Most of us spent part of our youth cutting up construction paper to make snowflakes which tended to look nothing like the globs of white moving past our windows. Check under a microscope, though, and you’ll see that snowflakes are beautiful six-sided crystals. Why is this?
The beautiful hexagonal crystals that water forms when it freezes in the clouds have been much celebrated by those with electron microscope, and without a driveway that they need to shovel clear. Because within the giant globs of freezing water that the world playfully throws at us, there are intricate hexagonal patterns.
Most of the time we’re too big to see those patterns and too cold to care about them, but when we get inside, warm our feet, and maybe get our hand around a hot mug of something, we contemplate. There are billions of swirling molecules of water up there in the clouds. Why do they always form into hexagonal structures? And if there is some preference, on the atomic level, for hexagonal structures, why are each of those tiny structures so different from one another?
The answer to the hexagonal shape of the snowflake lies in the shape of the water molecule itself. A water molecule is two electron-deprived hydrogen atoms clinging to a central, also-electron-deprived oxygen atom. By pooling their outermost electrons they get a stable formation. That stable formation requires the hydrogen atoms to be pinned at specific points to the oxygen atom. They can’t simply hang out at either end of the oxygen atom, 180 degrees apart. They hug relatively close to each other, only 104.5 degrees apart, giving the overall molecule a V shape. The oxygen tugs the electrons close to itself, leaving the angle of the V with a slightly negative charge while the hydrogen atoms, with the protons in their nuclei exposed to god and everyone, are slightly positive. (Exhibitionists? You decide.) Positive charges are attracted to negative ones, so the hydrogen end of one V will be attracted to the oxygen apex of another. Even when it’s splashing around in a bowl, or whirling through a cloud, water is structured.
When it gets cold, some real organization happens. Water doesn’t just spontaneously freeze. It freezes via a process called nucleation. One water molecule shifts its structure slightly and freezes solid with another, and as that structure moves through a whirling cloud of water, more water molecules hop on board, until the whole structure gets too heavy to fly and drops down. This process is not a free-for-all. A frozen water molecule will still repel, and be repelled by, any other molecule that approaches it from the wrong angle. Get a group of bar magnets together and jumble them around, and they’ll ‘arrange’ themselves with north and south polls of different magnets linked. Water molecules do the same, just with a V shape. The patterns grow from that shape, and they arrange themselves into hexagons.
And yet, different snowflakes look radically different from each other. Nucleation doesn’t generally start on its own. The ‘seeds’ of nucleation are dust molecules whirling with the water through the atmosphere. They’re one of the reasons why we see differently-shaped snowflakes whenever we care to look. Different patterns appear depending on the size and density of dust. The snowflakes also depend on temperature, air pressure, wind speed, and a thousand other factors. Scientists are still studying the exact mechanism of the different shapes. There are so many variables involved. In the meantime, they’re still pretty.