X-ray Diffraction
Definition and meaning of X-ray Diffraction in chemistry.
X-ray diffraction is an analytical technique used to determine the atomic structure of a crystal. It works by aiming a beam of X-rays at a crystalline sample and measuring how the X-rays scatter off the electrons in its atoms. The resulting pattern reveals exactly how the atoms are arranged in three dimensions.
In more detail
A crystal is built from atoms arranged in a repeating, orderly pattern. When X-rays strike this pattern, they scatter in specific directions rather than randomly, because the atoms act like a three-dimensional diffraction grating. Bragg's Law describes the exact angles at which this scattering, called diffraction, produces a detectable signal.
By measuring the angles and intensities of the diffracted beams, scientists can calculate the electron density at every point in the crystal. Since electrons cluster tightly around atomic nuclei, this electron density map shows precisely where each atom sits. From that map, researchers can measure bond lengths, bond angles, and the overall three-dimensional shape of a molecule.
Modern labs use instruments called X-ray diffractometers, which rotate the crystal and detector to collect thousands of data points automatically. X-ray diffraction requires a well-ordered crystal, since scattering from a disordered sample would not produce a clear, readable pattern. This is why growing high-quality crystals is often the hardest step in a diffraction study.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Key equation | Bragg's Law (nλ = 2d sinθ) |
| Primary use | Determining crystal and molecular structures |
| Sample requirement | A well-ordered single crystal, or powder for polycrystalline analysis |
| Famous application | Revealed the double helix structure of DNA |
X-ray diffraction was famously used by Rosalind Franklin in the early 1950s to photograph fibers of DNA. Her image, known as Photo 51, showed an X-shaped pattern that revealed DNA's helical structure. This evidence was essential when James Watson and Francis Crick proposed the double helix model in 1953. The work later contributed to a Nobel Prize in Medicine awarded to Watson, Crick, and Maurice Wilkins in 1962.
Frequently asked questions
Can X-ray diffraction be used on non-crystalline samples?
Standard single-crystal X-ray diffraction needs an ordered crystal, but powder X-ray diffraction can analyze polycrystalline materials and identify what phases are present in a sample.
Why are X-rays used instead of visible light?
X-ray wavelengths are roughly the same size as the distances between atoms in a crystal. That match in scale is what makes diffraction possible.
What is Bragg's Law?
Bragg's Law, nλ = 2d sinθ, relates the X-ray wavelength, the spacing between crystal planes, and the angle at which diffraction produces a strong, detectable signal.