Chemists use X-ray crystallography to determine the exact structure of chemical compounds. This involves taking a solid crystal of a pure compound and diffracting X-rays off of it, which produces a complex diffraction pattern. Using computer software, the diffraction pattern can be processed to yield a structure that describes the structure of an individual molecule of the compound making up the crystal. This is a powerful technique, but it can only be used on compounds that can be crystallized. It’s also worth pointing out that the solid phase structure of a molecule is not always the same as that in a solution, so caution should be used when relating crystal structures to chemical reactivity in the solution phase.