The atom

• John Dalton thought of the atom as a solid sphere that could not be divided into smaller parts. His model did not contain any protons, neutrons or electrons and was called the billiard ball model
• J.J Thompson‘s plum pudding model was the first to include the subatomic particle electron. In his model, the atom was a ball of positive charge with negatively charged electrons embedded in it.
• In the alpha scattering experiment, scientists fired small, positively charged alpha particles at a sheet of gold foil, a few atoms thick. They expected the alpha particles to go straight through the gold, but many were deflected and bounced back. To explain why, the scientists suggested that the positive charge and mass of an atom was concentrated in a small space at its centre. They called this space the nucleus
• After the alpha scattering experiment, the plum pudding model was replaced by the nuclear model, in which electrons orbited the nucleus, but not at set distances.
• Niels Bohr calculated that the electrons must orbit the nucleus from set distances. These orbits were called shells or energy levels, and further experiments provided evidence that the nucleus had smaller particles called protons, oppositely charged to an electron
• James Chadwick discovered the neutron, and scientists concluded that protons and neutrons were found in the nucleus and electrons orbit the nucleus in shells
• The atom has a radius of 1 x 10^-10m. Nuclei are about 10,000 times smaller and have a radius of 1 x 10^-14.
• Isotopes are atoms of the same element with a different number of neutrons, meaning they have a different overall mass number
• The relative atomic mass = (abundance of isotope 1 x mass of isotope 1) + (abundance of isotope 2 x mass of isotope 2) / 100
• You can separate mixtures through:

  Filtration - insoluble solids and a liquid

  Crystallisation - soluble solid from a solution

  Simple distillation - solvent from a solution

  Fractional distillation - two liquids with similar boiling points

  Paper chromatography - identify substances from a mixture in a solution