Dictionary of Chemistry [6th Ed.]

hydrideA chemical compound of
hydrogen and another element or el-
ements. Non-metallic hydrides (e.g.
ammonia, methane, water) are cova-
lently 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. Hy-
dride-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 hy-
dride ions as ligands; many are pow-
erful reducing agents.

hydriodic acid See hydrogen io-
dide.

hydrobromic acid See hydrogen
bromide.

hydrocarbons Chemical com-
pounds 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 effec-
tive 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 viscos-
ity. If the solvent is water, the hydro-

dynamic 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. Experi-

ments involving *nuclear magnetic

resonance (NMR) and isotope tracers

indicate that there is considerable

movement between solvent mol-

ecules within the hydrodynamic ra-

dius and the rest of the solution.

hydroÛuoric acidSee 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 pre-

sent in water and in all organic com-

pounds. There are three isotopes:

naturally occurring hydrogen con-

sists of the two stable isotopes hydro-

gen–1 (99.985%) and *deuterium. The

radioactive *tritium is made artiÜ-

cially. The gas is diatomic and has

two forms: orthohydrogen, in which

the nuclear spins are parallel, and

parahydrogen, in which they are an-

tiparallel. 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. Hydro-

gen is also used in various other

275 hydrogen

h

RC

O

R′

+

H

H

NN

R′′

H

• H 2 O
  R C

N

R′

N

R′′

H

ketone hydrazine hydrazone

Hydrazones