Chemistry · Ch 1 — Some Basic Concepts of Chemistry
Dalton's Atomic Theory
Dalton's Atomic Theory
Dalton’s Atomic Theory
The idea that matter is made of tiny, indivisible particles is ancient — the Greek philosopher Democritus called them atomos, meaning “uncuttable” or “indivisible”. But that was philosophical speculation, not science. It took until 1808, when John Dalton published A New System of Chemical Philosophy, for the atomic theory to be placed on a firm experimental footing. Dalton’s theory was the first to explain the laws of chemical combination that had been established in the preceding century — the law of conservation of mass, the law of definite proportions, and the law of multiple proportions — by proposing a concrete model of the atom.
Dalton’s theory rests on four postulates.
Postulate 1. Matter consists of indivisible atoms.
An atom is the smallest particle of an element that can take part in a chemical reaction. It cannot be subdivided further by chemical means. This postulate directly reflects the ancient Greek notion, but now it is grounded in experimental evidence from quantitative chemical analysis.
Postulate 2. All atoms of a given element have identical properties, including identical mass. Atoms of different elements differ in mass.
This is the crucial claim that gives each element its unique identity. If every atom of carbon has the same mass, and every atom of oxygen has a different (but also fixed) mass, then the mass ratios in compounds become predictable. The property of atomic mass — the relative mass of an atom of one element compared to an atom of another — becomes the central quantitative tool of chemistry.
We now know that atoms of the same element can have different masses — these are called isotopes. Dalton’s postulate is strictly true only for a pure sample of a single isotope. For naturally occurring elements, the atomic mass we use is a weighted average of all isotopes. But for the purpose of explaining the laws of chemical combination, Dalton’s assumption works perfectly.
Postulate 3. Compounds are formed when atoms of different elements combine in a fixed ratio.
When elements react to form a compound, the atoms combine in a simple, whole-number ratio. For example, water is always formed from hydrogen and oxygen atoms in the ratio 2:1. This is the atomic explanation of the law of definite proportions — the fixed ratio of masses is a direct consequence of the fixed ratio of atoms.
Postulate 4. Chemical reactions involve reorganisation of atoms. These are neither created nor destroyed in a chemical reaction.
Atoms are indestructible. In a chemical reaction, the atoms present in the reactants are simply rearranged to form the products. No new atoms are created, and no existing atoms vanish. This is the atomic explanation of the law of conservation of mass — the total mass of the reactants equals the total mass of the products because the same atoms are present before and after the reaction, just in different arrangements.
The four postulates together provide a complete atomic explanation for the three fundamental laws of chemical combination:
- Conservation of mass (Postulate 4)
- Definite proportions (Postulate 3)
- Multiple proportions (Postulate 2, combined with Postulate 3)
What Dalton’s Theory Could and Could Not Explain
Dalton’s theory was a triumph. It unified the known laws of chemical combination under a single, simple model. For the first time, chemists had a mental picture of what was happening at the atomic level during a reaction.
However, the theory had limitations. …