X-ray crystallography The use of
*X-ray diffraction to determine the
structure of crystals or molecules.
The technique involves directing a
beam of X-rays at a crystalline sample and recording the diffracted Xrays on a photographic plate. The
diffraction pattern consists of a pattern of spots on the plate, and the
crystal structure can be worked out
from the positions and intensities of
the diffraction spots. X-rays are diffracted by the electrons in the molecules and if molecular crystals of a
compound are used, the electron
density distribution in the molecule
can be determined.
X-ray diffraction The diffraction
of X-rays by a crystal. The wavelengths of X-rays are comparable in
size to the distances between atoms
in most crystals, and the repeated
pattern of the crystal lattice acts like
a diffraction grating for X-rays. Thus,
a crystal of suitable type can be used
to disperse X-rays in a spectrometer.
X-ray diffraction is also the basis of Xray crystallography.
X-ray Ûuorescence The emission
of *X-rays from excited atoms produced by the impact of high-energy
electrons, other particles, or a primary beam of other X-rays. The
wavelengths of the Ûuorescent X-rays
can be measured by an X-ray spectrometer as a means of chemical
analysis. X-ray Ûuorescence is used in
such techniques as electron-probe
microanalysis.
X-rays Electromagnetic radiation of
shorter wavelength than ultraviolet
radiation produced by bombardment
of atoms by high-quantum-energy
particles. The range of wavelengths is
10
–11 m to 10
–9 m. Atoms of all the elements emit a characteristic X-ray
spectrum when they are bombarded
by electrons. The X-ray photons are
emitted when the incident electrons
knock an inner orbital electron out
of an atom. When this happens an
outer electron falls into the inner
shell to replace it, losing potential
energy (∆E) in doing so. The wavelength λ of the emitted photon will
then be given by λ = ch/∆E, where c is
the speed of light and h is the Planck
constant.
X-rays can pass through many
forms of matter and they are therefore used medically and industrially
to examine internal structures. X-rays
are produced for these purposes by
an X-ray tube.
X-ray spectrum See x-rays.
xylenes See dimethylbenzenes.
xylenol (hydroxydimethylbenzene)
Any of six isomeric solid aromatic
compounds, C 6 H 3 (CH 3 ) 2 OH. An impure mixture of isomers is a liquid
made from coal tar and employed as
a solvent. The pure substances resemble *phenol in their reactions.
They are used to make thermosetting
polymer resins, and the chloroderivative of 1,2,5-xylenol is an industrial disinfectant.
X-ray crystallography
560
x
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*X-ray diffraction to determine the
structure of crystals or molecules.
The technique involves directing a
beam of X-rays at a crystalline sample and recording the diffracted Xrays on a photographic plate. The
diffraction pattern consists of a pattern of spots on the plate, and the
crystal structure can be worked out
from the positions and intensities of
the diffraction spots. X-rays are diffracted by the electrons in the molecules and if molecular crystals of a
compound are used, the electron
density distribution in the molecule
can be determined.
X-ray diffraction The diffraction
of X-rays by a crystal. The wavelengths of X-rays are comparable in
size to the distances between atoms
in most crystals, and the repeated
pattern of the crystal lattice acts like
a diffraction grating for X-rays. Thus,
a crystal of suitable type can be used
to disperse X-rays in a spectrometer.
X-ray diffraction is also the basis of Xray crystallography.
X-ray Ûuorescence The emission
of *X-rays from excited atoms produced by the impact of high-energy
electrons, other particles, or a primary beam of other X-rays. The
wavelengths of the Ûuorescent X-rays
can be measured by an X-ray spectrometer as a means of chemical
analysis. X-ray Ûuorescence is used in
such techniques as electron-probe
microanalysis.
X-rays Electromagnetic radiation of
shorter wavelength than ultraviolet
radiation produced by bombardment
of atoms by high-quantum-energy
particles. The range of wavelengths is
10
–11 m to 10
–9 m. Atoms of all the elements emit a characteristic X-ray
spectrum when they are bombarded
by electrons. The X-ray photons are
emitted when the incident electrons
knock an inner orbital electron out
of an atom. When this happens an
outer electron falls into the inner
shell to replace it, losing potential
energy (∆E) in doing so. The wavelength λ of the emitted photon will
then be given by λ = ch/∆E, where c is
the speed of light and h is the Planck
constant.
X-rays can pass through many
forms of matter and they are therefore used medically and industrially
to examine internal structures. X-rays
are produced for these purposes by
an X-ray tube.
X-ray spectrum See x-rays.
xylenes See dimethylbenzenes.
xylenol (hydroxydimethylbenzene)
Any of six isomeric solid aromatic
compounds, C 6 H 3 (CH 3 ) 2 OH. An impure mixture of isomers is a liquid
made from coal tar and employed as
a solvent. The pure substances resemble *phenol in their reactions.
They are used to make thermosetting
polymer resins, and the chloroderivative of 1,2,5-xylenol is an industrial disinfectant.
X-ray crystallography
560
x
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