and absorption occurs only in the
surface layer. ATRS is used in forensic science for analysis of thin layers
(e.g. paint).
atto- Symbol a. A preÜx used in the
metric system to denote 10
–18
. For example, 10
–18 second = 1 attosecond
(as).
attractor The set of points in phase
space to which the representative
point of a dissipative system (i.e. one
with internal friction) tends as the
system evolves. The attractor can be:
a single point; a closed curve (a limit
cycle), which describes a system with
periodic behaviour; or a strange attractor, in which case the system exhibits *chaos.
Aufbau principle A principle that
gives the order in which orbitals are
Ülled in successive elements in the
periodic table. The order of Ülling is
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,
6s, 4f, 5d, 6p, 7s, 5f, 6d. See atom.
Auger effect The ejection of an
electron from an atom as a result of
the de-excitation of an excited electron within the atom. An electron is
Ürst ejected from an atom by a photon, electron impact, ion impact, or
some other process, thus creating a
vacancy. In the subsequent rearrangement of the electronic structure of the atom, an electron from a
higher energy level falls into the vacancy. This process is associated with
excess energy, which is released by
the ejection of a second electron
(rather than by emission of a photon). This second electron is called
the Auger electron. Auger spectroscopy is a form of electron spectroscopy using this effect to study the
energy levels of ions. It is also a form
of analysis and can be used to identify the presence of elements in surface layers of solids. The effect was
discovered by the French physicist
Pierre Auger (1899–1994) in 1925.
Auger electron See auger effect.
auric compounds Compounds of
gold in its higher (+3) oxidation state;
e.g. auric chloride is gold(III) chloride
(AuCl 3 ).
aurous compounds Compounds
of gold in its lower (+1) oxidation
state; e.g. aurous chloride is gold(I)
chloride (AuCl).
austenite See steel.
autocatalysis *Catalysis in which
one of the products of the reaction is
a catalyst for the reaction. Reactions
in which autocatalysis occurs have a
characteristic S-shaped curve for reaction rate against time – the reaction starts slowly and increases as
the amount of catalyst builds up,
falling off again as the products are
used up.
autoclave A strong steel vessel
used for carrying out chemical reactions, sterilizations, etc., at high temperature and pressure.
automated tensor low-energy
electron diffraction See atleed.
autoprotolysis A transfer of a hydrogen ion (H
+
) between molecules of
an amphiprotic *solvent, one molecule acting as a Brønsted acid and
the other as a Brønsted base. It occurs in the autoionization of water.
autoprotolysis constant See
ionic product.
autoradiography An experimental technique in which a radioactive
specimen is placed in contact with
(or close to) a photographic plate, so
as to produce a record of the distribution of radioactivity in the specimen.
The Ülm is darkened by the ionizing
radiation from radioactive parts of
the sample. Autoradiography has a
51
autoradiography
a
www.AzShimi.ir www.AzShimi.com
surface layer. ATRS is used in forensic science for analysis of thin layers
(e.g. paint).
atto- Symbol a. A preÜx used in the
metric system to denote 10
–18
. For example, 10
–18 second = 1 attosecond
(as).
attractor The set of points in phase
space to which the representative
point of a dissipative system (i.e. one
with internal friction) tends as the
system evolves. The attractor can be:
a single point; a closed curve (a limit
cycle), which describes a system with
periodic behaviour; or a strange attractor, in which case the system exhibits *chaos.
Aufbau principle A principle that
gives the order in which orbitals are
Ülled in successive elements in the
periodic table. The order of Ülling is
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p,
6s, 4f, 5d, 6p, 7s, 5f, 6d. See atom.
Auger effect The ejection of an
electron from an atom as a result of
the de-excitation of an excited electron within the atom. An electron is
Ürst ejected from an atom by a photon, electron impact, ion impact, or
some other process, thus creating a
vacancy. In the subsequent rearrangement of the electronic structure of the atom, an electron from a
higher energy level falls into the vacancy. This process is associated with
excess energy, which is released by
the ejection of a second electron
(rather than by emission of a photon). This second electron is called
the Auger electron. Auger spectroscopy is a form of electron spectroscopy using this effect to study the
energy levels of ions. It is also a form
of analysis and can be used to identify the presence of elements in surface layers of solids. The effect was
discovered by the French physicist
Pierre Auger (1899–1994) in 1925.
Auger electron See auger effect.
auric compounds Compounds of
gold in its higher (+3) oxidation state;
e.g. auric chloride is gold(III) chloride
(AuCl 3 ).
aurous compounds Compounds
of gold in its lower (+1) oxidation
state; e.g. aurous chloride is gold(I)
chloride (AuCl).
austenite See steel.
autocatalysis *Catalysis in which
one of the products of the reaction is
a catalyst for the reaction. Reactions
in which autocatalysis occurs have a
characteristic S-shaped curve for reaction rate against time – the reaction starts slowly and increases as
the amount of catalyst builds up,
falling off again as the products are
used up.
autoclave A strong steel vessel
used for carrying out chemical reactions, sterilizations, etc., at high temperature and pressure.
automated tensor low-energy
electron diffraction See atleed.
autoprotolysis A transfer of a hydrogen ion (H
+
) between molecules of
an amphiprotic *solvent, one molecule acting as a Brønsted acid and
the other as a Brønsted base. It occurs in the autoionization of water.
autoprotolysis constant See
ionic product.
autoradiography An experimental technique in which a radioactive
specimen is placed in contact with
(or close to) a photographic plate, so
as to produce a record of the distribution of radioactivity in the specimen.
The Ülm is darkened by the ionizing
radiation from radioactive parts of
the sample. Autoradiography has a
51
autoradiography
a
www.AzShimi.ir www.AzShimi.com