2.6 Taking Up Space

One other way to represent molecules that I want to describe is the space-filling model. Atoms have a definite radius, called the van der Waals radius, and it's different for each element. Normally, no two atoms can approach with their centers closer together than the sum of their van der Waals radii, unless they share a bond. Space-filling models represent that van der Waals radius as a solid sphere, so that we can tell at a glance whether atoms are too close together or how much room there is for them to move around. Here's the model for vanillin just to demonstrate how strange these things look:

I've always thought they kind of look like caterpillars. If we could see molecules directly, they probably wouldn't look like this. Molecules have been imaged using electron microscopes, and the result looks something like a skeletal formula or a ball-and-stick model, perhaps surrounded by a ghostly space-filling model. But that is how molecules look in electron beams, and we don't see electron beams, we see photons. So we can't really say which type of model is closest to how molecules would actually look. In truth, molecules are too small to actually look like anything.

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