X-rays are very short wavelength electromagnetic radiation and the wavelengths are similar to the interatomic distances in solids.
When a beam of monochromatic X-rays (i.e. rays with a single wavelength) is directed at a crystal, some of the X-rays are diffracted by the planes of atoms in the crystal. This produces a pattern of intensities that can be interpreted to give electron density maps of that crystal. The most concentrated electron density is around the largest atoms (the ones with the most electrons).
An overview of X-ray Crystallography Technique
This technique allows us to measure bond lengths and bond angles of molecules but does not show up hydrogen atoms since they only possess one electron and are swamped by other larger atoms.
Things to be noted:
- X-rays are diffracted by planes of atoms in a crystal because they have similar wavelengths to the separation of the atoms in the molecules
- The intensity of the interaction depends on the electron density around a given atom. For this reason, hydrogen does not show up
- By using X-ray crystallography, bond lengths and bond angles in molecules can be measured