The element molybdenum (Mo) has an atomic number of 42, which means that it has 42 protons in its nucleus. The number of neutrons in an atom can vary, and molybdenum has several isotopes with different neutron numbers. The most common isotope of molybdenum, molybdenum-98 (Mo-98), has 56 neutrons. Therefore, molybdenum-98 has a total of 42 protons and 56 neutrons.