2. The Bohr Rutherford Model I wonder what your house is like?

3. So…what is a model A model is a way of looking at something What kind of house do you live in ? Hmmm – Well, I know you live in Nova Scotia so you have to have a roof and walls – You probably have a house that is made of wood, although I’llbet some of you live in brick or stone houses – You probably have steps leading into the house – You all have a bathroom with a toilet, sink and tub or shower Am I getting close?

4. What they knew then… By the late 1800’s scientists had a working model of the atom. Dalton proposed that the atom was a sold sphere JJ Thomson (remember him?) came up with another model…mmm, plum pudding

5. Radiation As scientists learned about matter they also began to learn about RADIATION 1895 – Wilhelm Roetgen discovers x-rays He accidentally exposed a sheet of paper coated with fluorescent material to a cathode ray tube. The paper glowed in the darkened room when the tube was turned on. Doctors started using the x-ray just a few years after it was discovered.

6. Radioactivity 1896 – French scientist Henri Becquerel wonders if there is a link between fluorescence and X-rays. – Do substances exposed to sunlight give off X-rays? He put samples of crystal (and some uranium) on well wrapped photographic plates and left them out in the sun. One day he put them all in a drawer Wow – some of the film was exposed even in the drawer!

7. Radioactivity So what happened? Becquerel reasoned that X-rays were not responsible since the sun had not touched the plates that day. Some new kind of ray must be responsible. Tests showed that Uranium was emitting these rays. Marie Curie was very interested in these rays and coined the term radioactivity. Of course she died horribly from radiation poisoning

8. Searching for Radioactive Elements Marie Curie and her husband Pierre discovered a number of radioactive elements by breaking down something called “pitchblend” Uranium Polonium Radium

9. Rutherford and Thomson Rutherford was a scientist who also explored radiation. He discovered three kinds of radiation and decided that different particles were causing each kind. Alpha Beta Gamma

10. Probing the atom…again! 1909 – Rutherford designs an experiment where he fires radioactivity through gold foil to see what happens…

11. So What did Rutherford find out? 1) The nucleus is so small that the odds are overwhelmingly in favor of a given alpha particle motoring right on through the gold foil as if nothing were there. It turns out that the atom is a very empty place, indeed!

12. 2) Some alphas, by pure random chance, will pass near some gold atom nuclei during their passage through the foil and will be slightly deflected.

13. 3) A very, very few alphas, by pure, random chance, will hit a nucleus almost head-on. The alpha, traveling at 10% the speed of light, penetrates the atom and gets very close to the nucleus. However, the repulsion between the alpha and the atom nucleus is so great that the atom flings the alpha back out, and it does so in a hyperbolic path. Depending on various factors, this occasionally results in the alpha being turned around 90° or more. The very heavy nucleus recoils a bit from the impact, but essentially goes nowhere.

14. Inferring Rutherford inferred the nucleus of the atom like so... – How could the foil repel the dense, strongly positive alpha particles? – Well…the atoms must possess something that is very dense and very positive – But … these things must be very small because most of the particles get through the foil. – So… the atoms must have a large area that is mostly empty.

15. Technology break… Nucleur medicine uses radiation as its main tool – X-rays – Therapy – to kill cancer cells – Radioactive injections to look inside the body and trace pathways.

16. But what about the Neutron? Rutherford calculated that GOLD must have 79 protons in its neucleus. BUT…he also calculated that GOLD must have a mass of around double that. Where is the missing mass??????? Negative charge is already accounted for Positive charge is already accounted for Must be something neutral – ah the Neutron!

17. Bohr and the Elctrons Many scientists thought Rutherford was full of something other than protons! They wondered why the negative electrons didn’t crash into the positively charged nucleus. 1912 – Neils Bohr goes to study with Rutherford. He wonders the same thing.

18. Just Like the Solar System By looking at the light emitted by the elements Bohr inferred that the atom must be like the solar system. The rapid movement of the electron keeps it from crashing into the nucleus – just like the planets and the sun.