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How Atomic Models Evolved
Dalton
Atom is a solid, indivisible particle
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Thomson
Electrons embedded in a positive sphere (plum pudding)
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Rutherford
Tiny dense nucleus; electrons revolve around it
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Bohr
Electrons in fixed energy levels → atom is stable
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Modern model — electrons as “electron clouds”
- Atoms
- The building blocks of matter.
- Thomson’s model
- Electrons embedded in a positively charged sphere.
- Rutherford’s model
- Mostly empty space with a dense, positive nucleus at the centre and electrons orbiting.
- Bohr’s model
- Electrons move in fixed energy levels (shells) around the nucleus.
- Shells
- Named K, L, M, N… from the nucleus outward.
- Subatomic particles
- Electrons (−1), protons (+1) and neutrons (0); neutron found by Chadwick.
- Octet
- A full outermost shell of 8 electrons (2 for helium) → stable, unreactive.
- Valency
- The combining capacity — electrons gained, lost or shared for a stable configuration.
- Atomic number (Z)
- Number of protons in the nucleus.
- Mass number (A)
- Total nucleons = protons + neutrons.
- Isotopes
- Same atomic number, different mass numbers.
- Average atomic mass
- Based on the relative abundance of the isotopes in nature.
- Isobars
- Different elements with the same mass number but different atomic numbers.
