P4

P4

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

2. Solids cannot be squashed, therefore the atoms which make them up must be solid throughout

3. Rubbing two solids together often results in static charge so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide.

4. 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.

5. Alpha particles are a form of nuclear radiation with a large positive charge.

6. Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking.

7. Most of the alpha particles did pass straight through the foil.

8. A small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil.

9. A very small number of alpha particles came straight back off the foil.

10. The fact that most alpha particles went straight through the foil is evidence for the atom being mostly empty space.

11. A small number of alpha particles being deflected at large angles suggested that there is a concentration of positive charge in the atom - like charges repel, so the positive alpha particles were being repelled by positive charges.

12. The very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus) - the tiny number doing this means the chance of being on that exact collision course was very small, and so the 'target' being aimed at had to be equally tiny.

13. The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford. The nucleus was calculated to be about 1/10,000th the size of the atom.

14. 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.

15. 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.

16. 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.

17. The proton had been proposed as being the nuclear particle responsible for the positive charge of the nucleus and for some of the nuclear mass. However, there was still a difference between the atomic number of the atom and the atomic mass.

18. Bohr and other scientists knew that there also had to be a neutral particle the same size as a proton to keep the nucleus stable and to make up the mass. This was accepted and scientists used models that included the neutron for a further 20 years before it was actually discovered. It wasn't until 1932 that James Chadwick was able to prove its existence.

19. Chadwick used a version of Rutherford's experiment, using a sheet of beryllium and a paraffin block instead of gold foil. He was able to prove that a proton-sized neutral particle - now known as the neutron - existed.

20. Atoms are very small, they have a radius of around 1 × 10-10 metres.

21. The modern view of the atom is of a nucleus containing protons and neutrons with smaller electrons orbiting outside the nucleus.

22. Protons and neutrons are the heaviest particles in an atom and as a result they make up most of the mass of the atom. The mass of electrons is often not considered to be significant.

23. The total number of protons and neutrons is called the mass number and the number of protons is called the atomic number.