last update 01 oct 08
* The actinide rare earths, or just "actinides", occupy a side row off the last row of the periodic table. The actinides are all radioactive, though some have very long half-lives and so are not particularly dangerous to deal with. However, some are so radioactive that they decay too quickly to accumulate in any quantity, making properties such as boiling point difficult to determine. They are moderately to highly reactive.
The members of the lower part of the actinide series can be obtained in
quantity:
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RADIUM / Ra / 88
A soft, silvery, lustrous, very radioactive metal. Over two dozen
isotopes are known, all radioactive, the longest-lived being
Ra<226/88>, with a half-life of 1,620 years. Despite the relatively short
half life, this isotope is found in nature since it is part of a decay
series, as are the other two naturally occurring isotopes, Ra<223/88> --
with a half-life of 11.5 days -- and Ra<224/88> -- with a half-life of 3.7
days.
atomic weight: 226.0254
abundance: 86th
density: 5 g/cc
melting point: 700 C
boiling point: 1,140 C
valence: 2
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ACTINIUM / Ac / 89
A soft, silvery, reactive, very radioactive metal. There are
dozens of known isotopes of actinium, all radioactive. The most
stable is Ac<227/89>, with a half-life of 21.8 years. The next
most stable, Ac<225/89>, has a far shorter half-life of ten days,
and some isotopes only survive for moments.
atomic weight: 227.0278
abundance: negligible
density: 10.07 g/cc
melting point: 1,050 C
boiling point: ~3,200 C
valence: 3
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THORIUM / Th / 90
A silvery, radioactive metal. In bulk form, it resists corrosion
through formation of an oxide layer, but powdered thorium can
spontaneously ignite. There are 24 isotopes, all radioactive, but
the primary isotope, Th<232/90>, has a half-life of 14 billion
years and so it is found in reasonable quantities. Th<232/90> is
potentially useful because, like U<235/92> and Pu<238/92>, it can
be used as a fission fuel, though thorium reactors built to date
have always been experimental units.
atomic weight: 232.0381
abundance: 39th
density: 11.72 g/cc
melting point: 1,750 C
boiling point: ~3,800 C
valence: 2? 3? 4
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PROTACTINIUM / Pa / 91
A silvery, radioactive, reactive metal with no stable isotopes. A
few dozen radioactive isotopes are known, the most stable being
Pa<231/91>, with a half-life of 23,760 years.
atomic weight: 231.0388
abundance: negligible
density: 15.4 g/cc
melting point: ~1,600 C
boiling point: ~4,000 C
valence: 4 5
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URANIUM / U / 92
A silvery, ductile, malleable, reactive metal. It is popularly
thought of as highly radioactive, but the primary isotope found
in nature -- U<238/92>, 99.3% -- has a half-life of 4.5 billion
years, and the second most common isotope -- U<235/92>, 0.7% --
has a half-life of 700 million years. Most of the remainder is
U<234/92>, with a half-life of 245,000 years. U<235/92> is
useful because, like Pu<238/92 and Th<232/90>, it can be used
as a fission fuel.
atomic weight: 238.0289
abundance: 48th
density: 18.95 g/cc
melting point: 1,132 C
boiling point: 3,818 C
valence: 2 3 4 5 6
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NEPTUNIUM / Np / 93
A silvery, reactive metal, with over 20 isotopes. The
longest-lived is Np<237/93>, with a half-life of 2.14 million
years, followed by Np<236/93>, with a half-life of 155,000 years.
All the rest have half-lives of half a year or less.
atomic weight: 237.0482
abundance: negligible
density: 20.25 g/cc
melting point: 640 C
boiling point: 3,902 C
valence: 3 4 <5> 6
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PLUTONIUM / Pu / 94
A silvery, reactive metal, so radioactive that it will actually
seem warm to the touch. A good-sized lump of it will boil water.
All isotopes are radioactive, with the most common being
Pu<239/94>, with a half-life of 24,000 years. There are actually
longer-lived isotopes -- Pu<244/94> has a half-life of 80 million
years and Pu<242/94> has a half-life of 376,000 years -- but they
are produced by relatively unusual atomic processes and so less
common.
All other isotopes of plutonium are much more unstable; the most
useful isotope, Pu<238/94>, has a half-life of 88 years. Pu<238/94>
is useful because, like U<235/92> and Th<232/90>, it can be used as
a fission fuel.
atomic weight: 244.0642
abundance: negligible
density: 19.8 g/cc
melting point: 641 C
boiling point: 3,232 C
valence: 3 4 <5> 6
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AMERICIUM / Am / 95
A silvery, shiny radioactive metal; the most stable isotope --
Am<243/95>, with a half-life of 7,370 years -- can be produced in
kilogram quantities.
atomic weight: 243.1
abundance: negligible
density: 13.67 g/cc
melting point: 994 C
boiling point: 2,607 C
valence: 2 <3> 4 5 6
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CURIUM / Cm / 96
A silvery, corrosive metal. There are over a dozen known isotopes,
the most stable being Cm<247/96>, with a half-life of 16 million
years. However, only Cm<244/96>, with a half-life of 18 years,
and Cm<242/96>, with a half-life of 163 days, have been synthesized
in kilogram quantities.
atomic weight: 247.1
abundance: negligible
density: 13.51 g/cc
melting point: 1,340 C
boiling point: ? C
valence: 3 4
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The actinides uranium and plutonium have significant application as nuclear
fuels; some of the other actinides are useful as radioactive sources. As
mentioned, uranium is not particularly radioactive. For for most practical
purposes, uranium can be handled without taking any precautions. The major
threat in handling a brick of ordinary uranium is dropping the thing on your
foot; it presents more risk as a heavy-metal toxin than as a radioactive
material. The usefulness of uranium as a nuclear fuel is due to its ability
of the U<235/92> isotope to support nuclear "chain reactions", with the
breakdown of nuclei from neutron impacts generating more neutrons to generate
more breakdowns.
Uranium is obtained from natural deposits of uranium oxides, such as UO2, U2O8, and UO3. Uranium actually has some applications outside of its use as nuclear fuel, mostly because of its density; "depleted" uranium -- made up mostly of U<238/92> -- is used as the core of anti-armor gun projectiles, and it also used as ballast and even, ironically, as radiation shielding. Plutonium, in contrast, is noticeably radioactive, so much so that only small traces of it are found in nature. It is the pre-eminent nuclear fuel, produced in "breeder reactors" where uranium is bombarded with neutrons to become plutonium. It has no real uses other than nuclear fuels.
The other actinides are more obscure. Thorium is the most common, in fact about as common as lead -- as with the lanthanide rare earths, the actinide rare earths are not really all that rare in general -- but though it could in principle be used as nuclear reactor fuel, nobody's done so yet on a practical basis, and it has no other real uses. Neptunium is fairly rare, though the Np<237/93> isotope is only mildly radioactive, and also has no real uses. Actinium, americium, and curium are highly radioactive, and used in some applications as radiation sources.
* All the other actinides are too unstable to be very useful, though
californium has been used in cancer therapy to an extent. Most are so
transient that it is difficult to determine their chemical properties, and in
fact some have only been synthesized in terms of a handful of atoms. They
are of little interest in practical chemistry and are only mentioned here for
the sake of completeness:
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BERKELIUM / Bk / 97
CALIFORNIUM / Cf / 98
EINSTEINIUM / Es / 99
FERMIUM / Fm / 100
MENDELIVIUM / Md / 101
NOBELIUM / No / 102
LAWRENCIUM / Lr / 103
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