or hydriodic acid). It is also a reduc-
ing agent.hydrogen ion See acids.
hydrogen molecule ion The sim-
plest type of molecule (H 2 +), consist-
ing of two hydrogen nuclei and one
electron. In the *Born–Oppenheimer
approximation, in which the nuclei
are regarded as beingÜxed, the
*Schrödinger equation for the hy-
drogen molecule ion can be solved
exactly. This enables ideas and ap-
proximation techniques concerned
with chemical bonding to be tested
quantitatively.hydrogen peroxide A colourless
or pale blue viscous unstable liquid,
H 2 O 2 ; r.d. 1.44; m.p. –0.41°C; b.p.
150.2°C. As with water, there is con-
siderable hydrogen bonding in the
liquid, which has a high dielectric
constant. It can be made in the labo-
ratory by adding dilute acid to bar-
ium peroxide at 0°C. Large quantities
are made commercially by electroly-
sis of KHSO 4 .H 2 SO 4 solutions. An-
other industrial process involves
catalytic oxidation (using nickel, pal-
ladium, or platinum with an an-
thraquinone) of hydrogen and water
in the presence of oxygen. Hydrogen
peroxide readily decomposes in light
or in the presence of metal ions to
give water and oxygen. It is usually
supplied in solutions designated by
volume strength. For example, 20-
volume hydrogen peroxide would
yield 20 volumes of oxygen per vol-
ume of solution. Although the *per-
oxides are formally salts of H 2 O 2 , the
compound is essentially neutral.
Thus, the acidity constant of the ion-
ization
H 2 O 2 + H 2 O ˆH 3 O++ HO 2 –
is 1.5 × 10 –12mol dm–3. It is a strong
oxidizing agent, hence its use as a
mild antiseptic and as a bleaching
agent for cloth, hair, etc. It has alsobeen used as an oxidant in rocket
fuels.
hydrogen spectrum The atomic
spectrum of hydrogen is character-
ized by lines corresponding to radia-
tion quanta of sharply deÜned
energy. A graph of the frequencies at
which these lines occur against the
ordinal number that characterizes
their position in the series of lines,
produces a smooth curve indicating
that they obey a formal law. In 1885
J. J. Balmer (1825–98) discovered the
law having the form:
1/λ= R(1/n 12 + 1/n 22 )
This law gives the so-called Balmer
seriesof lines in the visible spectrum
in which n 1 = 2 and n 2 = 3,4,5..., λis
the wavelength associated with the
lines, and R is the *Rydberg constant.
In the Lyman series, discovered by
Theodore Lyman (1874–1954), n 1 = 1
and the lines fall in the ultraviolet.
The Lyman series is the strongest fea-
ture of the solar spectrum as ob-
served by rockets and satellites above
the earth’s atmosphere. In the
Paschen series, discovered by F.
Paschen (1865–1947), n 1 = 3 and the
lines occur in the far infrared. The
Brackett series (n 1 = 4), Pfund series
(n 1 = 5), and Humphreys series (n 1 = 6)
also occur in the far infrared.
A- Balmer’s paper
hydrogensulphate (bisulphate)A
salt containing the ion HSO 4 – or an
ester of the type RHSO 4 , where R is
an organic group. It was formerly
called hydrosulphate.hydrogen sulphide (sulphuretted
hydrogen) A gas, H 2 S, with an odour
of rotten eggs; r.d. 1.54 (liquid); m.p.
–85.5°C; b.p. –60.7°C. It is soluble in
water and ethanol and may be pre-
pared by the action of mineral acids
on metal sulphides, typically hy-
drochloric acid on iron(II) sulphidehydrogen ion 278h