What were the main ideas in John Dalton's atomic theory?

John Dalton’s atomic theory, developed between 1803 and 1808, transformed earlier philosophical ideas into a scientific framework that provided a physical basis for understanding chemical combinations. His main ideas included:12

• Matter consists of atoms: All matter is composed of extremely small, indivisible particles called atoms.2
• Indestructibility: Atoms cannot be created, destroyed, divided into smaller pieces, or transformed into atoms of other elements.2
• Elemental identity: Each element is characterized by the unique mass of its atoms. All atoms of a specific element are identical in shape, size, and mass.12
• Chemical combinations: In chemical reactions, atoms combine in small whole-number ratios to form molecules. This concept helped explain the Law of Multiple Proportions, which states that when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers.21

While Dalton’s assumption that atoms were indivisible and his specific rules for determining molecular formulas were later refined, his foundational argument remains a key pillar of chemistry.312