Dirk Coster (1889–1950) and George
de Hevesey (1885–1966) in 1923.
A
• Information from the WebElements site
Hahn, Otto (1879–1968) German
chemist, who studied in London
(with William *Ramsay) and Canada
(with Ernest *Rutherford) before returning to Germany in 1907. In 1917,
together with Lise *Meitner, he discovered protactinium. In the late
1930s he collaborated with Fritz
Strassmann (1902–80) and in 1938
bombarded uranium with slow neutrons. Among the products was barium, but it was Meitner (now in
Sweden) who the next year interpreted the process as nuclear Üssion.
In 1944 Hahn received the Nobel
Prize for chemistry.
hahnium See transactinide elements.
half cell An electrode in contact
with a solution of ions, forming part
of a *cell. Various types of half cell
exist, the simplest consisting of a
metal electrode immersed in a solution of metal ions. Gas half cells have
a gold or platinum plate in a solution
with gas bubbled over the metal
plate. The commonest is the *hydrogen half cell. Half cells can also be
formed by a metal in contact with an
insoluble salt or oxide and a solution.
The *calomel half cell is an example
of this. Half cells are commonly referred to as electrodes.
half chair See ring conformations.
half-life See decay.
half sandwich See sandwich compound.
half-thickness The thickness of a
speciÜed material that reduces the intensity of a beam of radiation to half
its original value.
half-width Half the width of a
spectrum line (or in some cases the
full width) measured at half its height.
halide A compound of a halogen
with another element or group. The
halides of typical metals are ionic
(e.g. sodium Ûuoride, Na
+
F
– ). Metals
can also form halides in which the
bonding is largely covalent (e.g. aluminium chloride, AlCl 3 ). Organic compounds are also sometimes referred
to as halides; e.g. the alkyl halides
(see haloalkanes) and the *acyl
halides. Halides are named Ûuorides,
chlorides, bromides, or iodides.
halite (rock salt) Naturally occurring *sodium chloride (common salt,
NaCl), crystallizing in the cubic system. It is chieÛy colourless or white
(sometimes blue) when pure but the
presence of impurities may colour it
grey, pink, red, or brown. Halite
often occurs in association with anhydrite and gypsum.
Hall–Heroult cell An electrolytic
cell used industrially for the extraction of aluminium from bauxite. The
bauxite is Ürst puriÜed by dissolving
it in sodium hydroxide and Ültering
off insoluble constituents. Aluminium hydroxide is then precipitated (by adding CO 2 ) and this is
decomposed by heating to obtain
pure Al 2 O 3 . In the Hall–Heroult cell,
the oxide is mixed with cryolite (to
lower its melting point) and the
molten mixture electrolysed using
graphite anodes. The cathode is the
lining of the cell, also of graphite.
The electrolyte is kept in a molten
state (about 850°C) by the current.
Molten aluminium collects at the
bottom of the cell and can be tapped
off. Oxygen forms at the anode, and
gradually oxidizes it away. The cell is
named after the US chemist Charles
Martin Hall (1863–1914), who discovered the process in 1886, and the
French chemist Paul Heroult (1863–
Hahn, Otto
260
h
www.AzShimi.ir www.AzShimi.com
de Hevesey (1885–1966) in 1923.
A
• Information from the WebElements site
Hahn, Otto (1879–1968) German
chemist, who studied in London
(with William *Ramsay) and Canada
(with Ernest *Rutherford) before returning to Germany in 1907. In 1917,
together with Lise *Meitner, he discovered protactinium. In the late
1930s he collaborated with Fritz
Strassmann (1902–80) and in 1938
bombarded uranium with slow neutrons. Among the products was barium, but it was Meitner (now in
Sweden) who the next year interpreted the process as nuclear Üssion.
In 1944 Hahn received the Nobel
Prize for chemistry.
hahnium See transactinide elements.
half cell An electrode in contact
with a solution of ions, forming part
of a *cell. Various types of half cell
exist, the simplest consisting of a
metal electrode immersed in a solution of metal ions. Gas half cells have
a gold or platinum plate in a solution
with gas bubbled over the metal
plate. The commonest is the *hydrogen half cell. Half cells can also be
formed by a metal in contact with an
insoluble salt or oxide and a solution.
The *calomel half cell is an example
of this. Half cells are commonly referred to as electrodes.
half chair See ring conformations.
half-life See decay.
half sandwich See sandwich compound.
half-thickness The thickness of a
speciÜed material that reduces the intensity of a beam of radiation to half
its original value.
half-width Half the width of a
spectrum line (or in some cases the
full width) measured at half its height.
halide A compound of a halogen
with another element or group. The
halides of typical metals are ionic
(e.g. sodium Ûuoride, Na
+
F
– ). Metals
can also form halides in which the
bonding is largely covalent (e.g. aluminium chloride, AlCl 3 ). Organic compounds are also sometimes referred
to as halides; e.g. the alkyl halides
(see haloalkanes) and the *acyl
halides. Halides are named Ûuorides,
chlorides, bromides, or iodides.
halite (rock salt) Naturally occurring *sodium chloride (common salt,
NaCl), crystallizing in the cubic system. It is chieÛy colourless or white
(sometimes blue) when pure but the
presence of impurities may colour it
grey, pink, red, or brown. Halite
often occurs in association with anhydrite and gypsum.
Hall–Heroult cell An electrolytic
cell used industrially for the extraction of aluminium from bauxite. The
bauxite is Ürst puriÜed by dissolving
it in sodium hydroxide and Ültering
off insoluble constituents. Aluminium hydroxide is then precipitated (by adding CO 2 ) and this is
decomposed by heating to obtain
pure Al 2 O 3 . In the Hall–Heroult cell,
the oxide is mixed with cryolite (to
lower its melting point) and the
molten mixture electrolysed using
graphite anodes. The cathode is the
lining of the cell, also of graphite.
The electrolyte is kept in a molten
state (about 850°C) by the current.
Molten aluminium collects at the
bottom of the cell and can be tapped
off. Oxygen forms at the anode, and
gradually oxidizes it away. The cell is
named after the US chemist Charles
Martin Hall (1863–1914), who discovered the process in 1886, and the
French chemist Paul Heroult (1863–
Hahn, Otto
260
h
www.AzShimi.ir www.AzShimi.com