angstrom Symbol Å. A unit of
length equal to 10
–10 metre. It was
formerly used to measure wavelengths and intermolecular distances
but has now been replaced by the
nanometre. 1 Å = 0.1 nanometre. The
unit is named after A. J. *Ångström.
Angström, Anders Jonas (1814–
74) Swedish astronomer and physicist who became professor of physics
at the University of Uppsala from
1858 until his death. He worked
mainly with emission *spectra,
demonstrating the presence of hydrogen in the sun. He also worked out
the wavelengths of *Fraunhofer lines.
Since 1905 spectral wavelengths
have been expressed in *angstroms.
angular momentum A property
of a rotating body. In the case of a
rigid rotating body the angular momentum is given by Iω, where I is the
*moment of inertia of the body and
ω is its angular velocity. The quantum theory of angular momentum is
closely associated with the *rotation
group and has important applications in the electronic structure of
atoms and diatomic molecules and
*rotational spectroscopy of molecules. Electron spin is a type of angular momentum.
anharmonicity The extent to
which the oscillation of an oscillator
differs from simple harmonic motion. In molecular vibrations the anharmonicity is very small near the
equilibrium position, becomes large
as the vibration moves away from
the equilibrium position, and is very
large as dissociation is approached.
Anharmonicity is taken into account
in molecular vibrations by adding an
anharmonicity term to the potential
energy function of the molecule.
For a harmonic oscillator the potential energy function U is given by
U = f(r – r e )
2 where r is the interatomic distance, r e is the equilibrium
interatomic distance, and f is a constant. Anharmonicity is taken into
account by adding a cubic term g(r –
r e )
3 to the quadratic term, where g is
much smaller than f. Higher terms in
(r – r e ) can be added to improve the
description of anharmonicity.
anharmonic oscillator An oscillating system (in either classical mechanics or *quantum mechanics)
that is not oscillating in simple harmonic motion. In general, the problem of an anharmonic oscillator is
not exactly soluble, although many
systems approximate to harmonic oscillators and for such systems the
*anharmonicity can be calculated
using *perturbation theory.
anhydride A compound that produces a given compound on reaction
with water. For instance, sulphur trioxide is the (acid) anhydride of sulphuric acid SO 3 + H 2 O → H 2 SO 4 .
See also acid anhydrides.
anhydrite An important rockforming anhydrous mineral form of
calcium sulphate, CaSO 4 . It is chemically similar to *gypsum but is
harder and heavier and crystallizes in
the rhombic form (gypsum is monoclinic). Under natural conditions anhydrite slowly hydrates to form
gypsum. It occurs chieÛy in white
and greyish granular masses and is
often found in the caprock of certain
salt domes. It is used as a raw material in the chemical industry and in
the manufacture of cement and fertilizers.
anhydrous Denoting a chemical
compound lacking water: applied
particularly to salts lacking their
water of crystallization.
aniline See phenylamine.
anilinium ion The ion C 6 H 5 NH 3
+ ,
derived from *phenylamine.
37
anilinium ion
a
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length equal to 10
–10 metre. It was
formerly used to measure wavelengths and intermolecular distances
but has now been replaced by the
nanometre. 1 Å = 0.1 nanometre. The
unit is named after A. J. *Ångström.
Angström, Anders Jonas (1814–
74) Swedish astronomer and physicist who became professor of physics
at the University of Uppsala from
1858 until his death. He worked
mainly with emission *spectra,
demonstrating the presence of hydrogen in the sun. He also worked out
the wavelengths of *Fraunhofer lines.
Since 1905 spectral wavelengths
have been expressed in *angstroms.
angular momentum A property
of a rotating body. In the case of a
rigid rotating body the angular momentum is given by Iω, where I is the
*moment of inertia of the body and
ω is its angular velocity. The quantum theory of angular momentum is
closely associated with the *rotation
group and has important applications in the electronic structure of
atoms and diatomic molecules and
*rotational spectroscopy of molecules. Electron spin is a type of angular momentum.
anharmonicity The extent to
which the oscillation of an oscillator
differs from simple harmonic motion. In molecular vibrations the anharmonicity is very small near the
equilibrium position, becomes large
as the vibration moves away from
the equilibrium position, and is very
large as dissociation is approached.
Anharmonicity is taken into account
in molecular vibrations by adding an
anharmonicity term to the potential
energy function of the molecule.
For a harmonic oscillator the potential energy function U is given by
U = f(r – r e )
2 where r is the interatomic distance, r e is the equilibrium
interatomic distance, and f is a constant. Anharmonicity is taken into
account by adding a cubic term g(r –
r e )
3 to the quadratic term, where g is
much smaller than f. Higher terms in
(r – r e ) can be added to improve the
description of anharmonicity.
anharmonic oscillator An oscillating system (in either classical mechanics or *quantum mechanics)
that is not oscillating in simple harmonic motion. In general, the problem of an anharmonic oscillator is
not exactly soluble, although many
systems approximate to harmonic oscillators and for such systems the
*anharmonicity can be calculated
using *perturbation theory.
anhydride A compound that produces a given compound on reaction
with water. For instance, sulphur trioxide is the (acid) anhydride of sulphuric acid SO 3 + H 2 O → H 2 SO 4 .
See also acid anhydrides.
anhydrite An important rockforming anhydrous mineral form of
calcium sulphate, CaSO 4 . It is chemically similar to *gypsum but is
harder and heavier and crystallizes in
the rhombic form (gypsum is monoclinic). Under natural conditions anhydrite slowly hydrates to form
gypsum. It occurs chieÛy in white
and greyish granular masses and is
often found in the caprock of certain
salt domes. It is used as a raw material in the chemical industry and in
the manufacture of cement and fertilizers.
anhydrous Denoting a chemical
compound lacking water: applied
particularly to salts lacking their
water of crystallization.
aniline See phenylamine.
anilinium ion The ion C 6 H 5 NH 3
+ ,
derived from *phenylamine.
37
anilinium ion
a
www.AzShimi.ir www.AzShimi.com