John Dalton argued for the existence of atoms mainly by using quantitative chemical evidence , especially how elements combine by mass in chemical reactions.

Core Evidence Dalton Used

  1. Law of Conservation of Mass (Lavoisier’s work)
    • In chemical reactions, the total mass of reactants equals the total mass of products.
 * Dalton reasoned that this makes sense if matter is made of tiny, indestructible particles (atoms) that are just rearranged, not created or destroyed.
  1. Law of Definite Proportions (Proust’s law)
    • A given compound always contains the same elements in the same fixed mass ratio (for example, water is always the same ratio of hydrogen to oxygen by mass).
 * Dalton explained this by saying compounds are made from atoms combining in **simple whole‑number ratios** (like 2 H atoms for every 1 O atom in water).
  1. Law of Multiple Proportions (Dalton’s own key insight)
    • When two elements form more than one compound (like carbon monoxide vs carbon dioxide), the masses of one element that combine with a fixed mass of the other are in simple whole‑number ratios (e.g., 1:2).
 * Dalton argued this only makes sense if you’re adding whole atoms: you can’t have “half an atom,” so the combining ratios must be 1:1, 1:2, 2:3, etc., not messy fractions.
  1. Consistent relative atomic masses
    • Dalton compared masses in reactions and assigned relative atomic weights to elements (like hydrogen = 1, oxygen ≈ 16), which allowed him to write simple formulas for compounds.
 * The fact that these relative masses and formulas worked consistently across many reactions supported his claim that elements are made of distinct kinds of atoms with different masses.

How He Turned This into Atomic Theory

From that evidence, Dalton proposed that:

  • All matter is made of tiny, indivisible atoms.
  • Atoms of the same element are identical in mass and properties; atoms of different elements have different masses and properties.
  • Atoms combine in simple whole‑number ratios to form compounds, and in reactions they are just rearranged.

So, Dalton’s “evidence” wasn’t seeing atoms directly , but showing that the measured masses in chemical reactions obeyed simple laws that are naturally explained if matter is made of discrete, indivisible atoms combining in whole numbers.

Information gathered from public forums or data available on the internet and portrayed here.