Abstract:
Isotopes are atoms that belong to the same element,
but have different numbers of neutrons. For example,
hydrogen can have 0 neutrons (protium) or 1 neutron
(deuterium) or 2 neutrons (tritium). The mass changes
because of the extra neutrons, so protium hydrogen,
with only a proton, has an atomic mass of about
1 amu (atomic mass unit), deuterium hydrogen,
with one proton and one neutron, has a mass of
about 2 amu and tritium, with one proton and two
neutrons, is about 3 amu. Remember – if you change
the number of protons, you change the element.