Periodic Table of the Elements

تاريخ التسجيل : 22/11/2009

History
(Gr. hydro, water, and genes, forming) Hydrogen was prepared many years before it was
recognized as a distinct substance by Cavendish in 1776.
Named by Lavoisier, hydrogen is the most abundant of all elements in the universe. The heavier
elements were originally made from Hydrogen or from other elements that were originally
made from Hydrogen.
Sources
Hydrogen is estimated to make up more than 90% of all the atoms or three quarters of the mass
of the universe. This element is found in the stars, and plays an important part in powering the
universe through both the proton-proton reaction and carbon-nitrogen cycle -- stellar hydrogen
fusion processes that release massive amounts of energy by combining Hydrogen to form
Helium.
Production of hydrogen in the U.S. alone now amounts to about 3 billion cubic feet per year.
Hydrogen is prepared by
steam on heated carbon,
decomposition of certain hydrocarbons with heat,
action of sodium or potassium hydroxide on aluminum
electrolysis of water, or
displacement from acids by certain metals.
Liquid hydrogen is important in cryogenics and in the study of superconductivity, as its melting
point is only 20 degrees above absolute zero.
Tritium is readily produced in nuclear reactors and is used in the production of the hydrogen
bomb.
Hydrogen is the primary component of Jupiter and the other gas giant planets. At some depth in the
Hydrogen
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planet's interior the pressure is so great that solid molecular hydrogen is converted to solid metallic
hydrogen.
In 1973, a group of Russian experimenters may have produced metallic hydrogen at a pressure of 2.8
Mbar. At the transition the density changed from 1.08 to 1.3 g/cm3. Earlier, in 1972, at Livermore,
California, a group also reported on a similar experiment in which they observed a pressure-volume point
centered at 2 Mbar. Predictions say that metallic hydrogen may be metastable; others have predicted it
would be a superconductor at room temperature.
Compounds
Although pure Hydrogen is a gas we find very little of it in our atmosphere. Hydrogen gas is so
light that uncombined Hydrogen will gain enough velocity from collisions with other gases that
they will quickly be ejected from the atmosphere. On earth, hydrogen occurs chiefly in
combination with oxygen in water, but it is also present in organic matter such as living plants,
petroleum, coal, etc. It is present as the free element in the atmosphere, but only to the extent of
less than 1 ppm by volume. The lightest of all gases, hydrogen combines with other elements --
sometimes explosively -- to form compounds.
Uses
Great quantities are required commercially for the fixation of nitrogen from the air in the Haber
ammonia process and for the hydrogenation of fats and oils. It is also used in large quantities in
methanol production, in hydrodealkylation, hydrocracking, and hydrodesulfurization. Other
uses include rocket fuel, welding, producing hydrochloric acid, reducing metallic ores, and
filling balloons.
The lifting power of 1 cubic foot of hydrogen gas is about 0.07 lb at 0C, 760 mm pressure.
The Hydrogen Fuel cell is a developing technology that will allow great amounts of electrical
power to be obtained using a source of hyrogen gas.
Consideration is being given to an entire economy based on solar- and nuclear-generated
hydrogen. Public acceptance, high capital investment, and the high cost of hydrogen with
respect to today's fuels are but a few of the problems facing such an economy. Located in
remote regions, power plants would electrolyze seawater; the hydrogen produced would travel
to distant cities by pipelines. Pollution-free hydrogen could replace natural gas, gasoline, etc.,
and could serve as a reducing agent in metallurgy, chemical processing, refining, etc. It could
also be used to convert trash into methane and ethylene.
Forms
Quite apart from isotopes, it has been shown that under ordinary conditions hydrogen gas is a
mixture of two kinds of molecules, known as ortho- and para-hydrogen, which differ from one
another by the spins of their electrons and nuclei.
Hydrogen
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Normal hydrogen at room temperature contains 25% of the para form and 75% of the ortho
form. The ortho form cannot be prepared in the pure state. Since the two forms differ in energy,
the physical properties also differ. The melting and boiling points of parahydrogen are about
0.1oC lower than those of normal hydrogen.
Isotopes
The ordinary isotope of hydrogen, H, is known as Protium, the other two isotopes are
Deuterium (a proton and a neutron) and Tritium (a protron and two neutrons). Hydrogen is the
only element whose isotopes have been given different names. Deuterium and Tritium are both
used as fuel in nuclear fusion reactors. One atom of Deuterium is found in about 6000 ordinary
