Copernicium (Cn)

The state of matter listed for copernicium at room temperature is a convention, not an observation. Nobody has ever had enough of element 112 to see whether it is a solid, and the calculations disagree with each other: some predict a metal that melts near room temperature, others a substance volatile enough to be a gas. The most defensible summary is that copernicium is extremely volatile for a metal, and that whether it would be liquid or gaseous in a jar is genuinely open.

That uncertainty is the whole interest of the element, because mercury sits directly above it.

Two atoms on a gold surface

Mercury is the one metal that is liquid at room temperature, and the reason is relativistic: in a mercury atom the 6s electrons are contracted and held so tightly that they participate weakly in metallic bonding, leaving the atoms only loosely attached to each other. Element 112 has the same electronic arrangement one shell further out, with relativistic effects roughly four times stronger. The obvious question is whether copernicium takes mercury's weirdness to its logical conclusion and stops being a metal at all — some early calculations suggested it might behave like a noble gas.

The test was direct. Gold amalgamates readily with mercury; a mercury atom landing on a gold surface sticks hard. A radon atom does not stick at all. Between 2006 and 2008 a team led by Robert Eichler ran copernicium atoms produced at Dubna along a gold-coated detector channel with a temperature gradient and recorded where they came to rest.

Two atoms gave the answer, and it was intermediate. Copernicium stuck to gold — so it is a metal, forming a genuine if weak metallic bond — but at a much lower temperature than mercury would, making it far more volatile. It is not a noble gas. It is a metal that behaves more like a mercury vapour that never quite condenses.

Cp, and why it could not be used

The discoverers proposed the symbol Cp. IUPAC would not accept it, for a reason buried in early twentieth-century nomenclature: Cp had been the symbol for cassiopeium, the name German chemists used for element 71 until lutetium prevailed around 1949. Reviving a retired symbol for a different element invites exactly the confusion that symbols exist to prevent. The abbreviation is also thoroughly established in organometallic chemistry, where Cp denotes the cyclopentadienyl ligand and appears in thousands of formulae.

The discoverers accepted Cn instead, and the name was made official on 19 February 2010 — the 537th anniversary of Nicolaus Copernicus's birth.

An astronomer beneath mercury

Copernicus was not a chemist and had nothing to do with element 112. The Darmstadt group's stated reasoning was that heliocentrism reordered a picture of the world that had looked settled, in the way that a periodic table built on electron structure reordered a chemistry built on observed resemblances. They also pointed out a small pleasing coincidence: the element directly above theirs is named for a planet, and the planet is named for a god, so a square below mercury is not a strange place to put an astronomer.

The element was made on 9 February 1996 by Sigurd Hofmann's group, from a lead target under a zinc beam. The claim took fourteen years to become a name, in part because the original evidence was a single decay chain, and in part because Darmstadt subsequently withdrew one of its early chains during an internal review of its own archived data — a correction the laboratory made itself. The discovery survived on later, independently confirmed events, including work at RIKEN in 2004.

Isotopes, and the limit on what can be asked

Copernicium-285 lasts around half a minute, which is the longest of any known isotope of the element and is long enough for the gas-phase experiments described above to work. Nothing longer is available, so the questions that can be put to element 112 are limited to those a single atom can answer in thirty seconds: does it stick to this surface, at what temperature, and how does that compare with a lighter element run through the same apparatus.

Everything else about copernicium — its density, its melting point, whether it would form compounds with fluorine as mercury does — remains a computation waiting for a longer-lived isotope that may never be found.