Dalton's atomic theory

John Dalton's early nineteenth-century theory held that matter consists of indivisible atoms, with each element made of its own kind of atom and compounds formed in fixed whole-number ratios.

Law of conservation of mass: In a closed system, the total mass remains constant during a chemical reaction. Dalton explained this law by treating reactions as rearrangements of atoms, not their creation or destruction.
Chemical reaction: A process in which substances change as atoms are rearranged into new combinations. Dalton pictured reactions as rearrangements that preserve each kind of atom.
John Dalton's law of partial pressures: A gas mixture’s total pressure equals the sum of the pressures each gas would exert alone. Dalton’s studies of gas mixtures helped prompt his search for a particle-based account of matter.
Subatomic particle: A particle smaller than an atom, such as an electron, proton, or neutron. Subatomic particles disproved Dalton’s claim that atoms are indivisible.
Law of definite proportions: A pure chemical compound contains its elements in a fixed proportion by mass. Dalton’s fixed combinations of atoms supplied a particle-level explanation for this constant composition.
Atomic mass: The mass of an atom, commonly expressed in unified atomic mass units. Comparing atomic masses lets chemical proportions reveal possible atom counts.
Meteorology: The scientific study of the atmosphere and its weather. Dalton’s investigations of air and gases began within his meteorological work.
Isotope: One of two or more forms of an element whose atoms have different neutron counts. Isotopes show that atoms of one element need not all have the same mass.
Law of multiple proportions: When two elements form several compounds, their masses combine in ratios of small whole numbers. This regularity was Dalton’s strongest quantitative evidence for atoms combining in discrete counts.
Molecular formula: A formula specifying the actual number of each type of atom in one molecule. It expresses the discrete combinations that Dalton’s account proposed for compounds.

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