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taskmasters
The element Hydrogen has three isotopes. The hydrogen (1H), the deuterium (2H), and tritium (3H) differs in number of their neutrons where hydrogen has none while deuterium has one and tritium has two. They also differ in mass numbers where hydrogen has one while deuterium has two and tritium has three. 

Its just that the number of neutrons range from 0 to 2.  (distance from 1 to 3 is 2)

Not totally sure about this one... I had it as a question in a 25 question test, and this was my answer. I got a 96% on that test, which means I only got one wrong,  so this is probably right.