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Elements occur as various isotopes, variations of elements with different atomic weights. Harold Urey wondered if the smallest atom, hydrogen, had different isotopes, and he calculated how they ought to be constituted if that were the case. By distilling liquid hydrogen, a hydrogen isotope was extracted in 1932 that was twice as heavy as regular hydrogen. It was called deuterium. Water that contains deuterium, so-called heavy water, proved to have other chemical properties that differed from regular water, and in various ways deuterium became significant in nuclear technology.