protium

Pronunciation: /ˈpɹəʊ.tiəm/

The English word “protium” has 2 meanings listed here.

noun

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.
An atom of this isotope.

From proto- + -ium. From Ancient Greek πρῶτος (prôtos, “first”).

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