Crystals.

Step 1 of 7

One small box

Pour some table salt onto dark paper and look closely: most grains are little cubes, with flat faces and sharp corners. Nobody cut them that way. A crystal is a solid whose shape comes from inside: its particles are arranged in a pattern that repeats, exactly, in every direction.

In the Scene Zoom to the atoms is at Atoms, and Show unit cell outlines the smallest box that builds the whole crystal when copied again and again: the unit cell. For salt it is a cube 0.564 nm along each edge. How many such cells fit across a grain of salt one millimetre wide?

Step 2 of 7

Six-sided snowflakes

The shape of the cell shows in the shape of the crystal. Salt’s cell is a cube, and a grain’s flat faces are planes of ions lying exactly along that cube. The Scene now shows Ice. Switch between Salt, Ice and Quartz, and compare the outlined cell with the crystal’s shape: choose Crystals under Zoom to the atoms to see the whole crystal.

Step 3 of 7

Neighbours in ice

In ice the water molecules are joined by hydrogen bonds, dashed in sky blue. They form puckered hexagonal rings, hence six-sided plates and snowflakes. Pick one water molecule in the middle of the crystal and count how many molecules it is hydrogen-bonded to.

Step 4 of 7

Slow cooling

A crystal starts from a seed: a few particles that happen to settle into the right pattern. Below its freezing point a liquid is ready to crystallise, but a seed that is too small falls apart again. The colder the liquid gets below its freezing point, the faster new seeds appear, and meanwhile every seed that survives grows, layer by layer.

In the Scene Cooling rate is at 1, the slowest, and salt crystals have grown on a rock. Count how many crystals there are.

Step 5 of 7

Fast cooling

How big the crystals end up depends on the race between new seeds and growing crystals. Predict what happens when you move Cooling rate up to 100, a hundred times faster.

Step 6 of 7

Rocks remember

Rocks keep a record of this. Granite cooled deep underground over thousands of years or more, and its crystals are big enough to see. Basalt is lava that cooled at the surface within days to years, and its crystals are too small to pick out. In geodes, mineral-rich water fills a hollow slowly, so their quartz has time to grow into long columns. Metals are crystals too: a steel beam is a mosaic of tiny grains, and their size is set by how fast it cools.

Step 7 of 7

Ions that touch

The size of the unit cell is measured with X-rays. It can also be worked out from the size of the ions.