Isotopes and Atomic Masses

Isotopes have the same number of protons, but different number of neutrons. For example, Hydrogen-2 has 1 proton and one neutron, but Hydrogen-3 has 1 proton and 2 neutrons. The isotope of the element Hydrogen shown in the periodic table has 1 proton and no neutrons, and can be also called Hydrogen-1. An isotope's mass number is an integer. The mass number shown in the periodic table is nature's mix of isotopes.