Chemistry Module 4-Atoms

Chemistry Module 4-Atoms

1. Background

   • Atom-a very small part of matter
     • nucleus contains positive protons and neutral neutrons and is positively charged
       • very heavy
         • electrons revolve on the shells
           • no mass,negatively charged
         • the whole mass of an atom is in the nucleus
     • their mass is about 10^-23g
       • their radius is about 10^-10m
     • mass number is the total no. of protons and neutrons
     • atomic number is the number of protons
       • no. of protons is equal to no. of electrons
         • to get the number of neutrons, subtract the atomic number from the mass number
   • isotopes
     • has the same atomic number but different mass number
       • we change the different number of neutrons

2. The history of the atom

   • John Dalton described atoms as solid spheres and that different spheres made up the different elements
     • One correct theory was that all matter is made of atoms
       • In 1897 J J Thomson showed that this was wrong and that an atom contained smaller pieces
         • discovered the electron
           • This was known as the plum pudding model
             • In 1909 Rutherford showed that the plum pudding model was wrong
               • he used an experiment to show that an atom must contain a central nucleus
                 • with his assistants Geiger and Marsden
                   • in 1905 they tested the atom imagined as a sphere of positive charge with negatively charged electrons dotted around insided
                     • Beam of alpha particles was aimed at very thin gold foil
                       • they expected the alpha particles to pass straight through
                         • However, some emerged from the foil at different angles, some even deflected straight back and some passed through
                           • they realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom
                             • They concluded, most of the atom is an empty space
                               • some particles were repelled by the nucleus
                                 • most of the atoms mass is concentrated in the central nucleus
                                   • nucleus surround by a 'cloud' of negative electrons
                                     • plum pudding replaced with nuclear model of atom
             • Bohrs electron orbits
               • he used experimental evidence
                 • to support the idea that electrons occupy particular orbits or shells around the nucleus
                   • therefore electrons in a 'cloud' would be attracted to nucleus
                     • but the atom would collapse?
                       • Therefore came up with a new model
                         • electrons only exist in fixed orbits or shells and not anywhere inbetween
                           • Tested by scientists
                             • helped to explain other observations and we know is now true
                             • more experiments, new evidence was found to fit the theory
                               • new evidence means new ideas
                                 • make predictions which if proved correct are an indication that ideas are right
                                   • Scientists put up their ideas for peer review
                                     • other scientists can test and help develop
                                 • scientific knowledge
                           • each shell has a fixed energy

3. Ions

   • ions are formed when atoms loose or gain electrons
     • metal atoms form positive ions
       • non-metal atoms form negative ions
         • by gaining electrons
         • group 6 gains 2 electrons
           • group 7 gains 1 electron
       • by losing electrons
       • all of atoms in group 1 will lose 1 electron
         • group 2 will lose 2 electrons
           • group 3 will lose 3 electrons

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