Most Common Isotope of Hydrogen
Puzzle 2 of Celtic Mythology Hard Pack
Question is: Most Common Isotope of Hydrogen
and the answer is: PROTIUM
Word Meaning:
PROTIUM — (physics) The lightest and most common isotope of hydrogen, having a single proton and no neutrons: 11H. It is the only stable nuclide with no neutrons.
Definitions from Wiktionary.