chemistry is the way in which these
molecules are formed and the way
in which they interact with clouds of
interstellar dust.
A
• The website of the astrochemistry work
group of the International Astronomical
Union
asymmetric atom See optical activity.
asymmetric induction The preferential formation of one particular
enantiomer or diastereoisomer in a
reaction as a result of a chiral element in one of the reactants or in
the catalyst used.
asymmetric top See moment of
inertia.
atactic polymer See polymer.
ATLEED (automated tensor lowenergy electron diffraction) A form
of *LEED (low-energy electron diffraction) in which the information
obtained can readily be stored and
analysed using a computer.
atmolysis The separation of a mixture of gases by means of their different rates of diffusion. Usually,
separation is effected by allowing the
gases to diffuse through the walls of
a porous partition or membrane.
atmosphere 1. (atm.) A unit of
pressure equal to 101 325 pascals.
This is equal to 760.0 mmHg. The actual *atmospheric pressure Ûuctuates
around this value. The unit is usually
used for expressing pressures well in
excess of standard atmospheric pressure, e.g. in high-pressure chemical
processes. 2. See earth’s atmosphere.
atmospheric pressure The pressure exerted by the weight of the air
above it at any point on the earth’s
surface. At sea level the atmosphere
will support a column of mercury
about 760 mm high. This decreases
with increasing altitude. The standard value for the atmospheric pressure at sea level in SI units is 101 325
pascals.
atom The smallest part of an element that can exist chemically.
Atoms consist of a small dense nucleus of protons and neutrons surrounded by moving electrons. The
number of electrons equals the number of protons so the overall charge
is zero. The electrons are considered
to move in circular or elliptical orbits
(see bohr theory) or, more accurately, in regions of space around the
nucleus (see orbital).
The electronic structure of an atom
refers to the way in which the electrons are arranged about the nucleus,
and in particular the *energy levels
that they occupy. Each electron can
be characterized by a set of four
quantum numbers, as follows:
(1) The principal quantum number n
gives the main energy level and has
values 1, 2, 3, etc. (the higher the
number, the further the electron
from the nucleus). Traditionally,
these levels, or the orbits corresponding to them, are referred to as shells
and given letters K, L, M, etc. The Kshell is the one nearest the nucleus.
The maximum number of electrons
in a given shell is 2n
2 .
(2) The orbital quantum number l,
which governs the angular momentum of the electron. The possible values of l are (n – 1), (n – 2), … 1, 0.
Thus, in the Ürst shell (n = 1) the electrons can only have angular momentum zero (l = 0). In the second shell (n
= 2), the values of l can be 1 or 0, giving rise to two subshells of slightly
different energy. In the third shell (n
= 3) there are three subshells, with l
= 2, 1, or 0. The subshells are denoted by letters s(l = 0), p(l = 1), d(l =
2), f(l = 3). The number of electrons in
47
atom
a
www.AzShimi.ir www.AzShimi.com
molecules are formed and the way
in which they interact with clouds of
interstellar dust.
A
• The website of the astrochemistry work
group of the International Astronomical
Union
asymmetric atom See optical activity.
asymmetric induction The preferential formation of one particular
enantiomer or diastereoisomer in a
reaction as a result of a chiral element in one of the reactants or in
the catalyst used.
asymmetric top See moment of
inertia.
atactic polymer See polymer.
ATLEED (automated tensor lowenergy electron diffraction) A form
of *LEED (low-energy electron diffraction) in which the information
obtained can readily be stored and
analysed using a computer.
atmolysis The separation of a mixture of gases by means of their different rates of diffusion. Usually,
separation is effected by allowing the
gases to diffuse through the walls of
a porous partition or membrane.
atmosphere 1. (atm.) A unit of
pressure equal to 101 325 pascals.
This is equal to 760.0 mmHg. The actual *atmospheric pressure Ûuctuates
around this value. The unit is usually
used for expressing pressures well in
excess of standard atmospheric pressure, e.g. in high-pressure chemical
processes. 2. See earth’s atmosphere.
atmospheric pressure The pressure exerted by the weight of the air
above it at any point on the earth’s
surface. At sea level the atmosphere
will support a column of mercury
about 760 mm high. This decreases
with increasing altitude. The standard value for the atmospheric pressure at sea level in SI units is 101 325
pascals.
atom The smallest part of an element that can exist chemically.
Atoms consist of a small dense nucleus of protons and neutrons surrounded by moving electrons. The
number of electrons equals the number of protons so the overall charge
is zero. The electrons are considered
to move in circular or elliptical orbits
(see bohr theory) or, more accurately, in regions of space around the
nucleus (see orbital).
The electronic structure of an atom
refers to the way in which the electrons are arranged about the nucleus,
and in particular the *energy levels
that they occupy. Each electron can
be characterized by a set of four
quantum numbers, as follows:
(1) The principal quantum number n
gives the main energy level and has
values 1, 2, 3, etc. (the higher the
number, the further the electron
from the nucleus). Traditionally,
these levels, or the orbits corresponding to them, are referred to as shells
and given letters K, L, M, etc. The Kshell is the one nearest the nucleus.
The maximum number of electrons
in a given shell is 2n
2 .
(2) The orbital quantum number l,
which governs the angular momentum of the electron. The possible values of l are (n – 1), (n – 2), … 1, 0.
Thus, in the Ürst shell (n = 1) the electrons can only have angular momentum zero (l = 0). In the second shell (n
= 2), the values of l can be 1 or 0, giving rise to two subshells of slightly
different energy. In the third shell (n
= 3) there are three subshells, with l
= 2, 1, or 0. The subshells are denoted by letters s(l = 0), p(l = 1), d(l =
2), f(l = 3). The number of electrons in
47
atom
a
www.AzShimi.ir www.AzShimi.com