Mendelevium · element 101
The first element discovered one atom at a time, from just seventeen atoms in a Berkeley laboratory
Key facts
Mendelevium was the first element made and identified atom by atom. In February 1955 a team at Berkeley fired alpha particles at a target holding roughly a billion atoms of einsteinium-253, a quantity far too small to see. Across a series of overnight runs they caught seventeen atoms of the new element. Each announced itself by a single spontaneous fission event, recorded on a detector wired to ring a bell.
The technique they invented, catching recoiling atoms on a foil and rushing them through chemistry before they decayed, became the template for every heavier element found since. Mendelevium is named after Dmitri Mendeleev, the Russian chemist whose periodic table predicted elements before they were discovered.
Where it is found
Mendelevium does not exist in nature and never has in any detectable amount. Its most stable isotope, mendelevium-258, lasts about 51 days, and most others decay within hours. It also cannot be made in a reactor by neutron capture, because the fermium gap blocks the route.
It is produced in particle accelerators by bombarding einsteinium with alpha particles, or more rarely by firing heavier ions at lighter actinide targets. Even the best experiments yield only a few million atoms, which sounds a lot but weighs less than a trillionth of a gram. Every mendelevium atom ever made would fit, unseen, on the head of a pin many times over.
How it was discovered
Albert Ghiorso, Bernard Harvey, Gregory Choppin, Stanley Thompson and Glenn Seaborg carried out the discovery at the Berkeley 60-inch cyclotron in 1955. They plated einsteinium onto a thin gold foil and bombarded it with alpha particles. Atoms that fused with an alpha particle were knocked out of the target and landed on a second gold foil behind it. This catcher foil, free of the intensely radioactive target, was dissolved and passed down an ion-exchange column.
Fractions from the column were checked for spontaneous fission from the decay product of mendelevium-256. Over the experiment they logged seventeen events. The team drove the samples from the cyclotron to the chemistry laboratory by car because time was so short. They proposed the name mendelevium, and because it honoured a Russian during the Cold War, Seaborg later recalled seeking approval from the US government first. The symbol was initially Mv and changed to Md in 1957.
What we use it for
Mendelevium has no uses beyond research and never will, given its half-lives and the atom-scale quantities available. Its scientific value lies in what it reveals about the end of the actinide series. Experiments with a few thousand atoms at a time have shown that mendelevium forms +3 ions like its neighbours but can be reduced to a stable +2 state, and there is evidence it can reach +1 under some conditions.
That growing preference for lower oxidation states, moving from fermium through mendelevium to nobelium, is the signature of the 5f shell becoming complete and the 7s electrons being held more tightly by relativistic effects. Mendelevium data help theorists calibrate models of heavy-element chemistry that are then used to predict the behaviour of elements nobody can study in detail.
Reading the numbers
Mendelevium's melting point of about 830 °C is an estimate, not a measurement, and no boiling point or density has ever been determined, because a solid sample has never been made. The atomic mass of 258 is that of its longest-lived isotope. Its configuration [Rn] 5f13 7s2 sits under thulium, and its solution chemistry as a +3 ion agrees, though the ease with which it forms +2 ions is more pronounced than in thulium. The first ionisation energy of about 635 kilojoules per mole is a calculated value, continuing the slow climb across the actinides. Its electron affinity in our data is positive and comparatively large, which suggests a free mendelevium atom can accept an extra electron, though this too comes from theory rather than experiment.
Safety and the environment
Mendelevium is intensely radioactive, but so little has ever existed that it has no environmental footprint whatsoever. It decays within hours or weeks into fermium and lighter elements. The genuine hazard in mendelevium experiments comes from the einsteinium targets, which are among the most radioactive materials handled anywhere, and from the accelerator itself. Work is done behind shielding with remote handling.
Did you know?
- The discovery relied on just seventeen atoms, recorded one at a time as spontaneous fission events.
- The einsteinium target contained about a billion atoms, too few to be visible and weighing far less than a nanogram.
- A bell was wired to the detector so that the team would hear each mendelevium atom as it decayed.
- Mendelevium was the first element named after a Russian by American scientists during the Cold War, and the team sought government clearance beforehand.
- Its symbol was Mv for two years before being changed to Md in 1957.
More in the actinides
The nearest neighbours of mendelevium in the family, by atomic number.
See all 15 actinides on one page, or browse all 118 elements.
Frequently asked questions
Who discovered mendelevium?
A team at Berkeley led by Albert Ghiorso, with Glenn Seaborg, in February 1955. They made seventeen atoms by bombarding einsteinium with alpha particles.
Why is it called mendelevium?
It honours Dmitri Mendeleev, who created the periodic table in 1869 and predicted the existence of elements not yet found.
What is mendelevium used for?
Nothing outside research. Only atoms at a time have ever been made, so it is studied to understand how the heaviest elements behave chemically.
Is mendelevium found in nature?
No. Its isotopes decay too fast, and no natural process creates it. It is made only in particle accelerators.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Mendelevium reference article. Spotted an error? Tell us.