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Johanes Kepler was the astronomer who first understand the beautiful eliptical orbits of planets around the sun. He had a wonderfully restless mind and in 1611 he composed a charming, learned little book speculating on why snowflakes have six points. Brilliant though he was, Kepler didn’t have the knowledge of atoms and molecules that we do today, so his book, fascinating as it is, doesn’t come up with the answer.
Here’s how we get those delicate snowflakes, some of which landed on Kepler’s coat as he walked across the Charles bridge in December of 1610. A molecule of water is composed of two little atoms of hydrogen linked to a bigger atom of oxygen. The two hydrogen atoms are positioned 104.5 degrees from each other, and that gives the three atoms taken together a shape rather like a three-sided pyramid. That’s a water molecule.
That’s not the same as what we call water, which is a bunch of water molecules hanging out together. The comparatively large oxygen atom is composed such that the side opposite the two hydrogen atoms is able to link up with the hydrogen atoms of other water molecules. When a water molecule links up with four other water molecules, it arranges into a nice four-sided pyramid – called a tetrahedron in your geometry class.
Now we’re getting someplace. Because as the temperature drops, these four-sided arrangements of water molecules draw closer together and form a six-sided, or hexagonal, structure – which is what we see when we look at an ice crystal. That ice crystal is the heart of the combined molecules which add up and spread out to a clearly visible snow flake. Now you know.
Another version of why snowflakes have six points can be found in the very short tale The Queen of the Rain Was in Love with the Prince of the Sky. Furthermore, that little story also explains why no two snowflakes are precisely alike. And this is shameless self-promotion, because that little gem is by Eugene Mirabelli, who writes all these unsigned posts at Critical Pages.
If you’d like to read the mini-book The Queen of the Rain Was in Love with the Prince of the Sky, which is considerably lighter and more entertaining than molecular chemistry, click on the title at the start of this sentence. On the other hand, if you’re a chemistry buff and can’t follow the lucid explanation of the arrangement of atoms and molecules posted here and insist on seeing diagrams, you can find some really good ones on the web. And if you’d like to read that book by Kepler, check out the offerings at Paul Dry Books.