number of applications, particularly
in the study of living tissues and
cells.auxochrome A group in a dye mol-
ecule that inÛuences the colour due
to the *chromophore. Auxochromes
are groups, such as –OH and –NH 2 ,
containing lone pairs of electrons
that can be delocalized along with
the delocalized electrons of the chro-
mophore. The auxochrome inten-
siÜes the colour of the dye. Formerly,
the term was also used of such
groups as –SO 2 O–, which make the
molecule soluble and affect its appli-
cation.Avogadro, Amedeo (1776–1856)
Italian chemist and physicist. In 1811
he published his hypothesis (see avo-
gadro’s law), which provided a
method of calculating molecular
weights from vapour densities. The
importance of the work remained
unrecognized, however, until cham-
pioned by Stanislao Cannizzaro
(1826–1910) in 1860.Avogadro constant Symbol NAor
L. The number of atoms or molecules
in one *mole of substance. It has the
value 6.022 1367(36) × 1023. Formerly
it was called Avogadro’s number.
Avogadro’s law Equal volumes of
all gases contain equal numbers of
molecules at the same pressure and
temperature. The law, often called
Avogadro’s hypothesis, is true only
for ideal gases. It wasÜrst proposed
in 1811 by Amedeo *Avogadro.
axialSee ring conformations.Axilrod–Teller formula An ex-
pression for three-body interactions
in intermolecular forces. These di-
pole dispersion terms, called
Axilrod–Teller terms(named after
B. M. Axilrod and E. Teller, who dis-
covered them in 1943) are of impor-
tance in the third virial coefÜcient.At low temperatures, the Axilrod–
Teller terms can have a comparable
size to two-body interactions, making
them of importance to such proper-
ties of liquids as pressure and surface
tension. The Axilrod–Teller formula
for the dispersion energy V of three
closed-shell atoms A, B, and C is
given by
V = – C 6 /rAB^6 – C 6 /rBC^6 – C 6 /rCA^6 +
C′/(rABrBCrCA)^3 ,
where C 6 is a coefÜcient depending
on the identity of the atoms, C′is a
coefÜcient depending on another
constant and the angles between the
atoms, and the r terms give distances
between the indicated atoms.AX spectrumA general pattern of
the nuclear magnetic resonance spec-
trum of a molecule AX, where A and
X are both spin-½nuclei. If X is spin
α, the spin–spin interaction between
the nuclei of A and X results in one
line in the spectrum of A being
shifted by J/2 from the frequency of
precession it has in the absence of
coupling. If X is spin β, the change in
frequency of precession of A is –J/2.
Thus, single lines in the resonances
of both A and X are split into dou-
blets, with splittings J. The A reso-
nance in molecules of the type AnX is
also a doublet with the splitting J.azeotrope (azeotropic mixture; con-
stant-boiling mixture) A mixture of
two liquids that boils at constant
composition; i.e. the composition of
the vapour is the same as that of the
liquid. Azeotropes occur because of
deviations in Raoult’s law leading to
a maximum or minimum in the
*boiling-point–composition diagram.
When the mixture is boiled, the
vapour initially has a higher propor-
tion of one component than is pre-
sent in the liquid, so the proportion
of this in the liquid falls with time.
Eventually, the maximum and mini-auxochrome 52a