ment of the other three groups is in
descending priority, the compound
belongs to the r-series; if the descending order is anticlockwise it is
in the s-series. d-(+)-glyceraldehyde is
r-(+)-glyceraldehyde. See illustration.
absolute temperature See absolute; temperature.
absolute zero Zero of thermodynamic *temperature (0 kelvin) and
the lowest temperature theoretically
attainable. It is the temperature at
which the kinetic energy of atoms
and molecules is minimal. It is equivalent to –273.15°C or –459.67°F. See
also zero-point energy.
absorption 1. (in chemistry) The
take up of a gas by a solid or liquid,
or the take up of a liquid by a solid.
Absorption differs from adsorption
in that the absorbed substance permeates the bulk of the absorbing
substance. 2. (in physics) The conversion of the energy of electromagnetic
radiation, sound, streams of particles,
etc., into other forms of energy on
passing through a medium. A beam
of light, for instance, passing
through a medium, may lose intensity because of two effects: scattering
of light out of the beam, and absorption of photons by atoms or molecules in the medium. When a
photon is absorbed, there is a transition to an excited state.
absorption coefÜcient 1. (in
spectroscopy) The molar absorption
coefÜcient (symbol ε) is a quantity
that characterizes the absorption of
light (or any other type of electromagnetic radiation) as it passes
through a sample of the absorbing
material. It has the dimensions of
1/(concentration × length). ε is dependent on the frequency of the incident light; its highest value occurs
where the absorption is most intense. Since absorption bands usually
spread over a range of values of the
frequency ν it is useful to deÜne a
quantity called the integrated absorption coefÜcient, A, which is the
integral of all the absorption coefÜcients in the band, i.e. A = ∫ε(ν)dν.
This quantity characterizes the intensity of a transition. It was formerly
called the extinction coefÜcient. See
also beer–lambert law. 2. The volume of a given gas, measured at standard temperature and pressure, that
will dissolve in unit volume of a
given liquid.
absorption indicator See adsorption indicator.
absorption spectrum See spectrum.
absorption tower A long vertical
column used in industry for absorbing gases. The gas is introduced at
the bottom of the column and the
absorbing liquid, often water, passes
in at the top and falls down against
the countercurrent of gas. The towers are also known as scrubbers.
ABS plastic Any of a class of
plastics based on acrylonitrile–
butadiene–styrene copolymers.
abstraction A chemical reaction
that involves bimolecular removal of
an atom or ion from a molecule. An
example is the abstraction of hydrogen from methane by reaction with a
radical:
CH 4 + X . → H 3 C . + HX.
abundance 1. The ratio of the total
mass of a speciÜed element in the
earth’s crust to the total mass of the
earth’s crust, often expressed as a
percentage. For example, the abundance of aluminium in the earth’s
crust is about 8%. 2. The ratio of the
number of atoms of a particular isotope of an element to the total number of atoms of all the isotopes
present, often expressed as a percent3
abundance
a
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descending priority, the compound
belongs to the r-series; if the descending order is anticlockwise it is
in the s-series. d-(+)-glyceraldehyde is
r-(+)-glyceraldehyde. See illustration.
absolute temperature See absolute; temperature.
absolute zero Zero of thermodynamic *temperature (0 kelvin) and
the lowest temperature theoretically
attainable. It is the temperature at
which the kinetic energy of atoms
and molecules is minimal. It is equivalent to –273.15°C or –459.67°F. See
also zero-point energy.
absorption 1. (in chemistry) The
take up of a gas by a solid or liquid,
or the take up of a liquid by a solid.
Absorption differs from adsorption
in that the absorbed substance permeates the bulk of the absorbing
substance. 2. (in physics) The conversion of the energy of electromagnetic
radiation, sound, streams of particles,
etc., into other forms of energy on
passing through a medium. A beam
of light, for instance, passing
through a medium, may lose intensity because of two effects: scattering
of light out of the beam, and absorption of photons by atoms or molecules in the medium. When a
photon is absorbed, there is a transition to an excited state.
absorption coefÜcient 1. (in
spectroscopy) The molar absorption
coefÜcient (symbol ε) is a quantity
that characterizes the absorption of
light (or any other type of electromagnetic radiation) as it passes
through a sample of the absorbing
material. It has the dimensions of
1/(concentration × length). ε is dependent on the frequency of the incident light; its highest value occurs
where the absorption is most intense. Since absorption bands usually
spread over a range of values of the
frequency ν it is useful to deÜne a
quantity called the integrated absorption coefÜcient, A, which is the
integral of all the absorption coefÜcients in the band, i.e. A = ∫ε(ν)dν.
This quantity characterizes the intensity of a transition. It was formerly
called the extinction coefÜcient. See
also beer–lambert law. 2. The volume of a given gas, measured at standard temperature and pressure, that
will dissolve in unit volume of a
given liquid.
absorption indicator See adsorption indicator.
absorption spectrum See spectrum.
absorption tower A long vertical
column used in industry for absorbing gases. The gas is introduced at
the bottom of the column and the
absorbing liquid, often water, passes
in at the top and falls down against
the countercurrent of gas. The towers are also known as scrubbers.
ABS plastic Any of a class of
plastics based on acrylonitrile–
butadiene–styrene copolymers.
abstraction A chemical reaction
that involves bimolecular removal of
an atom or ion from a molecule. An
example is the abstraction of hydrogen from methane by reaction with a
radical:
CH 4 + X . → H 3 C . + HX.
abundance 1. The ratio of the total
mass of a speciÜed element in the
earth’s crust to the total mass of the
earth’s crust, often expressed as a
percentage. For example, the abundance of aluminium in the earth’s
crust is about 8%. 2. The ratio of the
number of atoms of a particular isotope of an element to the total number of atoms of all the isotopes
present, often expressed as a percent3
abundance
a
www.AzShimi.ir www.AzShimi.com