Hydrogen has no neutron, deuterium has one, and tritium has two neutrons. The isotopes of hydrogen have, respectively, mass numbers of one, two, and three.
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edit The atoms of these isotopes have one electron to balance the charge of the one proton.
Hydrogen has three isotopes, called hydrogen or protium, heavy hydrogen or deuterium, and tritium.

Heavy hydrogen is used in the preparation of the hydrogen bomb.

Tritium is used in nuclear fusion reactions. 

Ordinary hydrogen or protium is represented as H. One of the applications of hydrogen is the hydrogenation of materials, such as nitrogen (to produce ammonia).

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