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 re-
turning to Germany in 1907. In 1917,
together with Lise *Meitner, he dis-
covered protactinium. In the late
1930s he collaborated with Fritz
Strassmann (1902–80) and in 1938
bombarded uranium with slow neu-
trons. Among the products was bar-
ium, but it was Meitner (now in
Sweden) who the next year inter-
preted the process as nuclearÜssion.
In 1944 Hahn received the Nobel
Prize for chemistry.
hahniumSee transactinide el-
ements.half cellAn 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 solu-
tion 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 *hydro-
gen 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 re-
ferred to as electrodes.
half chairSee ring conforma-
tions.half-lifeSee decay.
half sandwichSee sandwich com-
pound.
half-thicknessThe thickness of a
speciÜed material that reduces the in-
tensity of a beam of radiation to half
its original value.half-widthHalf the width of a
spectrum line (or in some cases the
full width) measured at half its height.halideA compound of a halogen
with another element or group. The
halides of typical metals are ionic
(e.g. sodiumÛuoride, Na+F–). Metals
can also form halides in which the
bonding is largely covalent (e.g. alu-
minium chloride, AlCl 3 ). Organic com-
pounds 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 occur-
ring *sodium chloride (common salt,
NaCl), crystallizing in the cubic sys-
tem. 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 an-
hydrite and gypsum.Hall–Heroult cell An electrolytic
cell used industrially for the extrac-
tion of aluminium from bauxite. The
bauxite isÜrst puriÜed by dissolving
it in sodium hydroxide andÜltering
off insoluble constituents. Alu-
minium hydroxide is then precipi-
tated (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 discov-
ered the process in 1886, and the
French chemist Paul Heroult (1863–Hahn, Otto 260h