Evolution of the Atomic Model

George Dalton's Atomic Theory (1807)

• George Dalton was the first person to propose an atomic theory. His model stated:
• All matter is made up of tiny hard spheres known as atoms
• Atoms cannot be created or destroyed
• Atoms cannot be broken down into smaller particles
• All the atoms in an element are identical

J.J. Thomson's Discovery of the Electron (1897)

• Performed many experiments involving applying high voltages across vacuum tubes - which causes the tube to glow
• These vacuum tubes are known as cathode ray tubes
• Thomson measured the mass of the particles in these rays
• Found that they were much lightest than the smallest atom and therefore must consist of sub-atomic particles
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Rutherford's Discovery of Nucleus (1909)

• Before Rutherford's experiments: The atom was considered using the Thomson's Plum Pudding Model - a sphere of positive charge with electrons distributed randomly in it
• Rutherford's experiments: Involved bombarding gold foil with positively charged particles known as alpha particles
• What the atomic model at the time would have predicted: Most of the alpha particles would be deflected straight back
• What actually happened: Most of the alpha particles passed straight through, some were slightly deflected and a very small number bounced straight back
• Rutherford's conclusions: The atom must be mostly empty space, with a positive central nucleus that contains most of the mass
• This is sometimes known as the Geiger and Marsden experiment as it was carried out by two of Rutherford's assistant with those names.
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Chadwick's Discovery of the Neutron (1932)

• James Chadwick discovered the neutron in 1932
• This led to the discovery of isotopes and thus nuclear energy
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Nuclear energy

• By firing neutrons at U-235, a nucleus can be split in a process called nuclear fission
• Produces new elements and transfers large amounts of energy
• This energy is used to produce electricity in nuclear power plants
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