Ask A Scientist Chemistry Archive

(Created prior to 1993)

Question: Why is a hydrogen proton considered an isotope?
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Strictly speaking an isotope is one of two or more atoms that 
have the same atomic number but different mass numbers.  The element hydrogen 
has three isotopes known respectively as hydrogen, deuterium, and tritium.  
Hydrogen has one proton in its nucleus, deuterium has a proton and one 
neutron, and tritium has one proton and two neutrons.  Each has one electron 
to balance the charge of the single proton, but differ in mass due to the 
different number of neutrons in the nucleus.  Very little deuterium and 
tritium occur in nature (approx. 1/100 of a percent or less). 
woodford
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