(C 2 H 5 COOH), etc., form a homolo-
gous series in which each member
differs from the next by CH 2. Succes-
sive members of such a series are
called homologues.
homolyticÜssionThe breaking of
a bond in a compound in which the
fragments are uncharged free radi-
cals. For example, Cl 2 →Cl.+ Cl..
Compare heterolytic fission.
homonuclearDenoting a molecule
in which the atoms are of the same
element.
homopolar bondSee chemical
bond.
homopolymer See polymer.
hormone A substance that is man-
ufactured and secreted in very small
quantities into the bloodstream by
an endocrine gland or a specialized
nerve cell and regulates the growth
or functioning of a speciÜc tissue or
organ in a distant part of the body.
For example, the hormone insulin
controls the rate and manner in
which glucose is used by the body.
hornblende Any of a group of
common rock-forming minerals
of the amphibole group with
the generalized formula
(Ca,Na) 2 (Mg,Fe,Al) 5 (Al,Si) 8 O 22 (OH,F) 2.
Hornblendes consist mainly of cal-
cium, iron, and magnesium silicate.
host–guest chemistry See
supramolecular chemistry.
HPLCSee high-performance liquid
chromatography.
HRELSSee high-resolution elec-
tron-loss spectroscopy.
HSAB principleA method of classi-
fying Lewis acids and bases (See acid)
developed by Ralph Pearson in the
1960s. The acronym stands for ‘hard
and soft acids and bases’. It is based
in empirical measurements of stabil-
ity of compounds with certain lig-
ands. Hard acids tend to complex
with halide ions in the order
F–>Cl–>Br–>I–
Soft acids complex in the opposite
order. Compounds that complex
with hard acids are hard bases; ones
that more readily form complexes
with soft acids are called soft bases.
In general, soft acids and bases are
more easily polarized than hard acids
and bases and consequently have
more covalent character in the bond.
The idea is an extension of the *type
A and B metals concept to com-
pounds other than metal complexes.Hückel, Erich(1896–1980) German
physicist and theoretical chemist
who worked with Peter *Debye at
Zürich on the theory of electrolytes.
Later he moved to Copenhagen to
work with Niels *Bohr and here he
produced his work on bonding in
aromatic molecules. See hückel ap-
proximation; aromatic compound.
Hückel approximation A set of
approximations used to simplify the
molecular orbital analysis of conju-
gated molecules, suggested by Erich
Hückel in 1931. In the Hückel theory,
the σorbitals are treated separately
from the πorbitals, the shape of the
molecule being determined by the σ
orbitals. The Hückel theory makes
the approximation that interactions
between non-neighbouring atoms
are taken to be zero. It enables calcu-
lations to be made for conjugated
molecules. In particular, the *delocal-
ization energy of such molecules can
be estimated. Hückel explained the
stability of benzene associated with
aromaticity in this way. The theory
can also be used to analyse delocal-
ized bonding in solids.Hückel ruleSee aromatic com-
pound.humectantA substance used to273 humectant
h