* 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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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 gm/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 all been experimental units.
atomic weight: 232.0381
abundance: 39th
density: 11.72 gm/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 gm/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, meaning it's hardly radioactive at all, 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 gm/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 gm/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, 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 gm/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 gm/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 gm/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 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 one's foot; it presents more hazard 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. It is generally produced by solution mining -- pumping the ore to the surface in a solution into ponds, where it is separated. Work has been done on extracting uranium (and thorium) from coal ash from coal-fired powerplants -- the concentration of the fissionable elements is low, but the material doesn't have to be mined -- as well as from sea water -- the concentrations are very low, but seawater extraction is particularly attractive to nations who are trying to keep their nuclear development programs secret.
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:
__________________________ BERKELIUM / Bk / 97 CALIFORNIUM / Cf / 98 EINSTEINIUM / Es / 99 FERMIUM / Fm / 100 MENDELIVIUM / Md / 101 NOBELIUM / No / 102 LAWRENCIUM / Lr / 103 __________________________
