GCSE Physics · Edexcel 1PH0 · Topic 6A
Atomic structure
Key points
- An atom is about m across; the nucleus is about 100 000 times smaller but contains almost all the mass.
- Protons (charge +1, mass 1), neutrons (charge 0, mass 1), electrons (charge −1, very small mass).
- Atomic number = number of protons; mass number = protons + neutrons. Atoms are neutral because protons = electrons.
- Isotopes of an element have the same number of protons but different numbers of neutrons.
- Electrons sit in energy levels; absorbing EM radiation moves them up, and they emit radiation when they drop back. Losing an electron makes a positive ion.
- The model developed from Dalton's solid spheres to Thomson's plum pudding, then Rutherford's nuclear model (alpha scattering) and Bohr's energy levels.
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Atoms are made of a tiny, dense nucleus surrounded by electrons. Knowing what is in the nucleus explains isotopes and radioactivity, and the story of how we found out is a classic example of how science works.
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