The Bohr Rutherford ModelThe Bohr Rutherford Model

I wonder what your house is like?

So…what is a modelSo…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?

What they knew then…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

RadiationRadiation

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.

RadioactivityRadioactivity

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!

RadioactivityRadioactivity 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

Searching for Radioactive Searching for Radioactive ElementsElements

Marie Curie and her husband Pierre discovered a number of radioactive elements by breaking down something called “pitchblend”

UraniumPoloniumRadium

Rutherford and ThomsonRutherford 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.

AlphaBetaGamma

Probing the atom…again!Probing the atom…again!1909 – Rutherford designs an experiment

where he fires radioactivity through gold foil to see what happens…

So What did Rutherford find So What did Rutherford find out?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!

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.

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.

InferringInferring

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.

Technology break…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.

But what about the Neutron?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!

Bohr and the ElctronsBohr 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.

Just Like the Solar SystemJust Like the Solar SystemBy 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.