Until the early 20th century, physicists thought that the atom was a positively charged globule with negatively charged electrons sprinkled in it. This "plum pudding" model was called thomson model as it was proposed by J. J. Thomson, who discovered the electron in 1897, was proposed in 1904 before the discovery of the atomic nucleus in order to add the electron to the atomic model. This all changed in 1909 when the Rutherford scattering experiment was done. Rutherford's scattering experiment: A stream of alpha particels from a radioactive source was fired at a very thin gold foil. A flash at the fluorescent screen is observed when the alpha particles strikes the screen. The experiment was carried out in a vaccum so this reduces the chance of collision between aplha particles and the air molecules so indeed reduces the chance of losing energy. Conclusions: * Most of fast moving alpha particles went straight through the gold foil. Therefore, the atom is mostly empy space. * Some of the alpha particles were deflected back through significant angles, this suggest that the centre of the atom must be tiny but contain a lot of mass and it was named nucleus by Ernest Rutherford. He was awarded Nobel prize for his discovery. * The alpha particels were repelled, so the nucleus must be postive charge as the charge on the alpha particle is positive so postive- postive repels. * Atoms are overall neutral so the electrons must be on the outside of the atom, separating one atom from the next. Basic structure of the atom: An atom is made up of nucleus containing protons and neutrons and the nucleus is surrounded by the cloud of electrons. The relative sizes: Atom= 10^-10 Radius of nucleus= 10^-14- 10^-15 Radius of molecule= 10^-10 - 10^-6 Definitions: Nucleon number: A nucleon is a proton or neutron and is also know as mass number. Nuclide: A nuclide is a particular combination of neutrons and protons.
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