Mendelevium, << mehn duh LEE vee uhm or mehn duh LAY vee uhm, >> is an artificially produced radioactive element. It has an atomic number (number of protons) of 101. Its chemical symbol is Md.
Scientists have discovered 16 isotopes of mendelevium. Each isotope is a form of the element with 101 protons but a different number of neutrons. The atomic mass numbers (total numbers of protons and neutrons) of these isotopes range from 247 to 260. The most stable isotope has a mass number of 258 and a half-life of 51.5 days—that is, due to radioactive decay, only half the atoms in a sample of isotope 258 would be atoms of that isotope after 51.5 days.
Chemists classify mendelevium in the actinide group among the transuranium elements. For information on the position of mendelevium on the periodic table, see the article Periodic table.
In 1955, scientists at the Lawrence Radiation Laboratory (now Lawrence Berkeley National Laboratory) in Berkeley, California, claimed that they had produced element 101. They had bombarded einsteinium, whose atomic number is 99, with helium, whose atomic number is 2. The Berkeley scientists suggested the name mendelevium in honor of Russian chemist Dmitri Mendeleev. This name was accepted by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC is the recognized authority in crediting the discovery of elements and assigning names to them.
In 1986, the IUPAC and the International Union of Pure and Applied Physics formed a working group to review the histories of elements with atomic numbers from 101 to 109. In 1993, the IUPAC accepted the working group’s decision to credit the discovery of mendelevium to work done, not in 1955, but in 1958 by a different group at the Berkeley Laboratory.
