protium, isotope of hydrogen (q.v.) with atomic weight of approximately 1; its nucleus consists of only one proton. Ordinary hydrogen is made up almost entirely of protium.
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The Editors of Encyclopaedia Britannica. "protium". Encyclopedia Britannica, 6 Aug. 2024, https://www.britannica.com/science/protium-isotope. Accessed 8 April 2025.