The Atomic Nucleus
Abstract:
To learn what the structure
of the atom
is by doing an experiment on a much larger scale that helps us find hidden
structure.
Apparatus:
1)
Box with paper
ÒdetectorÓ windows.
2)
Squirt bottle
3)
Ink
Procedure:
1) Do NOT lift the box up at any time.
2)
Stick the squirt
bottle's nozzle into the small hole in the box.
3)
Repeatedly squirt
ink into the box. Try to hit as much of the white paper (our ÒdetectorsÓ) at
the back and sides of the box.
4)
When you are
satisfied that you have fired in every direction possible within the box call your teacher to help you remove the detector
windows and record your results in the Data section.
Data:
You may have noticed that you
couldn't wet all parts of the white paper. This is because there were some
obstacles inside the box. Record what areas were wet on each sheet of paper by
shading in the blocks below. If there is just a few drops in
an area leave it blank as this was probably just from some splashing off an
obstacle.
Back Screen
| Left Screen | Right Screen |
Analysis:
Looking at your results. You should see that the
obstacles blocked some of the water. Try figure out what the obstacles looked
like. Think about their size and position in the box. You could even try guess
the shape. Draw the obstacles into the box below.
Top view of your box
– Draw in your obstacles
Are you sure you found all
the obstacles? Is it possible that some were hidden? Explain your answer.
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If you could never open the
box, what would you do to ensure you were right? Are there any other methods
you can think of that would help? Describe them below
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In 1909 scientists were
trying to figure out what the atom looked like. They thought atoms were made up
of a big ball of positive charge with some small balls of negative charge
floating around (called electrons). They tried to test this idea by
shooting tiny positive particles (called alpha particles) at atoms that
were heavy and big. They expected these alpha particles to be deflected
a little bit by the negative electrons but since they were small they
expected to miss them most of the time. The positive charge was spread out over
the whole atom so they didn't think it would be strong to block the alpha
particles (it's like if you take a block of wood and spread it over a large
area it becomes as thin as paper and so wouldn't stop you if you ran into it).
So they expected something like this
And so when they ran the experiment they
expected to see this
Instead though they got this
Notice that most alpha
particles got through but some of alpha particles (which are
positive) were deflected almost right back where they came from. Imagine your
experiment used a ping pong ball instead of water, what would it mean if the
ball bounced back to where you shot it from?
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If most of the alpha
particles got through without bouncing back, what can you say about the
size of the obstacle responsible for bouncing them back?
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The scientist in charge of
the experiment, Ernest Rutherford, calculated that there was no way the tiny
negative electrons could bounce the alpha particles back like
that. The electrons were known to exist so he couldn't change that part
of the picture. But if the positive charge was thinly spread out then he
couldn't bounce the alpha particles back, in fact he said it was like
firing a gun at a piece of tissue paper and having the bullet bounce back and
hit you. This was obviously not correct.
What do you think was going on?
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Draw a picture in the space
below of what you think the atom looked like from those results. Show where the
positive charge is and where the negative charge is.
Conclusion:
Compare your picture to the
picture on pg 252 of your textbook. The orange dots
are the negative electrons, the green balls are the positive charged
particles called protons. Later on they realized there was also the
purple balls called neutrons which had no
charge. How close was your picture? If it wasn't close redraw a correct one at
the end of the lab labelling all the parts.
When all the calculations
were finished scientists worked out that all the positive charge and almost all
the mass was located in a tiny area about 10 000 times smaller than the atom. This
part of the atom was called the nucleus. The electrons orbited
around the nucleus. They had all the negative charge and only a tiny bit of the
mass, but they moved around in orbits the size of the whole atom, unlike the protons
and neutrons that were found only in the tiny area in the center called
the nucleus.
Summarize below what you now
know about the structure of an atom. Use diagrams with labels too.
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