Project Gutenberg #49819
Worlds Within Worlds: The Story of Nuclear Energy, Volume 1
Isaac Asimov
1972The first volume of Asimov's accessible history of nuclear energy, prepared section by section from Project Gutenberg HTML.
Project Gutenberg #49819 Public domain in the United States Cover source Local typographic cover created for MojiMori from public-domain source metadata
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oxygen became 15.9994, which is only very slightly less than 16.
The radioactive elements did not escape this new view either. The atomic weight of uranium (chemical symbol U) is just about 238 and, indeed, most of its atoms are ²³⁸U. In 1935, however, the Canadian-American physicist, Arthur Jeffrey Dempster (1886-1950), found that 0.7% of its atoms were a lighter isotope, ²³⁵U.
These differed considerably in radioactive properties. The common uranium isotope, ²³⁸U, had a half-life of 4500 million years, while ²³⁵U had a half-life of only 700 million years. Furthermore ²³⁵U broke down in three stages to actinium. It was ²³⁵U, not actinium itself, that was the beginning of the actinium radioactive series.
As for thorium (atomic symbol Th) with an atomic weight of 232, it did indeed turn out that in the naturally occurring element virtually all the atoms were ²³²Th.
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