P4 - Atomic structure

P4 - Atomic structure

1. After discovering the electron in 1897, J J Thomson proposed that the atom looked like a plum pudding.

   • he suggested that the atom consisted of positive 'dough' with a lot of negative electrons stuck in it. This was consistent with the evidence available at the time. He did not know about neutrons so there aren't any neutrons in this model
   • .

2. In 1905, Ernest Rutherford did an experiment to test the plum pudding model. His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum.

   • The vacuum is important because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else. Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking.
   • It was thought that the alpha particles could pass straight through the thin foil, or possibly puncture it. If the plum pudding model had been correct then all of the fast, highly charged alpha particles would have whizzed straight through undeflected. The scientists were very surprised when other things happened:
     • most of the alpha particles did pass straight through the foil a small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil a very small number of alpha particles came straight back off the foil

3. Even though Rutherford had proven the existence of the nucleus, scientists were unsure how electrons fitted into this new model. In 1913, Niels Bohr revised Rutherford's model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus.

   • By doing this, he was able to explain that since particular chemicals burn with certain-coloured flames, the pattern of energy released by electrons in the chemical reaction must be the same for every single atom of that element. Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom.
     • When atoms absorb energy, the electrons at a particular level are pushed up to higher levels (at bigger distances from the nucleus). In time, they jump back down to a lower level releasing light of definite frequencies.