101.4°C. Deuterium oxide, D 2 O, oc-
curs to a small extent (about 0.003%
by weight) in natural water, from
which it can be separated by frac-
tional distillation or by electrolysis. It
is useful in the nuclear industry be-
cause of its ability to reduce the ener-
gies of fast neutrons to thermal
energies and because its absorption
cross-section is lower than that of hy-
drogen and consequently it does not
appreciably reduce the neutronÛux.
In the laboratory it is used for *la-
belling other molecules for studies of
reaction mechanisms. Water also
contains the compound HDO.
hecto- Symbol h. A preÜx used in
the metric system to denote 100
times. For example, 100 coulombs =
1 hectocoulomb (hC).
Heisenberg, Werner Karl
(1901–76) German physicist, who be-
came a professor at the University of
Leipzig and, after World War II, at
the Kaiser Wilhelm Institute in Göt-
tingen. In 1923 he was awarded the
Nobel Prize for his work on matrix
mechanics. But he is best known for
his 1927 discovery of the *uncer-
tainty principle.
Heisenberg uncertainty princi-
pleSee uncertainty principle.
helicateA type of inorganic mol-
ecule containing a double helix of
bipyridyl-derived molecules formed
around a chain of up toÜve copper(I)
ions. See also supramolecular chem-
istry.
heliumSymbol He. A colourless
odourless gaseous nonmetallic el-
ement belonging to group 18 of the
periodic table (see noble gases); a.n.
2; r.a.m. 4.0026; d. 0.178 g dm–3; m.p.
–272.2°C (at 20 atm.); b.p. –268.93°C.
The element has the lowest boiling
point of all substances and can be so-
lidiÜed only under pressure. Natural
helium is mostly helium–4, with a
small amount of helium–3. There are
also two short-lived radioactive iso-
topes: helium–5 and –6. It occurs in
ores of uranium and thorium and in
some natural-gas deposits. It has a va-
riety of uses, including the provision
of inert atmospheres for welding and
semiconductor manufacture, as a re-
frigerant for superconductors, and as
a diluent in breathing apparatus. It is
also used inÜlling balloons. Chemi-
cally it is totally inert and has no
known compounds. It was discovered
in the solar spectrum in 1868 by
Joseph Lockyer (1836–1920).
A- Information from the WebElements site
helium–neon laserA laser in
which the medium is a mixture of
helium and neon in the mole ratio
1:5 respectively. An electric discharge
is used to excite a He atom to the
metastable 1s^1 2s^1 conÜguration.
Since the excitation energy coincides
with an excitation energy of neon,
transfer of energy between helium
and neon atoms can readily occur
during collisions. These collisions
lead to highly excited neon atoms
with unoccupied lower energy states.
Thus, population inversion occurs
giving rise to laser action with a
wavelength of 633 nm. (Many other
spectral lines are also produced in
the process.)Helmholtz, Hermann Ludwig
Ferdinand von (1821–94) German
physiologist and physicist. In 1850 he
measured the speed of a nerve im-
pulse and in 1851 invented the oph-
thalmoscope. Helmholtz discovered
the conservation of energy (1847),
giving many examples of its applica-
tion, and also introduced the concept
of *free energy.Helmholtz free energy See free
energy.267 Helmholtz free energy
h