Atomic Masses of Nuclides
In 1815 Prout remarked that many atomic weights, expressed in the hydrogen atomic weight as a unit, were nearly whole numbers. He therefore suggested that the atoms of all elements were tightly bound combinations of hydrogen atoms. When slightly later it was discovered that some atomic weights were definitely non-integral numbers, one had either to abandon this hypothesis or to assume that atoms of one element could have different masses. The first alternative was chosen. In 1906, however, Boltwood 1 discovered ionium, which proved to be chemically inseparable from thorium, whereas the radioactive properties and the atomic weights of both substances were different. Within a short time many more examples of such a behaviour became known, and in 1910 Soddy 2 concluded that atoms of one element could have different masses; he suggested the name isotopes for bodies with the same chemical properties but different atomic weights. This discovery revived Prout’s hypothesis.
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