13h 55min, jan 1, 1913 y - Frederick Soddy and chemist Theodore Richards

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The concept of atomic weight. As people continued to study radioactivity, it had become increasingly clear that there were multiple varieties of elements. For example, there are both radioactive and nonradioactive versions of carbon. Soddy and Richards prove that the difference lies in the weight of the atomic nucleus - there can be different versions of the same element with different weights. The different versions are christened isotopes, from

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