Mendelevium – facts and properties

Position in the periodic table

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Occurrence and extraction

Mendelevium does not exist in nature. The longest-lived known isotope, mendelevium-258, has a half-life of a little over fifty days. Most other isotopes decay within hours or minutes.

The discovery team consisted of Albert Ghiorso, Bernard Harvey, Gregory Choppin, Stanley Thompson and Glenn Seaborg. They put roughly a billion atoms of einsteinium-253 on a gold foil and bombarded it with alpha particles in a cyclotron. The nuclear reaction produced single atoms of element 101. These were mendelevium-256, and they were knocked out of the foil and stuck to a second gold foil. That foil was dissolved, the solution was run through an ion-exchange column, and it was collected drop by drop. The team had predicted in which drop the new element should appear, and spontaneous fissions did turn up there. They came from the daughter nucleus fermium-256, into which mendelevium-256 changes by electron capture. Each run delivered only a few atoms, and the first runs yielded 17 in total.

The team chose the name to honour Mendeleev, who in 1869 arranged the elements by their properties and left gaps for unknown ones. In the era of the Cold War it was remarkable that American researchers named an element after a Russian chemist.

Properties and reactions

Mendelevium is in period 7 and is the thirteenth actinoid, in the actinoid row between fermium on its left and nobelium on its right. Thulium sits above it in the lanthanoid row. The atom's electrons follow the configuration [Rn] 5f¹³ 7s². The electronegativity of 1.3 and the melting point of 827 °C are estimates, because there is no metal sample to measure. Its atomic mass is given as 258 u, the mass number of the longest-lived isotope, mendelevium-258.

In solution plus three is the stable state. Mendelevium is among the first actinoids, though, for which the divalent state is easy to produce: a reducing agent turns Md³⁺ into the ion Md²⁺. This was achieved in 1967 with minute amounts. The trend continues to nobelium, where the divalent state is the most stable.

Mendelevium-258 decays by alpha radiation, shorter-lived isotopes such as mendelevium-256 by electron capture. Because at most traces form, the danger from the element itself is small. Even so, work follows the rules for radioactive materials.

Research and synthesis

Outside research mendelevium has no use. Its importance lies in what was learned from it. Because work had to be done with single atoms from the start, the Berkeley group developed methods that became standard: quick separation, detection through the decay of daughter nuclei, and assignment through behaviour in an ion-exchange column. The same principles are still used today to study the heaviest elements.

Mendelevium is now made in accelerators, from einsteinium or by fusing other atomic nuclei. With only a few atoms researchers determined how easily trivalent mendelevium can be reduced to the divalent form. Such values show how the filling of the 5f shell affects chemistry.

The discovery also began a principle that holds for all heavier elements: where not even a gram of material is available, a single atom is often enough to establish an element.

Quick question

What is the atomic number of mendelevium (Md)?

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