Do crystals violate Heisenberg's uncertainty principle? Atoms are arranged regularly in a crystal lattice and that means we know exactly where they are. Doesn't the uncertainty principle preclude the possibility of knowing a position of microscopic particles?
 A: It's understandable that someone would think we know exactly where atoms are when they are in a crystal, because a lot of problems intended for students are presented as if that is true.  But it isn't!
In a real crystal all the atoms are moving.  The movements get smaller if you cool the crystal down, but they never completely disappear.  See, for instance, the Wikipedia article on the quantum harmonic oscillator, which says:

[...] the lowest achievable energy (the energy of the n = 0 state, called the ground state) is not equal to the minimum of the potential well, but ħω/2 above it; this is called zero-point energy. Because of the zero-point energy, the position and momentum of the oscillator in the ground state are not fixed (as they would be in a classical oscillator), but have a small range of variance, in accordance with the Heisenberg uncertainty principle.

There is also a more philosophical issue to consider, which is how you can know the position of a fixed constituent atom (if it were fixed, which it isn't) without knowing the position of the crystal itself.
A: Good question. +1
The uncertainty principle limits how well you can predict both position and momentum. $\delta x\delta p > \hbar/2$.
Notice that for a small object, $p$ is small (unless it is traveling very fast), and therefore so is $\delta p$. That means $\delta x$ must be bigger. This means uncertainty of position is significant only for very small particles, such as an electron.
If you reduce the uncertainty of position of an electron by confining it near a nucleus, you increase the uncertainty of momentum. An electron that may have a high momentum isn't likely to stay near a nucleus very long. There is a size where these two competing uncertainties balance. This determines the size of an atom.
The smallest nucleus is just a proton. But that is 1800 times heavier than an electron. Other nuclei are much heavier than that. The uncertainty in position of an atom is much smaller that the size of an atom.
All the interesting physics of crystals is in the electrons. You do need quantum mechanics to predict the behavior. Around a nucleus, electrons occupy orbitals that are about the size of an atom. When atoms are near each other, orbitals overlap and create covalent bonds. Or they form an extended structure that allows electrons to freely move about in metals. All of these have a much bigger effect than the uncertainty of position of the atoms.
