In 1929 the American chemist William Francis Giauque (1895- ) found that oxygen was composed of more than 1 isotope. Its atomic weight had been set arbitrarily at 16.0000 so it was a relief that 99.76% of its atoms were ¹⁶O. However, 0.20% were ¹⁸O, and 0.04% were ¹⁷O.
As you see, ¹⁶O must have a mass number of slightly less than 16.0000 and it must be the more massive isotopes ¹⁷O and ¹⁸O that pull the average up to 16.0000. Disregarding this, chemists clung to a standard atomic weight of 16.000 for oxygen as it appeared in nature, preferring not to concern themselves with the separate isotopes.
Physicists, however, felt uneasy at using an average as standard for they were more interested in working with individual isotopes. They preferred to set ¹⁶O at 16.0000 so that the average atomic weight of oxygen was 16.0044 and all other atomic weights rose in proportion. Atomic weights determined by this system were “physical atomic weights”.
Finally, in 1961, a compromise was struck. Chemists and physicists alike decided to consider the atomic weight of ¹²C as exactly 12 and to use that as a standard. By this system, the atomic weight of 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.
ENERGY
The Law of Conservation of Energy
We have now gone as far as we conveniently can in considering the intertwining strands of the atom and of electricity. It is time to turn to the third strand—energy.