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Hmm. This is actually somewhat more complicated than it seems. In one sense, there is a lot of space between atoms. The question of how many spheres you can pack together is called "finding the filling factor". The filling factor is basically the ratio of volume of spheres to the volume of the smallest cube that can enclose them. (It's actually well-defined for other shapes, too.) For a sphere, f = 4pi/3 * r^ 3 (2r)^3 f = pi / 6, or approx 0.5! In other words, around half the volume is empty! But then again, _atoms_ are mostly empty space, too, because the nuclues (with almost all the mass) is about 1/10,000 the radius of the atom. So maybe this isn't too useful a distinction. And, in another real sense, there isn't any space at all. You see, under quantum physics, what we call "particles" aren't really hard spheres. They're collections of "wavicles" with properties of both particles and waves. The mathematical description of these waves gives the probability of finding an electron (or whatever) at a given point and time. But the equations say that these wavefunctions never _quite_ go to zero ... meaning electrons extend over the whole Universe (albeit with infinitesimal amplitude)! In that sense, the electon wavefunctions fill all of space, and there isn't _any_ empty room. Quantum Physics is weird, but (apparently) true.

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