Molybdenum is a chemical element with symbol Mo and atomic number 42. The name is from Neo-Latin molybdaenum, from Ancient Greek μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores.olybdenum-98 is the most abundant isotope, comprising 24.14% of all molybdenum. Molybdenum-100 has a half-life of about 10 19 y and undergoes double beta decay into ruthenium -100. Molybdenum isotopes with mass numbers from 111 to 117 all have half-lives of approximately 150 ns.