Mendelevium (Md)

Element 101 was discovered on the strength of seventeen atoms, and the reason that was sufficient is a genuinely elegant piece of reasoning. Chemistry usually needs a sample. Ion-exchange chromatography does not: it separates the actinides in a strict, reproducible order, so an atom's identity can be read off from when it comes off the column rather than from how much of it there is. If the atom appears in the slot immediately before fermium, it is element 101, and one atom establishes that as firmly as a milligram would.

Mendelevium was the first element found this way, one atom at a time, and the technique became the standard method for everything discovered afterwards.

The gold foil and the mile across campus

The 1955 experiment at Berkeley, run by Albert Ghiorso, Bernard Harvey, Gregory Choppin, Stanley Thompson and Glenn Seaborg, was constrained at both ends. The target was einsteinium-253 — perhaps a billion atoms of it, an invisible smear, and the largest quantity of element 99 anyone could spare. The product, mendelevium-256, has a half-life of about 77 minutes, which meant the whole chemical separation had to be finished before the sample decayed.

The solution to the first problem was a new trick. Instead of trying to dissolve the precious einsteinium target after bombardment, they made it thin enough that atoms of the new element recoiled clean out of it under the impact of the alpha beam, and caught them on a gold foil placed behind. The foil could then be dissolved and processed without touching the target at all, which meant the einsteinium could be used again. Recoil catchers have been part of heavy-element work ever since.

The second problem was solved by driving fast. After each three-hour bombardment, Ghiorso and Harvey pulled the foil out of the cyclotron vault and raced it roughly a mile across the Berkeley campus to the chemistry building, in Ghiorso's own car, so the separation could begin while there was still something left to separate.

Detection relied on a decay quirk. Mendelevium-256 captures an electron and becomes fermium-256, which promptly splits in half by spontaneous fission. Fission pulses are enormous compared with ordinary alpha decays and essentially unmistakable, so each atom announced itself unambiguously — one pulse at a time, through the night.

Naming a Russian chemist in 1955

The choice of name was not a neutral act. In 1955 the United States and the Soviet Union were several years into a nuclear arms race, and a Berkeley laboratory funded by the Atomic Energy Commission proposed to name a new element after Dmitri Mendeleev.

Seaborg was explicit that this was intended as a gesture: recognition of the Russian chemist who had built the framework the whole enterprise depended on, offered at a moment when scientific contact between the two countries had almost stopped. He recalled later that the proposal was not universally welcomed. It survived, and within a decade it looked prescient, because the Soviet laboratory at Dubna would soon be producing element claims of its own and the disputes that followed were bitter enough without an insult embedded in the table.

One detail did change. The discoverers originally proposed the symbol Mv. IUPAC replaced it with Md in 1957, which is why some papers from the mid-1950s use a symbol that no longer exists.

The +2 state, and a shell almost full

Mendelevium's chemistry is thin by necessity — everything known about it comes from tracer studies on at most a few thousand atoms — but it contains one result that matters for understanding where the actinide series is going.

Almost all actinides are stubbornly trivalent in solution. Mendelevium is not. It can be reduced to a +2 state readily, and mendelevium(II) is stable enough in aqueous solution to be separated on the basis of it. That behaviour is a direct consequence of electron count: taking mendelevium to +2 leaves a 5f¹³ configuration — a single electron short of the full shell — and that near-completion is starting to make the extra electron cheaper to keep than to give away. The same tendency strengthens sharply at element 102 and is one of the clearest demonstrations that the actinides are behaving as a genuine f-block series rather than as heavy transition metals.

A +1 state has been reported for mendelevium in older literature. It has not been substantiated by later work and is generally not accepted.

What exists today

Sixteen or so isotopes are known. Mendelevium-258 is the longest-lived at 51.5 days, and mendelevium-260 lasts about 32 days, both far more durable than the 77-minute isotope that was discovered first. Neither is produced in more than trace quantities, and the total amount of element 101 ever made across seventy years probably amounts to a few million atoms — still, collectively, far too little to see.

There is no mendelevium metal, no mendelevium compound in the solid state, and no application of any kind. What element 101 contributed was not a material but a method: after mendelevium, nobody expected to hold a new element again, and the question shifted from how much can we make to how few atoms can we identify with confidence. Every element above it has been claimed on that basis.