How many isotopes does hydrogen have? How do they differ?

Hydrogen is the lightest, simplest element on the whole periodic table of elements.  It has one proton in its nucleus, with one electron in the first energy level.  Hydrogen has three known isotopes, or variations.  The thing that makes the different forms of hydrogen different is the number of neutrons in the nucleus, which have no charge.  Simple hydrogen has no neutron, while deuterium has one neutron.  Tritium has two neutrons.  All these different forms of hydrogen are still hydrogen, because they each contain only one proton, which has a positive charge.  The number of protons is what ultimately establishes the identity of the element.  So, even though hydrogen has a mass number of 1, deuterium has a mass number of 2, and tritium has a mass number of 3, they all have an atomic number of 1, which means they are all different varieties of the same thing, hydrogen