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How can you tell what a particle is made of?

You Wrote:
"How can you tell what a particle is made of? For example, H2O--how do you know what the shape is if it is so small?"


My name is Chuck Brown and I am a scientist here at Fermilab.

How can you tell what a particle is made of? For example, H2O--how do you know what the shape is if it is so small?

The fact that water contains 2 hydrogen atoms for every 1 oxygen atom was discovered many years ago by chemists who were keeping careful track of what weight of different materials took part in a variety of chemical reactions - it always turned out that there was twice as much hydrogen as oxygen that appeared to be due to the water in the reaction.

How can you see two parts hydrogen and one part oxygen and what do they look like?

More recently, we have been able to scatter high energy particles off ice crystals to 'see' the shape of the H2O molecule. (A crystal of frozen water has the atoms locked in place and hence one can study the shape of the water molecule more easily). The pattern of particles scattered from the ice is related to the exact shape of the ice molecule. We now know that the H2O molecule has the oxygen between the two hydrogen atoms sort of like a dumbbell, H-O-H, except that the 3 atoms of the dumbbell are not in a straight line like the symbol H-O-H but rather are bent in the middle like the letter V. The science of studying the exact shape of molecules is called crystallography and this science has revealed many fascinating shapes from the simple bent dumbbell of the water molecule to the very complicated double-helix shape of the DNA molecule which forms the basis of the genetic material in the cells of all living things.

I hope you continue to be fasacinated by the beauty of the molecular world that is hidden from normal sight, but which can be revealed with modern chemistry and physics equipment. It has been many many years since I was a grade school student, but I have become more and more interested in the beautiful world around us as I have been able to study it, 'see' it, and learn more about it.

- Good luck, and enjoy your school years!

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last modified 11/21/1998   physicsquestions@fnal.gov