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Name: Mike
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Question:
How do you "see" inside an atom?



Replies:
We cannot really 'see' inside atoms, but we can watch how atoms affect other things to figure out what the atoms are like. Imagine "Harry Porter" with an invisibility cloak -- you cannot see him, but if you see a vase knocked over, you might deduce that he is there. Or perhaps a better example would be to launch tennis balls at him and see where they bounce to figure out his size, shape, etc. The same is true for atoms -- you hit atoms with electrons, light, other atoms, and more, and can learn about the atom based on what happens.

Safety note: DO NOT put a sheet over your little brother's head (telling him it is for 'science') and launch tennis balls at him. That is a bad idea.

Hope this helps,

Burr Zimmerman


I notice that you have put "see" in quotation marks in your question, so you must know that we cannot actually see atoms. They are so much smaller than light waves that they cannot be seen - so a good question - how do we see them?

In recent times scientist have been able to make use of an instrument called an electron microscope, which, as its name suggests, uses electrons to 'see' something rather than light. It uses magnets to focus instead of lenses, and it puts its image on a screen like a TV. With that we can 'see' incredibly tiny things - single molecules even - but while it can show us that there is an atom there, it cannot show us what is inside the atom. The atom is still way too small!

We have to enter a realm of guesses and speculation. A scientist will take a 'guess' at what he thinks the inside of an atom might look like, and then tests to see if his 'model' of the atom can be used to predict how the atom will behave.

One of the first 'models' of the atom was called the Plum Pudding Model. Scientist knew there were positive and negative bits inside the atom, and that the negative bits could make electricity, and they were called electrons. Someone suggested that the various bits were all just stuck in an atom ball, like sultanas and currants in a pudding. This model explained how positive and negative bits could both be there, but it did not explain other things.

In 1909 (100 years ago next year!) some scientists found that tiny radioactive particles called alpha particles, could get through a thin sheet of gold. This meant that if atoms were like plum puddings, then they were very soft and fluffy puddings, because they were soft enough that they could not stop the alpha particles. Think about firing an air gun pellet at a real pudding - it might get through - but fire it at a sponge cake??? So they set out to test how thick a sheet of gold was needed to stop the alpha particles. They PREDICTED that the particles would go straight through, or get knocked off course a little bit, but they found another very surprising result.

By counting the number of particles that passed through the gold they were able to work out that some particles were 'missing'. A bit more searching and they found that every now and again one particle in several thousand was knocked way off track, or even bounced back! This cannot be explained by 'plum pudding atoms'

One of the scientists, Ernest Rutherford, suggested that instead of being scattered through the atom, there must be a 'lump' of charged bits in the middle, and a bunch of other charged bits around the outside - +ve protons inside and -ve electrons outside. He had no idea HOW that could be, but he knew that the plum pudding had to be wrong. He suggested a core of positive particles, and the electrons spinning around the outside like the surface of a globe. He suggested that the core was very tiny - less than 1/3000th of the wide of the atom. A grapefruit sized core in the middle of a football field, with the electrons spinning around the fence. It is Rutherford's model of the atom that we usually see drawn in text books and where companies want to look 'scientific'. They draw a big dot in the middle, and a bunch of little dots making circles (that usually look oval shaped) around the edge.

Other scientists have done further tests and found that not all the electrons can be in one layer, because some can be taken away from the atom easily and some are almost impossible to get off, so then it was thought there might be several layers of electrons like the layers inside a gobstopper (all day sucker) (To help explain the model, the layers have been called shells, and if we just look at a small part of the circle, we can think of the layers like the shelves in a book case. If we put electrons (think marbles) on the top shelf, they can fall off easily, and land on the floor with lots of energy (make lots of noise). Electrons on the bottom shelf do not fall off nearly as easily, and land so gently we can hardly hear them - hence they have been called energy shelves or energy levels.)

And then someone else found that the electrons do not always go neatly on one layer or another all the time, sometimes they can be a little bit above or below the 'energy shelf'. The model has been changed again to suggest not layers of electrons, but clouds of electrons which not only spin around the centre, but move up and down a bit too - like the horses on a merry-go-round, except that there are several 'layers' of merry-go-rounds.

Can you 'see' that each time we try to describe an atom, we have to suggest something it might be like. These 'models' of the atom are not made of plastic and wood and string, but are ideas that scientists use to help them understand why things are behaving in certain ways. If the behaviour does not fit what the model suggests is should be, then the model has to be changed.

No one can 'see' inside an atom, we can only speculate and offer models to say that it might be like .... something.

This is all part of the 'scientific' way of looking at things we cannot see. Just as we can't 'see' an atom, we also cannot see what happens in a fire, or the inside of the sun, or why dogs bark at pushbikes. We can make models, and test to see if the model can predict. If my model is correct, then when I ..... , I should see .............. We try out an experiment, and compare what we predicted with what actually happened.

Let me see if I can give another example - My dog barks at the neighbour's son, Jake, when he rides past on his bicycle. I want to know why. I cannot see inside the dog's head to read his thoughts, but it might be that the dog does not like Jake because he threw a rock at him or something. If that is why the dog barks, then the dog should NOT BARK if someone else rides past. I test this idea by having Jake's sister ride past. The dog still barks. I try again with the Jake's little brother. The dog still barks. My 'model' of the dog's thinking must be wrong.

Perhaps the dog is barking because he doesn't like that particular bicycle. If that is correct, then the dog should NOT BARK at a different bicycle. I test this idea, by having the Jake ride past on a different bike. the dog still barks. I try another bike. The dog still barks. Again my model must be wrong. Perhaps the dog just does not like bicycles. If that is true, then the dog SHOULD BARK even if the bike is stationary. We place the bike on its stand in the road, and walk away. The dog does not bark! Again my model is wrong.

Perhaps the dog does not like things that move fast..........

Can you see that I might eventually be able to 'see' what is going on inside the dogs head, not by looking inside, but by observing behaviours and outcomes, and comparing them to predictions based on my model. It is a complicated and slow way to get an answer, but it usually gets you closer to the right answer than any other method we know.

This process - guessing answers and proposing and testing outcomes - is known as The Scientific method, and has been used for a few hundred years.

For more information, the entries in Wikipedia under ATOM and SCIENTIFIC METHOD are very useful

Nigel Skelton
AUSTRALIA


Mike,

The most common practice is to send things in and then see how they come back out. Do various things to an atom and figure out what CANNOT be true about the inside of the atom. Try different things, eliminating other possibilities. Some of the many things used are magnetic effects, electric effects, interaction with light, interaction with heat, interaction with other atoms, and interaction with electrons.

Eventually, you narrow it down to a small set of very similar possibilities. It is in many ways like sculpture. You continually eliminate what cannot be true until all you have left is what is probably true.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


You do not really "see" inside an atom in the usual way "see" is used. What is meant by "see" is how the inside of an atom "works" when it collides with very fast particles. Think of it this way. Suppose you have a peanut buried inside a cotton ball. How could you get some information about the shape of the peanut, even if you cannot see it? You could shoot at the cotton ball with a stream of particles and watch how the particles are deflected by the peanut. If the peanut is pointed toward the stream of particles, it would appear to be smaller than if it were lined up sideways. If you then looked at how the pattern of the particles changed as you changed the orientation of the peanut, you could get some idea about the shape of the peanut. If the stream of particles was too fast you might smash the peanut. Or if the stream of particles was too slow they might not even "see" the peanut, and just get buried in the cotton ball. Putting many hundreds or thousands of such measurements together you could get some idea of the size and shape of the peanut inside the cotton ball. This is approximately what is meant when scientists say that they "see" inside the atom.

Vince Calder



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