hydrogen atoms.
Deuterium is used as a moderator to slow down neutrons. Tritium atoms are also present but in
much smaller proportions. Tritium is readily produced in nuclear reactors and is used in the
production of the hydrogen (fusion) bomb. It is also used as a radioactive agent in making
luminous paints, and as a tracer.
Sources: CRC Handbook of Chemistry and Physics and the American Chemical Society.
Last Updated: 12/19/97, CST Information Services Team
Hydrogen
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Helium
For blimps
Atomic Number: 2
Atomic Symbol: He
Atomic Weight: 4.00260
Electron Configuration: 1s2
History
(Gr. helios, the sun). Janssen obtained the first evidence of helium during the solar eclipse of
1868 when he detected a new line in the solar spectrum. Lockyer and Frankland suggested the
name helium for the new element. In 1895 Ramsay discovered helium in the uranium mineral
clevite while it was independently discovered in cleveite by the Swedish chemists Cleve and
Langlet at about the same time. Rutherford and Royds in 1907 demonstrated that alpha particles
are helium nuclei.
Sources
Except for hydrogen, helium is the most abundant element found through out the universe.
Helium is extracted from natural gas. In fact, all natural gas contains at least trace quantities of
helium.
It has been detected spectroscopically in great abundance, especially in the hotter stars, and it is
an important component in both the proton-proton reaction and the carbon cycle, which account
for the energy of the sun and stars.
The fusion of hydrogen into helium provides the energy of the hydrogen bomb. The helium
content of the atmosphere is about 1 part in 200,000. While it is present in various radioactive
minerals as a decay product, the bulk of the Free World's supply is obtained from wells in Texas,
Oklahoma, and Kansas. The only known helium extraction plants, outside the United States, in
1984 were in Eastern Europe (Poland), the USSR, and a few in India.
Cost
The cost of helium fell from $2500/ft3 in 1915 to 1.5 cents /ft3 in 1940. The U.S. Bureau of Mines
has set the price of Grade A helium at $37.50/1000 ft3 in 1986.
Helium
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Properties
Helium has the lowest melting point of any element and is widely used in cryogenic research
because its boiling point is close to absolute zero. Also, the element is vital in the study of super
conductivity.
Using liquid helium, Kurti and co-workers and others, have succeeded in obtaining
temperatures of a few microkelvins by the adiabatic demagnetization of copper nuclei.
It has other peculiar properties. Helium is the only liquid that cannot be solidified by lowering
the temperature. It remains liquid down to absolute zero at ordinary pressures, but it can readily
be solidified by increasing the pressure. Solid 3He and 4He are unusual in that both can be
changed in volume by more than 30% by applying pressure.
The specific heat of helium gas is unusually high. The density of helium vapor at the normal
boiling point is also very high, with the vapor expanding greatly when heated to room
temperature. Containers filled with helium gas at 5 to 10 K should be treated as though they
contained liquid helium due to the large increase in pressure resulting from warming the gas to
room temperature.
While helium normally has a 0 valence, it seems to have a weak tendency to combine with
certain other elements. Means of preparing helium difluoride have been studied, and species
such as HeNe and the molecular ions He+ and He++ have been investigated.
Isotopes
Seven isotopes of helium are known: Liquid helium (He4) exists in two forms: He4I and He4II,
with a sharp transition point at 2.174K. He4I (above this temperature) is a normal liquid, but
He4II (below it) is unlike any other known substance. It expands on cooling; its conductivity for
heat is enormous; and neither its heat conduction nor viscosity obeys normal rules.
Uses
as an inert gas shield for arc welding;
a protective gas in growing silicon and germanium crystals and producing titanium and
zirconium;
as a cooling medium for nuclear reactors, and
as a gas for supersonic wind tunnels.
A mixture of helium and oxygen is used as an artificial atmosphere for divers and others
working under pressure. Different ratios of He/O2 are used for different depths at which the
diver is operating.
Helium is extensively used for filling balloons as it is a much safer gas than hydrogen. One of
the recent largest uses for helium has been for pressuring liquid fuel rockets. A Saturn booster,
like the type used on the Apollo lunar missions, required about 13 million ft3 of helium for a
firing, plus more for checkouts.
Helium

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Periodic Table of the Elements

Periodic Table of the Elements

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