How did John Dalton's theory explain why compounds have fixed compositions?

John Dalton’s atomic theory explained the fixed composition of chemical compounds by proposing that matter is composed of indivisible atoms, each characterized by a unique, distinct weight.12

Dalton reasoned that because elements consist of these discrete, constant-weight atoms, they must combine in specific, predictable ratios to form compounds. By examining the reacting weights of macroscopic quantities of elements, he inferred that compounds are formed by the combination of these atoms in fixed numerical proportions.21

This framework provided a physical basis for the law of definite proportions, which states that compounds contain constant proportions by mass of their constituent elements. Dalton further supported his theory with the law of multiple proportions, which observes that when two elements form more than one compound, the weights of one element that combine with a fixed weight of the other exist in a ratio of small whole numbers, reflecting the combination of whole atoms.31