hydride A chemical compound of
hydrogen and another element or elements. Non-metallic hydrides (e.g.
ammonia, methane, water) are covalently bonded. The alkali metals and
alkaline earths (*s-block elements)
form salt-like hydrides containing
the hydride ion H
–
, which produce
hydrogen on reacting with water. Hydride-forming *transition elements
form interstitial hydrides, with the
hydrogen atoms ‘trapped’ within the
gaps in the lattice of metal atoms.
Complex hydrides, such as *lithium
tetrahydroaluminate(III), have hydride ions as *ligands; many are powerful reducing agents.
hydriodic acid See hydrogen iodide.
hydrobromic acid See hydrogen
bromide.
hydrocarbons Chemical compounds that contain only carbon and
hydrogen. A vast number of different
hydrocarbon compounds exist, the
main types being the *alkanes,
*alkenes, *alkynes, and *arenes.
hydrochloric acid See hydrogen
chloride.
hydrochloride See amine salts.
hydrochloroÛuorocarbon (HCFC)
See halocarbons.
hydrocyanic acid See hydrogen
cyanide.
hydrodynamic radius The effective radius of an ion in a solution
measured by assuming that it is a
body moving through the solution
and resisted by the solution’s viscosity. If the solvent is water, the hydrodynamic radius includes all the water
molecules attracted to the ion. As a
result, it is possible for a small ion to
have a larger hydrodynamic radius
than a large ion – if it is surrounded
by more solvent molecules. Experiments involving *nuclear magnetic
resonance (NMR) and isotope tracers
indicate that there is considerable
movement between solvent molecules within the hydrodynamic radius and the rest of the solution.
hydroÛuoric acid See hydrogen
fluoride.
hydroÛuorocarbon (HFC) See
halocarbons.
hydrogen Symbol H. A colourless
odourless gaseous chemical element;
a.n. 1; r.a.m. 1.008; d. 0.0899 g dm
–3
;
m.p. –259.14°C; b.p. –252.87°C. It is
the lightest element and the most
abundant in the universe. It is present in water and in all organic compounds. There are three isotopes:
naturally occurring hydrogen consists of the two stable isotopes hydrogen–1 (99.985%) and *deuterium. The
radioactive *tritium is made artiÜcially. The gas is diatomic and has
two forms: orthohydrogen, in which
the nuclear spins are parallel, and
parahydrogen, in which they are antiparallel. At normal temperatures
the gas is 25% parahydrogen. In the
liquid it is 99.8% parahydrogen. The
main source of hydrogen is steam
*reforming of natural gas. It can also
be made by the Bosch process (see
haber process) and by electrolysis of
water. The main use is in the Haber
process for making ammonia. Hydrogen is also used in various other
275
hydrogen
h
R
C
O
R′
+
H
H
N
N
R″
H
– H 2 O
R
C
N
R′
N
R″
H
ketone
hydrazine
hydrazone
Hydrazones
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hydrogen and another element or elements. Non-metallic hydrides (e.g.
ammonia, methane, water) are covalently bonded. The alkali metals and
alkaline earths (*s-block elements)
form salt-like hydrides containing
the hydride ion H
–
, which produce
hydrogen on reacting with water. Hydride-forming *transition elements
form interstitial hydrides, with the
hydrogen atoms ‘trapped’ within the
gaps in the lattice of metal atoms.
Complex hydrides, such as *lithium
tetrahydroaluminate(III), have hydride ions as *ligands; many are powerful reducing agents.
hydriodic acid See hydrogen iodide.
hydrobromic acid See hydrogen
bromide.
hydrocarbons Chemical compounds that contain only carbon and
hydrogen. A vast number of different
hydrocarbon compounds exist, the
main types being the *alkanes,
*alkenes, *alkynes, and *arenes.
hydrochloric acid See hydrogen
chloride.
hydrochloride See amine salts.
hydrochloroÛuorocarbon (HCFC)
See halocarbons.
hydrocyanic acid See hydrogen
cyanide.
hydrodynamic radius The effective radius of an ion in a solution
measured by assuming that it is a
body moving through the solution
and resisted by the solution’s viscosity. If the solvent is water, the hydrodynamic radius includes all the water
molecules attracted to the ion. As a
result, it is possible for a small ion to
have a larger hydrodynamic radius
than a large ion – if it is surrounded
by more solvent molecules. Experiments involving *nuclear magnetic
resonance (NMR) and isotope tracers
indicate that there is considerable
movement between solvent molecules within the hydrodynamic radius and the rest of the solution.
hydroÛuoric acid See hydrogen
fluoride.
hydroÛuorocarbon (HFC) See
halocarbons.
hydrogen Symbol H. A colourless
odourless gaseous chemical element;
a.n. 1; r.a.m. 1.008; d. 0.0899 g dm
–3
;
m.p. –259.14°C; b.p. –252.87°C. It is
the lightest element and the most
abundant in the universe. It is present in water and in all organic compounds. There are three isotopes:
naturally occurring hydrogen consists of the two stable isotopes hydrogen–1 (99.985%) and *deuterium. The
radioactive *tritium is made artiÜcially. The gas is diatomic and has
two forms: orthohydrogen, in which
the nuclear spins are parallel, and
parahydrogen, in which they are antiparallel. At normal temperatures
the gas is 25% parahydrogen. In the
liquid it is 99.8% parahydrogen. The
main source of hydrogen is steam
*reforming of natural gas. It can also
be made by the Bosch process (see
haber process) and by electrolysis of
water. The main use is in the Haber
process for making ammonia. Hydrogen is also used in various other
275
hydrogen
h
R
C
O
R′
+
H
H
N
N
R″
H
– H 2 O
R
C
N
R′
N
R″
H
ketone
hydrazine
hydrazone
Hydrazones
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