Atomic structure

Atomic structure

1. Plum Pudding model

   • It was previously believed atoms were the smallest particles that existed
   • In 1804 Dalton agreed with Democritus, that matter was made up of atoms which couldn't be broken up but he thought each element was made of a different type of atom
     • In 1897 J.J Thompson discovered tiny, negatively charged particles(electrons) and that atoms can give out electrons so Dalton's theory was slightly wrong as atoms could be broken up
       • Thompson suggested atoms were spheres of positive charge with tiny negative electrons stuck in them - like plums in a plum pudding
         • However the 'plum pudding' theory didn't last long as in 1909 Rutherford and Marsden fired a beam of alpha particles at thin gold foil, a slight deflection of the positive alpha particles was expected to be caused by the electrons in the 'plum pudding' model
           • However - most alpha particles just went straight through with the odd one coming straight back at them, which was a bit of a shocker
             • Rutherford and Marsden realized this meant most of an atoms mass was in its centre in a tiny nucleus they also realized the nucleus must have a large positive charge since positive alpha particles were repelled at such large angles
               • The experiment also showed that most of an atom is just empty space which was also shocking as atoms make up everything
                 • In short Rutherford concluded: Most an atom is empty space, Most of an atoms mass is compressed into a tiny central volume called a nucleus and that an atoms nucleus has an overall positive charge
                   • Rutherford and Marsden came up with the nuclear model of the atom
                     • Nucleus is tiny but makes up most the atoms mass, it contains protons(positive charge) & neutrons(neutral charge) so overall positive charge on nucleus
                       • The rest of the atom is mostly empty space
                         • Negative electrons whizz round the outside of the nucleus and give the atom its overall size
                           • atoms have no overall charge an electrons charge size is the same as a protons but opposite
                             • This means the number of protons and electrons is always equal in a neutral atom
                               • If electrons are added or removed the atom becomes a charged particle called an ion