Curium (Cm)

Curium was made before americium, which is an accident of nuclear physics rather than of bookkeeping. Adding an alpha particle to plutonium jumps two squares at once and lands on element 96; getting to element 95 requires a slower route through neutron capture. So the summer of 1944 produced curium-242 at the Berkeley 60-inch cyclotron, and element 95 followed months later. The numbering on the periodic table and the order of discovery disagree here, and the discrepancy is in the shape of the nucleus, not the shape of the table.

A half-filled shell, and a name that follows from it

The element that occupies the corresponding position one row up in the lanthanides is gadolinium, and the resemblance is not merely positional. Gadolinium has a half-filled 4f shell — seven electrons in seven orbitals, one apiece — and half-filled shells are unusually stable. Curium has the same arrangement one shell out: seven 5f electrons, unpaired.

That configuration governs almost everything about curium's chemistry. It anchors the element hard in the +3 oxidation state, makes higher states difficult to reach, and gives curium(III) a sharp fluorescence that analytical chemists use to detect it. It also marks a genuine hinge in the actinide series. Below curium, the 5f electrons participate in bonding and the elements behave like awkward transition metals; from curium upward the 5f shell tucks inward and the remaining actinides behave far more like rare earths.

Gadolinium is named for the Finnish chemist Johan Gadolin, so Seaborg's group extended the parallel and named element 96 for Marie and Pierre Curie — the only element honouring two people, and, since both had died decades earlier, one that raised none of the objections that later attached to naming elements after the living.

Warm enough to see

Curium's defining physical property is heat output. Alpha decay deposits energy in whatever contains it, and curium's short-lived isotopes decay fast enough that the effect is dramatic rather than theoretical.

Curium-242, the first isotope made, has a half-life of 163 days and releases on the order of a hundred and twenty watts per gram — comparable, per unit mass, to the power density of a car engine at full load. Samples glow visibly from their own decay, and curium compounds are among the few substances that are self-luminous without any external excitation at all. The longer-lived curium-244, with an 18-year half-life, is far tamer at a few watts per gram, but even that is several times the output of the plutonium-238 used in spacecraft.

That power density made curium-244 an obvious candidate for radioisotope generators, and it was seriously studied for the role. It lost, for two reasons that are worth stating plainly: the half-life is too short for a mission lasting decades, and curium's spontaneous fission produces a neutron flux that requires far heavier shielding than an alpha emitter normally would.

The instrument that read the rocks of Mars

Curium's real career turned out to be in analysis rather than power. The alpha particle X-ray spectrometer, or APXS, is a small sensor head containing a set of curium-244 sources, placed directly against a rock or a patch of soil.

It works two ways at once. Some alpha particles bounce back off the nuclei in the sample, and the energy they retain identifies the light elements they hit. Meanwhile curium-244's decay daughter, plutonium-240, emits X-rays around 14 and 18 keV, which excite characteristic fluorescence from the heavier elements present. Between the two effects, a single passive instrument with no moving parts and no power-hungry X-ray tube can quantify everything from sodium to bromine.

Four Mars rovers have carried one: Sojourner in 1997, Spirit and Opportunity from 2004, and Curiosity from 2012. The basaltic composition of the Martian surface, the sulfate-rich soils, and the mineral evidence for water-altered rock in Gale Crater were all measured, in substantial part, by alpha particles from element 96. The same instrument design flew on the Philae lander to comet 67P/Churyumov–Gerasimenko and on India's Pragyan rover to the Moon.

It is a fair claim that curium has done more field geology off Earth than any other synthetic element.

Curium-247, and the isotopes nobody has enough of

Sixteen isotopes are known, and their half-lives cover an absurd range. Curium-247 lasts 15.6 million years; curium-250 is around nine thousand; curium-248, at 348,000 years, is the one heavy element chemists actually want, because it is stable enough to fabricate into a target and heavy enough to be a useful starting point for making elements further up the table. Curium-248 targets have been used to reach several of the transactinides.

None of these is abundant. World production of curium runs to grams, essentially all of it from Oak Ridge's high-flux reactor and its Russian counterpart, and it takes years of irradiation to work plutonium up to it. The scarcity is why curium chemistry lagged behind americium's for decades: the metal itself was not prepared until 1951, and structural work on curium compounds has proceeded a few micrograms at a time ever since.

Biologically, curium behaves as a bone-seeker, following the same chemical path as other trivalent actinides and concentrating in the skeleton. That is a property of the element rather than a property of any particular isotope, and it is the reason curium is handled only in facilities built for it.