Prout's hypothesis

Prout's Hypothesis suggested that all atomic weights are whole number multiples of hydrogen, proposing that hydrogen constitutes the fundamental atom and that other elements are composed of hydrogen atoms. This was a significant step in atomic theory, though it was later proven to be an oversimplification.
 William Prout England 1815 Prout's hypothesis

Prout's Hypothesis suggested that all atomic weights are whole number multiples of hydrogen, proposing that hydrogen constitutes the fundamental atom and that other elements are composed of hydrogen atoms. This was a significant step in atomic theory, though it was later proven to be an oversimplification.