Hydrogen has an atomic number of 1.
This means that hydrogen atoms have a single proton. They also have a
single electron: this is lost when a hydrogen atom turns into a hydrogen
ion.
However, there are three different isotopes of
hydrogen: normal hydrogen atoms do not have any neutrons; their atomic
mass is 1. Heavy hydrogen (deuterium) also has a single neutron that
makes it twice as heavy as a normal hydrogen atom; it has an atomic mass
of 2.
