OCR Chemistry 1.1.1 Atoms & Reactions

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  • OCR AS Chemistry 1.1.1 Atoms
    • History of the Atom
      • Ancient Greeks
        • all matter is made up of tiny particles (atoms)
        • if you were to repeatedly divide matter, you would reach a point where you could not divide any further
      • John Dalton 1808
        • atoms in any given element are all of the same kind
        • compound contains two or more elements combined in fixed proportions
        • during chemical reactions, atoms in the reactants rearrange to form reactants in the products
      • J.J Thompson 1897
        • found the electron
        • knew that atom is neutral, so part is - and part is +
          • plum pudding model
            • negatively charged electrons embedded in a large positively charged sphere
      • Ernest Rutherford 1909-1920
        • fired  positive alpha particles at gold foil
          • they surprisingly went straight through and some bounced back
            • found the proton
            • new model of atom with a very small positively charged nucleus in its centre
        • proton did not account for the whole mass of the nucleus (not accepted by scientists)
      • Henry Moseley 1914
        • determined charges on nuclei
        • atomic number is equal to the number of protons in the nucleus
          • this was used to reorganize Periodic table based in atomic number instead of atomic mass
    • Sub-Atomic Particles
      • proton mass=1 charge=+1
      • neutron mass=1 charge=0
      • electron mass=1/2000 charge=-1
      • behaviour in an electric field
        • neutron unaffected (no charge)
        • proton (+charge) attracted towards negative plate
        • electron (-charge) attracted towards positive plate
          • electron deflected more prominently than proton as it has a much lower mass
    • Definitions
      • atomic number = number of protons in the nucleus of an atom
      • mass number = the combined number of protons and neutrons in the nucleus of an atom
      • isotopes = atoms of the same element which have the same number of protons and electrons but different numbers of neutrons
        • isotopes have the same chemical properties as they have same number of electrons
        • isotopes have different physical properties because they have different masses
      • relative isotopic mass = the mass of one isotope of an element relative to one twelfth of an atom of Carbon 12
      • relative atomic mass = the weighted mean mass of an atom relative to one twelfth of an atom of Carbon 12
        • to calculate do the mass number x %abundance for each isotope. Add answers together. Divide by abundances of all isotopes
    • Mr or RFM?
      • for covalent molecules (only non metals) Relative Molecular Mass (Mr)
      • both calculated by adding up individual relative atomic masses of atoms
      • for ionic compounds (metals and non metals and ammonium compounds) Relative Formula Mass (RFM)
        • because they do not exist as molecules, formula represents ratio of positive to negative ions
    • Ionic Equations
      • focus only on ions that don't react together (spectator ions)
      • 1. rewrite equation showing individual ions present in ionic compounds only
        • 2. cancel out anything that is the same on both sides - must be identical including state

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