Molybdenum (Mo) is a chemical element with atomic number 42. It is a metal, but can't be found in its free metallic form in nature, only in oxidized form. One notable property of molybdenum is that it is relatively unaffected by high temperatures. When heated up, it expands less than most metals, and it is therefore useful in extreme conditions for example in military armors.
There are multiple stable molybdenum isotopes, but the most abundant one is 98Mo, with 42 protons and 56 neutrons.