Hydrogen has one proton in its nucleus but exists as three isotopes. One isotope has no neutrons, one has one neutron, and one has two neutrons.
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One isotope has no neutrons, H-1 "Hydrogen-1"
one has one neutron, H-2 "Hydrogen-2"
and one has two neutrons.H-3 "Hydrogen-3"
one has one neutron, H-2 "Hydrogen-2"
and one has two neutrons.H-3 "Hydrogen-3"