101.4°C. Deuterium oxide, D 2 O, occurs to a small extent (about 0.003%
by weight) in natural water, from
which it can be separated by fractional distillation or by electrolysis. It
is useful in the nuclear industry because of its ability to reduce the energies of fast neutrons to thermal
energies and because its absorption
cross-section is lower than that of hydrogen and consequently it does not
appreciably reduce the neutron Ûux.
In the laboratory it is used for *labelling 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 became a professor at the University of
Leipzig and, after World War II, at
the Kaiser Wilhelm Institute in Göttingen. 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 *uncertainty principle.
Heisenberg uncertainty principle See uncertainty principle.
helicate A type of inorganic molecule containing a double helix of
bipyridyl-derived molecules formed
around a chain of up to Üve copper(I)
ions. See also supramolecular chemistry.
helium Symbol He. A colourless
odourless gaseous nonmetallic element 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 solidiÜed only under pressure. Natural
helium is mostly helium–4, with a
small amount of helium–3. There are
also two short-lived radioactive isotopes: helium–5 and –6. It occurs in
ores of uranium and thorium and in
some natural-gas deposits. It has a variety of uses, including the provision
of inert atmospheres for welding and
semiconductor manufacture, as a refrigerant for superconductors, and as
a diluent in breathing apparatus. It is
also used in Ülling balloons. Chemically 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 laser A 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 impulse and in 1851 invented the ophthalmoscope. Helmholtz discovered
the conservation of energy (1847),
giving many examples of its application, and also introduced the concept
of *free energy.
Helmholtz free energy See free
energy.
267
Helmholtz free energy
h
www.AzShimi.ir www.AzShimi.com
by weight) in natural water, from
which it can be separated by fractional distillation or by electrolysis. It
is useful in the nuclear industry because of its ability to reduce the energies of fast neutrons to thermal
energies and because its absorption
cross-section is lower than that of hydrogen and consequently it does not
appreciably reduce the neutron Ûux.
In the laboratory it is used for *labelling 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 became a professor at the University of
Leipzig and, after World War II, at
the Kaiser Wilhelm Institute in Göttingen. 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 *uncertainty principle.
Heisenberg uncertainty principle See uncertainty principle.
helicate A type of inorganic molecule containing a double helix of
bipyridyl-derived molecules formed
around a chain of up to Üve copper(I)
ions. See also supramolecular chemistry.
helium Symbol He. A colourless
odourless gaseous nonmetallic element 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 solidiÜed only under pressure. Natural
helium is mostly helium–4, with a
small amount of helium–3. There are
also two short-lived radioactive isotopes: helium–5 and –6. It occurs in
ores of uranium and thorium and in
some natural-gas deposits. It has a variety of uses, including the provision
of inert atmospheres for welding and
semiconductor manufacture, as a refrigerant for superconductors, and as
a diluent in breathing apparatus. It is
also used in Ülling balloons. Chemically 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 laser A 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 impulse and in 1851 invented the ophthalmoscope. Helmholtz discovered
the conservation of energy (1847),
giving many examples of its application, and also introduced the concept
of *free energy.
Helmholtz free energy See free
energy.
267
Helmholtz free energy
h
www.AzShimi.ir www.AzShimi.com