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X-ray Crystallography
The size of the unit cell and the arrangement of atoms in a crystal may be determined from measurements of the diffraction of X-rays by the crystal, t…
In 1913, the father and son scientists William Henry Bragg and William Lawrence Bragg noticed that when X-rays strike a crystalline solid at a certain angle, the X-rays diffract and produce a pattern of regularly spaced spots.
This led to the development of X-ray crystallography, which uses this phenomenon to determine the structures of crystalline solids ranging from simple ionic compounds to complex macromolecules like nucleic acids and proteins.
Recall that diffracted electromagnetic waves undergo constructive and destructive interference. This produces interference patterns, or diffraction patterns, showing the varying intensity of the diffracted waves at different points in space.
X-rays are diffracted by the electrons of atoms if the atoms are regularly spaced and the X-ray wavelength is similar to the interatomic distance.
When X-rays diffract from atoms in different planes, the diffracted waves may or may not be in phase. This depends on the interplanar spacing, d, and the angle at which the X-rays struck the atoms, or the incidence angle, theta.
This is because the paths that the X-rays take from the source to the detector have different lengths. If the path difference is an integer multiple of the wavelength of the X-rays, then the X-rays constructively interfere.
This results in the pattern of regularly spaced spots of diffracted waves observed by the Braggs, where each spot represents a diffraction angle resulting in constructive interference.
The relationship between the angle of diffraction, the interplanar spacing, and the X-ray wavelength is expressed with Bragg’s equation. This relationship provides information about the underlying highly ordered arrangement of the atoms in the crystal.
Ultimately, the lattice parameters can be derived from this information via a series of calculations. Modern instruments collect diffraction patterns from many different orientations and use the patterns and spot intensities to identify the crystal structure that is most likely to produce the observed combination of results.
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Q1: What is X-ray crystallography and how does it determine crystal structure?
X-ray crystallography uses diffraction patterns produced when X-rays strike crystalline solids at specific angles to determine their atomic structures. When X-rays encounter regularly spaced atoms, they diffract and create interference patterns showing spots of varying intensity. By analyzing these diffraction patterns from multiple orientations and applying Bragg's equation, scientists can identify the crystal structure and derive lattice parameters that reveal the precise arrangement of atoms in the crystal.
Q2: Why do X-rays diffract when they pass through crystals?
X-rays diffract through crystals because their wavelengths are comparable to the distances between atoms in the crystal lattice, typically a few angstroms. When X-ray beams encounter atoms in different crystal planes, the waves scatter in all directions. The scattered waves then undergo interference, combining to produce either increased or decreased intensity depending on whether the path differences result in constructive or destructive interference patterns.
Q3: What is Bragg's equation and what does it describe?
Bragg's equation, expressed as nλ = 2d sin θ, describes the relationship between the X-ray wavelength (λ), interplanar spacing (d), diffraction angle (θ), and an integer factor (n). This equation shows that constructive interference occurs when the path difference between X-rays scattered from adjacent crystal planes equals an integer multiple of the wavelength. The equation enables scientists to calculate atomic spacing and crystal structure from measured diffraction angles.
Q4: How do constructive and destructive interference create diffraction patterns?
When X-rays scatter from atoms in different crystal planes, the diffracted waves may combine constructively or destructively depending on their path lengths. If the path difference equals an integer multiple of the X-ray wavelength, the waves constructively interfere, producing bright spots in the diffraction pattern. Conversely, when path differences are not integer multiples, destructive interference occurs, creating dark regions. This combination of bright and dark spots reveals the underlying atomic arrangement.
Q5: What information can scientists extract from X-ray diffraction spot intensities?
The intensities of diffraction spots in X-ray crystallography provide crucial information about the electron density and atomic positions within the crystal. Modern instruments collect diffraction patterns from many orientations and use both the spot positions and their intensities to identify which crystal structure is most likely to produce the observed combination of results. This data allows researchers to determine the precise three-dimensional arrangement of atoms in complex structures like proteins and nucleic acids.
Q6: How did the Braggs' discovery lead to modern crystallography?
In 1913, William Henry Bragg and William Lawrence Bragg observed that X-rays striking crystalline solids at certain angles produced regularly spaced diffraction spots. This observation led them to develop X-ray crystallography and formulate Bragg's equation, explaining the mathematical relationship governing diffraction. Their groundbreaking work, which earned them the Nobel Prize in Physics in 1915, established the foundation for determining structures of crystalline solids ranging from simple ionic compounds to complex macromolecules.
Q7: What role does interplanar spacing play in X-ray diffraction?
Interplanar spacing (d) is the distance between adjacent atomic planes in a crystal and is a critical variable in Bragg's equation. The spacing determines which X-ray wavelengths and diffraction angles will produce constructive interference. Different crystal structures have characteristic interplanar spacings, so by measuring diffraction angles and applying Bragg's equation, scientists can calculate the spacing values and use them to identify the crystal's lattice parameters and atomic arrangement.