John Dalton argued for the existence of atoms primarily based on experimental evidence and laws of chemistry known at his time:
- He observed that matter always combined in fixed ratios by weight or volume, reflecting the law of constant composition and the law of conservation of mass. Dalton explained these laws by proposing that matter is made up of tiny, indivisible particles called atoms, which combine in simple whole-number ratios to form compounds
- Dalton's experiments with gases, such as his discovery that oxygen combined with nitric oxide in fixed volume ratios, provided empirical evidence supporting his atomic ideas and the concept of atoms combining in definite proportions
- He also formulated Dalton's Law of Partial Pressures , showing that gases exert pressure independently, which supported the idea that gases are composed of discrete particles (atoms or molecules) moving and colliding in space
- Dalton assigned atomic weights to elements based on their combining ratios, proposing that atoms of different elements differ in mass and properties, which explained chemical behavior quantitatively
- Although Dalton could not see atoms directly, he used these consistent quantitative relationships in chemical reactions to argue that atoms must exist as fundamental building blocks of matter
In summary, Dalton's evidence for atoms was grounded in the consistent, reproducible laws of chemical combination and gas behavior, which he explained by hypothesizing indivisible particles (atoms) with characteristic weights that combine in fixed ratios during chemical reactions
