Mendelevium

Mendelevium is a radioactive actinide, the ninth element beyond uranium and the first that cannot be made by neutron capture in a reactor. Its atoms carry a 5f13 7s2 electron configuration, so like its lighter actinide neighbors it usually gives up three electrons to form the Md3+ ion. No visible amount has ever been isolated; every property has been measured on samples of a few thousand atoms or fewer, using tracer techniques where mendelevium is followed by its radioactivity.

Its chemistry has one twist. Because adding one more electron completes the 5f subshell at 5f14, mendelevium is unusually easy to reduce to the +2 state in water, more so than most actinides. In ion-exchange columns Md3+ elutes just before fermium, exactly where the actinide contraction predicts, which is how its position in the table was confirmed. The longest-lived isotope, mendelevium-258, has a half-life of about 51 days, long enough for slow, careful chemistry experiments.

Discovery

Albert Ghiorso, Bernard Harvey, Gregory Choppin, Stanley Thompson and Glenn Seaborg made mendelevium at Berkeley in early 1955 by bombarding about a billion atoms of einsteinium-253 with alpha particles in the 60-inch cyclotron. They identified 17 atoms in total, racing each sample to their detectors before its atoms decayed. It was named for Dmitri Mendeleev, the creator of the periodic table, a pointed choice by American scientists during the Cold War.