Hydrogen, in its most common isotope form (known as protium or hydrogen-1), has 0 neutrons. Its nucleus consists of only one proton, and it does not contain any neutrons. This is because the mass number of the most common hydrogen isotope is 1, which is the total number of protons and neutrons, and since there is 1 proton, there must be 0 neutrons. There are other isotopes of hydrogen, such as deuterium which has 1 neutron, and tritium which has 2 neutrons, but the typical hydrogen atom has 0 neutrons.