the atom

• Atom


  Smallest part of an element that can exist
• Dalton's model of the atom


  Atoms as solid spheres that could not be divided into smaller parts
• Plum pudding model of the atom


  Sphere of positive charge with negative electrons embedded in it
• Alpha scattering experiment


  • Some alpha particles were deflected by the gold foil, showing that an atom's mass and positive charge must be concentrated in one small space (the nucleus)
• Nuclear model of the atom


  Dense nucleus with electrons orbiting it
• Niels Bohr's discovery


  Electrons orbit in fixed energy levels (shells)
• James Chadwick's discovery


  Uncharged particle called the neutron
• Protons and neutrons are in the nucleus, and the electrons orbit the nucleus in shells
• Relative mass of sub-atomic particles


  • Proton: 1
  • Neutron: 1
  • Electron: 0 (very small)
• Relative charge of sub-atomic particles


  • Proton: +1
  • Neutron: 0
  • Electron: -1
• How to find the number of protons in an atom


  The atomic number on the Periodic Table
• How to calculate the number of neutrons in an atom

  Mass number - Atomic number
• Atoms have no overall charge because they have equal numbers of positive protons and negative electrons
• Number of electrons in the first, second, and third shells


  • Up to 2 in the first shell
  • Up to 8 in the second and third shells
• Element


  Substance made of one type of atom
• Compound


  Substance made of more than one type of atom chemically joined together
• Mixture


  Two or more substances not chemically combined
• Isotopes


  Atoms of the same element (same number of protons) with different numbers of neutrons
• Physical processes to separate mixtures


  • Filtration
  • Crystallisation
  • Distillation
  • Fractional distillation
  • Chromatography
• Relative mass


  The average mass of all the atoms of an element
• Food colourings are often used for the chromatography practical, but any coloured mixture could be used in an exam question
• Chromatography practical


  1. Separate coloured substances by making paper chromatograms

  2. Describe the method of chromatography, including the solutes and solvents involved

  3. Define the stationary and mobile phases

  4. Calculate R, values: distance moved by solute / distance moved by solvent
• Student A wanted to leave the chromatography experiment until the solvent front had moved three-quarters of the way up the paper


  Student B wanted to leave it for 15 minutes
• Pencil does not interact with the mobile phase, and therefore will not interfere with the chromatogram
• atomic number


  number of protons in the nucleus of an atom
• electron


  stable subatomic particle with charge of negative electricity
• energy level


  specific energies that electrons can have
• isotope


  variation of element occupying specific orbitals