CRC Handbook of Chemistry and Physics, 86th Edition

The Elements 4-23
correspond to HgNe, HgAr, HgKr, and HgXe. Mercury is a
virulent poison and is readily absorbed through the respiratory
tract, the gastrointestinal tract, or through unbroken skin.
It acts as a cumulative poison and dangerous levels are readily
attained in air. Air saturated with mercury vapor at 20°C contains
a concentration that exceeds the toxic limit many times.
The danger increases at higher temperatures. It is therefore
important that mercury be handled with care. Containers of
mercury should be securely covered and spillage should be
avoided. If it is necessary to heat mercury or mercury compounds,
it should be done in a well-ventilated hood. Methyl
mercury is a dangerous pollutant and is now widely found in
water and streams. The triple point of mercury, –38.8344°C,
is a fixed point on the International Temperature Scale (ITS-
90). Mercury (99.98%) is priced at about $110/kg. Native mercury
contains seven isotopes. Thirty-six other isotopes and
isomers are known.
Molybdenum — (Gr. molybdos, lead), Mo; at. wt. 95.94(1); at. no.
42; m.p. 2623°C; b.p. 4639°C; sp. gr. 10.22 (20°C); valence 2,
3, 4?, 5?, or 6. Before Scheele recognized molybdenite as a
distinct ore of a new element in 1778, it was confused with
graphite and lead ore. The metal was prepared in an impure
form in 1782 by Hjelm. Molybdenum does not occur native,
but is obtained principally from molybdenite (MoS 2 ). Wulfenite
(PbMoO 4 ) and powellite (Ca(MoW)O 4 ) are also minor commercial
ores. Molybdenum is also recovered as a by-product
of copper and tungsten mining operations. The U.S., Canada,
Chile, and China produce most of the world’s molybdenum
ores. The metal is prepared from the powder made by the hydrogen
reduction of purified molybdic trioxide or ammonium
molybdate. The metal is silvery white, very hard, but is softer
and more ductile than tungsten. It has a high elastic modulus,
and only tungsten and tantalum, of the more readily available
metals, have higher melting points. It is a valuable alloying
agent, as it contributes to the hardenability and toughness of
quenched and tempered steels. It also improves the strength
of steel at high temperatures. It is used in certain nickel-based
alloys, such as the “Hastelloys®” which are heat-resistant and
corrosion-resistant to chemical solutions. Molybdenum oxidizes
at elevated temperatures. The metal has found recent
application as electrodes for electrically heated glass furnaces
and forehearths. The metal is also used in nuclear energy applications
and for missile and aircraft parts. Molybdenum is
valuable as a catalyst in the refining of petroleum. It has found
application as a filament material in electronic and electrical
applications. Molybdenum is an essential trace element in plant
nutrition. Some lands are barren for lack of this element in the
soil. Molybdenum sulfide is useful as a lubricant, especially at
high temperatures where oils would decompose. Almost all
ultra-high strength steels with minimum yield points up to
300,000 psi(lb/in. 2 ) contain molybdenum in amounts from 0.25
to 8%. Natural molybdenum contains seven isotopes. Thirty
other isotopes and isomers are known, all of which are radioactive.
Molybdenum metal costs about $1/g (99.999% purity).
Molybdenum metal (99.9%) costs about $160/kg.
Neodymium — (Gr. neos, new, and didymos, twin), Nd; at. wt.
144.24(3); at. no. 60; m.p. 1021°C; b.p. 3074°C; sp. gr. 7.008
(25°C); valence 3. In 1841, Mosander, extracted from cerite a
new rose-colored oxide, which he believed contained a new
element. He named the element didymium, as it was an inseparable
twin brother of lanthanum. In 1885 von Welsbach
separated didymium into two new elemental components,
neodymia and praseodymia, by repeated fractionation of ammonium
didymium nitrate. While the free metal is in misch
metal, long known and used as a pyrophoric alloy for light
flints, the element was not isolated in relatively pure form until
1925. Neodymium is present in misch metal to the extent
of about 18%. It is present in the minerals monazite and bastnasite,
which are principal sources of rare-earth metals. The
element may be obtained by separating neodymium salts from
other rare earths by ion-exchange or solvent extraction techniques,
and by reducing anhydrous halides such as NdF 3 with
calcium metal. Other separation techniques are possible. The
metal has a bright silvery metallic luster. Neodymium is one
of the more reactive rare-earth metals and quickly tarnishes
in air, forming an oxide that spalls off and exposes metal to
oxidation. The metal, therefore, should be kept under light
mineral oil or sealed in a plastic material. Neodymium exists
in two allotropic forms, with a transformation from a double
hexagonal to a body-centered cubic structure taking place at
863°C. Natural neodymium is a mixture of seven isotopes, one
of which has a very long half-life. Twenty seven other radioactive
isotopes and isomers are recognized. Didymium, of which
neodymium is a component, is used for coloring glass to make
welder’s goggles. By itself, neodymium colors glass delicate
shades ranging from pure violet through wine-red and warm
gray. Light transmitted through such glass shows unusually
sharp absorption bands. The glass has been used in astronomical
work to produce sharp bands by which spectral lines may
be calibrated. Glass containing neodymium can be used as a
laser material to produce coherent light. Neodymium salts are
also used as a colorant for enamels. The element is also being
used with iron and boron to produce extremely strong magnets
having energy densities as high as 27 to 35 million gauss
oersteds. These are the most compact magnets commercially
available. The price of the metal is about $4/g. Neodymium
has a low-to-moderate acute toxic rating. As with other rare
earths, neodymium should be handled with care.
Neon — (Gr. neos, new), Ne; at. wt. 20.1797(6); at. no. 10; t.p.
–248.59°C; b.p. –246.08°C; t c –228.7°C (1 atm); density of gas
0.89990 g/l (1 atm, 0°C); density of liquid at b.p. 1.207 g/cm 3 ;
valence 0. Discovered by Ramsay and Travers in 1898. Neon
is a rare gaseous element present in the atmosphere to the extent
of 1 part in 65,000 of air. It is obtained by liquefaction
of air and separated from the other gases by fractional distillation.
Natural neon is a mixture of three isotopes. Fourteen
other unstable isotopes are known. It is very inert element;
however, it is said to form a compound with fluorine. It is still
questionable if true compounds of neon exist, but evidence
is mounting in favor of their existence. The following ions
are known from optical and mass spectrometric studies: Ne + ,
(NeAr) + , (NeH) + , and (HeNe + ). Neon also forms an unstable
hydrate. In a vacuum discharge tube, neon glows reddish orange.
Of all the rare gases, the discharge of neon is the most
intense at ordinary voltages and currents. Neon is used in
making the common neon advertising signs, which accounts
for its largest use. It is also used to make high-voltage indicators,
lightning arrestors, wave meter tubes, and TV tubes.
Neon and helium are used in making gas lasers. Liquid neon
is now commercially available and is finding important application
as an economical cryogenic refrigerant. It has over 40
times more refrigerating capacity per unit volume than liquid
helium and more than three times that of liquid hydrogen. It
is compact, inert, and is less expensive than helium when it