John Dalton's (1766-1844) atomic theory served scientists remarkably well for nearly a century after its announcement in 1803. But discoveries made in the late 1890s made it clear that Dalton's theory was incorrect or incomplete in some important details. The discovery of radioactivity by French physicist Antoine-Henri Becquerel (1852-1908) was among the most important of these. As scientists began to unravel the nature of this new phenomenon, they realized that atoms were far more complex than Dalton had imagined.

A particularly important discovery was made by English chemist Frederick Soddy (1877-1956) in 1913. During his investigation of naturally-occurring radioactive families, Soddy found that more than one form of an element could exist. These forms all had the same atomic number (and were, therefore, variations of a single element), but had different atomic weights. Soddy gave the name isotopes to these forms. Almost immediately, scientists began to ask whether...