Structure of the Atom

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The three fundamental sub-atomic particles are: 1. Electrons (e⁻) - negatively charged, negligible mass 2. Protons (p⁺) - positively charged, mass = 1 unit 3. Neutrons (n) - no charge, mass = 1 unit

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J.J. Thomson discovered the electron. He was awarded the Nobel Prize in Physics in 1906 for his work on the discovery of electrons.

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Thomson's model (Christmas pudding model): - Atom consists of a positively charged sphere - Electrons are embedded in this positive sphere like currants in a pudding - Negative and positive charges ar

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Key observations: 1. Most α-particles passed straight through the gold foil 2. Some α-particles were deflected by small angles 3. Very few (1 in 12,000) α-particles rebounded (deflected by 180°) This

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Rutherford's conclusions: 1. Most of the atom is empty space 2. Positive charge occupies very little space 3. All positive charge and mass are concentrated in a tiny nucleus 4. Nucleus radius is 10⁵ t

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The major drawback was that revolving electrons should radiate energy continuously and spiral into the nucleus, making atoms unstable. However, atoms are actually stable, so this model could not expla

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Bohr's postulates: 1. Only certain special orbits (discrete orbits) are allowed for electrons 2. Electrons do not radiate energy while revolving in these discrete orbits 3. These orbits are called ene

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J. Chadwick discovered neutrons in 1932. Neutrons have no charge and mass nearly equal to protons. They are present in the nucleus of all atoms except hydrogen.