Atomic Structure

Atomic Structure

1. What did Rutherford do?

   • 1. In 1804, John Dalton agreed with Democritus that matter was made up of tiny spheres (atoms) that couldn't be broken up, but he believed that each element was made up of a different type of 'atom'.
     • 2. JJ Thompson- discovered electrons could be removed from atoms. Dalton's Theory = not quite right. Thompson thus suggested that atoms were spheres of positive charge with tiny negative electrons stuck in them like plums in a plum pudding
       • 3. In 1909 Rutherford and Marsden tried firing a beam of alpha particles at thin gold foil- expected that the positively charged alpha particles would be slightly deflected by the electrons in the plum pudding model.
         • 4. However, most of the alpha particles went straight through but the odd one came straight back at them.
           • 5. Rutherford and Marsden realised this meant that MOST OF THE MASS of the atom was concentrated at the centre in a tiny NUCLEUS- must have a POSITIVE CHARGE, since it repelled some of the positive alpha particles.

2. Rutherford and Marsden thus came up with the nuclear model of the atom

   • 1. The nucleus is tiny but takes up most of the MASS of the atom
     • 2. It contains protons and neutrons- overall POSITIVE charge
       • 3. Rest of the atom is mostly empty space.
         • 4. The negative electrons travel around the outside of the atom, give the atom its overall SIZE

3. Radioactivity and Background Radiation

   • What are isotopes?
     • 1. Isotopes are different forms of the same element
       • 2.Isotopes are atoms with the same number of protons but a different number of neutrons
         • 3. Hence they have the same atomic number, but different mass numbers
           • 4. Atomic number is the NUMBER OF PROTONS in an atom
             • 5. Mass number is the NUMBER OF PROTONS + NUMBER OF NEUTRONS
               • 6. Most elements have different isotopes, but there's usually only 1 or 2 stable ones
                 • 7. The other isotopes tend to be RADIOACTIVE, which means they decay into other elements and give out radiation
   • Radioactivity is a totally random process
     • Give out radiation from their nuclei
       • Give out either alpha, beta or gamma radiation
   • Background radiation
     • What is it?
       • Background radiation is radiation that is present at all times- from unstable isotopes, in the air, in food, in building materials, in rocks
         • Radiation from space- cosmic rays from the Sun
           • Radiation from man-made resources e.g. nuclear waste