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Oct. 14, 2011

Science Dad on Crystals

by Vince Harriman

Click to enlarge images

Not two weeks after we posted our little experiments on the different states of matter, science went ahead and discovered another state. Dan Shechtman was awarded the Nobel Prize in Chemistry for his work on quasi-crystals. As I stated then and will re-state now, there are so many states of matter that didn't exist ten or twenty years ago that I hesitate to try to name them all. But Beckett and I did start talking and thinking about crystals. We decided to attempt a two part experiment: this week we will look at the structure that makes a crystal a crystal then next week we will actually grow some crystals.

First, what is a crystal? What makes a crystal different from other types of matter? The defining characteristic of a crystal is pattern. Beckett pulled out a box of blocks and we started arranging. First, he dumped them all out on his table. This is what matter with no form looks like. Dirt, sand, most amorphous solids:

unorganized 'atoms'

We decided that the blocks were atoms and would represent the different ways that matter can be organized. Here they have no discernible pattern, and no pattern that is likely to repeat in nature.

Next, Beckett arranged the blocks in the simplest pattern possible:

The simplest pattern with rectilinear tiles

The atoms all follow a simple pattern. But even a pattern this simple can have a variation:

Variations on a simple pattern

Very little has changed, but in a crystal, this would represent a monumental change! Imagine the blocks are atoms or molecules, and arranging them in this slightly different pattern represents atoms or molecules bonding on different sites, different electrons.

We decided to get fancy:

A simple herringbone pattern

The patterns were getting more interesting, the atoms and molecules were interacting in new and exciting ways. How many ways could there be to arrange so simple an atom or molecule? We went back to a square pattern:

Classic square basket weave

We were arranging the atoms of our crystals into patterns with no end in sight! I asked Beckett to see what else he could come up with:

Rotating out from center

This pattern is interesting because it has holes and gaps in it. It looks like the pattern can continue, but in a very different way than the very square patterns above. We tried another:

Two different 'atoms' or 'molecules'

Finally, I asked Beckett to think outside the box, way outside. What hadn't we done yet? Clearly there were hundreds of patterns that could be accomplished with a single basic tile. Were there more patterns though? We tried this:

Adding depth

There are half a dozen additional ways the blocks can be laid against one another giving hundreds more possibilities. I'm sure a mathematician will come along and reassure me that there are an infinite number of possibilities for how a single shape can be arranged in patterns. Our goal here was to see how a single shape-an atom or molecule-could be arranged in many different ways. This is not just an academic exercise -- scientists are finding new ways to arrange crystals all the time.

For your own crystal building exercise, start by looking around and noticing things that have patterns and those that don't. We noticed a box of straws in a restaurant, for example. Next, find something you can use to build patterns -- like building blocks -- and see what you can come up with. For added fun, try tiling with different size coins or using a variety of different but repeating shapes.

Next week we will move deeper into crystals -- we will grow some, and take another look at structure and pattern. If you come up with an interesting way to visualize crystal structure, please leave a comment below.

You can read the announcement of Shechtmann's prize on the Nobel Prize page. You can also learn about liquid crystals by playing this great game at the Nobel Prize website.

 

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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