ecules or crystal lattices at low tem-
peratures can be regarded as good ap-
proximations to harmonic oscillators
in quantum mechanics. Even if a sys-
tem is not exactly a harmonic oscilla-
tor the solution of the harmonic
oscillator is frequently a useful start-
ing point for treating such systems
using *perturbation theory. Compare
anharmonic oscillator.
harpoon mechanismA mecha-
nism suggested to explain the reac-
tion between such metal atoms as
sodium or potassium and such halo-
gen molecules as bromine, e.g. K +
Br 2 →KBr + Br. As the alkali atom ap-
proaches the bromine molecule its
valence electron moves to the
bromine molecule (thus providing a
‘harpoon’). There are then two ions
with a Coulomb attraction between
them. As a result the ions move to-
gether and the reaction takes place.
This mechanism, which has been
worked out quantitatively, explains
why the reaction occurs far more
readily than might be expected tak-
ing into account only mechanical col-
lisions between the alkali-metal
atoms and halogen molecules.
Hartree–Fock procedure A self-
consistentÜeld (SCF) procedure used
toÜnd approximate *wave functions
and energy levels in many-electron
atoms. This procedure was intro-
duced by the English mathematician
and physicist Douglas Hartree in
1928 and improved by the Soviet
physicist Vladimir Fock in 1930 (by
taking into account the Pauli exclu-
sion principle). The initial wave func-
tions can be taken to be hydrogenic
atomic orbitals. The resulting equa-
tions can be solved numerically using
a computer. The results of the
Hartree–Fock theory are sufÜciently
accurate to show that electron den-
sity occurs in shells around atoms
and can be used quantitatively to
show chemical periodicity.hashishSee cannabis.hassiumSymbol Hs. A radioactive
*transactinide element; a.n. 108. It
wasÜrst made in 1984 by Peter Arm-
bruster and a team in Darmstadt,
Germany. It can be produced by bom-
barding lead-208 nuclei with iron-58
nuclei. Only a few atoms have ever
been produced. The name comes
from the Latinized form of Hesse, the
German state where it wasÜrst syn-
thesized.
A- Information from the WebElements site
hazchem code (emergency action
code; EAC)A code designed to be dis-
played when hazardous chemicals
are transported or stored in bulk. It is
used to help the emergency services
to take action quickly in any acci-
dent. The code consists of a number
followed by one or two letters. The
number indicates the type of sub-
stance to be used in treating the acci-
dent (e.g. stream of water,Üne spray,
foam, dry agent). TheÜrst letter indi-
cates the type of protective clothing
needed along with information about
the possibility of violent reaction on
whether the substance should be
contained or diluted. The second let-
ter, where it exists, is letter E, indi-
cating that people have to be
evacuated from the neighbourhood
of the incident. In the UK, the code is
usually displayed as part of a panel,
which includes an international UN
number for the substance, a tele-
phone number for specialist advice,
the company name, and a symbol in-
dicating the danger (e.g., a skull and
crossbones for toxic substances).HCFC(hydrochloroÛuorocarbon) See
halocarbons.265 HCFC
h