scattering experiment showed a small, positive nucleus → Bohr's electron orbits → Chadwick discovered the neutron.
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An atom is mostly empty space, with a tiny, dense nucleus at its centre and electrons around it. This sub-topic covers the structure of the atom, how to describe particular atoms with numbers, and how scientists built up this model over…
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