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 ac-
tivity.
asymmetric inductionThe pref-
erential formation of one particular
enantiomer or diastereoisomer in a
reaction as a result of a chiral el-
ement in one of the reactants or in
the catalyst used.
asymmetric top See moment of
inertia.
atactic polymerSee polymer.
ATLEED (automated tensor low-
energy electron diffraction) A form
of *LEED (low-energy electron dif-
fraction) in which the information
obtained can readily be stored and
analysed using a computer.
atmolysisThe separation of a mix-
ture of gases by means of their differ-
ent 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 ac-
tual *atmospheric pressureÛuctuates
around this value. The unit is usually
used for expressing pressures well in
excess of standard atmospheric pres-
sure, e.g. in high-pressure chemical
processes. 2.See earth’s atmos-
phere.
atmospheric pressure The pres-
sure 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 stan-
dard value for the atmospheric pres-
sure at sea level in SI units is 101 325
pascals.atomThe smallest part of an el-
ement that can exist chemically.
Atoms consist of a small dense nu-
cleus of protons and neutrons sur-
rounded by moving electrons. The
number of electrons equals the num-
ber of protons so the overall charge
is zero. The electrons are considered
to move in circular or elliptical orbits
(see bohr theory) or, more accu-
rately, in regions of space around the
nucleus (see orbital).
The electronic structure of an atom
refers to the way in which the elec-
trons 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 correspond-
ing to them, are referred to as shells
and given letters K, L, M, etc. The K-
shell 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 momen-
tum of the electron. The possible val-
ues of l are (n – 1), (n – 2), ... 1, 0.
Thus, in theÜrst shell (n = 1) the elec-
trons can only have angular momen-
tum zero (l = 0). In the second shell (n
= 2), the values of l can be 1 or 0, giv-
ing rise to two subshellsof slightly
different energy. In the third shell (n
= 3) there are three subshells, with l
= 2, 1, or 0. The subshells are de-
noted by letters s(l = 0), p(l = 1), d(l =
2), f(l = 3). The number of electrons in47 atom
a