Basic chemistry

Dalton’s atomic theory

Dalton's atomic theory postulates


Already in the 6th century v. Greek philosophers like Heraklit, Anaximander and Thales thought about the nature and structure of matter. They suspected that the material world was made up of unchangeable, simple building blocks. Leukippus and Democritus also dealt with the essence of matter and assumed that all substances consist of the smallest, indivisible particles, the atoms (Gr. Atomos = indivisible). The ancient Greek theories were all based on purely abstract considerations, not experiments. These were only assumptions, not facts. Atomic theory was pure speculation for two thousand years. However, the existence of atoms has been widely accepted, for example by Robert Boyle in his workThe Skeptical Chymist (1661) or by Isaac Newton in his books Principia (1687) and Opticks (1704).

But it was not until 1808 that John Dalton succeeded in developing an atomic theory through scientific observations of chemical reactions and the recognition of laws.

Main postulates

The main postulates of Dalton’s atomic theory are:

    • Chemical elements consist of the smallest particles, the atoms, which cannot be further broken down. All atoms of an element are equal to each other, so they have the same mass and shape. Atoms of different elements have different properties. Each element consists of only one type of atom that is typical for the element.
    • Chemical compounds are created by the reaction of different elements. The atoms combine in simple numerical proportions.
    • In a chemical reaction, atoms are connected or separated from one another. Atoms are never destroyed or newly formed, and no atom of one element is converted into that of another element ( conservation of mass ).

Dalton’s theory is still valid today, even if the first postulate was modified somewhat. As far as we know today, most elements consist of different types of atoms that differ in their masses ( isotopes ).

This atomic theory suddenly explained some previously incomprehensible basic laws:

Basic Laws

Law of Conservation of Masses

In all chemical processes, the total mass of the substances involved in the reaction remains constant. There is therefore only one regrouping of the atoms. The total number of atoms remains the same.

The law of conservation of mass was drawn up in 1785 by A. Lavoisier.

Law of constant proportions

A chemical compound is always formed from constant mass ratios of the elements.

The law of constant proportions was established by J. Proust in 1799.


1 gram of carbon always combines with 1,333 grams of oxygen to form carbon monoxide CO, but not with quantities deviating from it.

Law of Multiple Proportions

If two elements form more than one connection, then the masses of the same element are related to each other in the ratio of small whole numbers.

The law of multiple proportions was established by J. Dalton in 1803.


1 gram of carbon + 1,333 grams of oxygen -> carbon monoxide CO

1 gram of carbon + 2,666 grams of oxygen -> carbon dioxide CO 2

The masses of carbon are in a ratio of 1: 1, those of oxygen in a ratio of 1: 2.

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