ecules or crystal lattices at low temperatures can be regarded as good approximations to harmonic oscillators
in quantum mechanics. Even if a system is not exactly a harmonic oscillator the solution of the harmonic
oscillator is frequently a useful starting point for treating such systems
using *perturbation theory. Compare
anharmonic oscillator.
harpoon mechanism A mechanism suggested to explain the reaction between such metal atoms as
sodium or potassium and such halogen molecules as bromine, e.g. K +
Br 2 → KBr + Br. As the alkali atom approaches 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 together 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 taking into account only mechanical collisions between the alkali-metal
atoms and halogen molecules.
Hartree–Fock procedure A selfconsistent Üeld (SCF) procedure used
to Ünd approximate *wave functions
and energy levels in many-electron
atoms. This procedure was introduced 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 exclusion principle). The initial wave functions can be taken to be hydrogenic
atomic orbitals. The resulting equations can be solved numerically using
a computer. The results of the
Hartree–Fock theory are sufÜciently
accurate to show that electron density occurs in shells around atoms
and can be used quantitatively to
show chemical periodicity.
hashish See cannabis.
hassium Symbol Hs. A radioactive
*transactinide element; a.n. 108. It
was Ürst made in 1984 by Peter Armbruster and a team in Darmstadt,
Germany. It can be produced by bombarding 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 was Ürst synthesized.
A
• Information from the WebElements site
hazchem code (emergency action
code; EAC) A code designed to be displayed when hazardous chemicals
are transported or stored in bulk. It is
used to help the emergency services
to take action quickly in any accident. The code consists of a number
followed by one or two letters. The
number indicates the type of substance to be used in treating the accident (e.g. stream of water, Üne spray,
foam, dry agent). The Ürst letter indicates 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 letter, where it exists, is letter E, indicating 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 telephone number for specialist advice,
the company name, and a symbol indicating the danger (e.g., a skull and
crossbones for toxic substances).
HCFC (hydrochloroÛuorocarbon) See
halocarbons.
265
HCFC
h
www.AzShimi.ir www.AzShimi.com
in quantum mechanics. Even if a system is not exactly a harmonic oscillator the solution of the harmonic
oscillator is frequently a useful starting point for treating such systems
using *perturbation theory. Compare
anharmonic oscillator.
harpoon mechanism A mechanism suggested to explain the reaction between such metal atoms as
sodium or potassium and such halogen molecules as bromine, e.g. K +
Br 2 → KBr + Br. As the alkali atom approaches 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 together 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 taking into account only mechanical collisions between the alkali-metal
atoms and halogen molecules.
Hartree–Fock procedure A selfconsistent Üeld (SCF) procedure used
to Ünd approximate *wave functions
and energy levels in many-electron
atoms. This procedure was introduced 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 exclusion principle). The initial wave functions can be taken to be hydrogenic
atomic orbitals. The resulting equations can be solved numerically using
a computer. The results of the
Hartree–Fock theory are sufÜciently
accurate to show that electron density occurs in shells around atoms
and can be used quantitatively to
show chemical periodicity.
hashish See cannabis.
hassium Symbol Hs. A radioactive
*transactinide element; a.n. 108. It
was Ürst made in 1984 by Peter Armbruster and a team in Darmstadt,
Germany. It can be produced by bombarding 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 was Ürst synthesized.
A
• Information from the WebElements site
hazchem code (emergency action
code; EAC) A code designed to be displayed when hazardous chemicals
are transported or stored in bulk. It is
used to help the emergency services
to take action quickly in any accident. The code consists of a number
followed by one or two letters. The
number indicates the type of substance to be used in treating the accident (e.g. stream of water, Üne spray,
foam, dry agent). The Ürst letter indicates 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 letter, where it exists, is letter E, indicating 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 telephone number for specialist advice,
the company name, and a symbol indicating the danger (e.g., a skull and
crossbones for toxic substances).
HCFC (hydrochloroÛuorocarbon) See
halocarbons.
265
HCFC
h
www.AzShimi.ir www.AzShimi.com