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The laws of chemical combination, established by Lavoisier and Proust, demonstrated that chemical reactions follow definite rules concerning mass. But what was the underlying explanation for these observations? The answer came from a British chemist named John Dalton, who, in 1808, presented his groundbreaking atomic theory. This theory marked a pivotal moment in the study of matter, transforming the ancient philosophical concept of the atom into a scientifically testable framework.

John Dalton, born in a humble weaver's family in 1766, began his career as a teacher at a young age. He spent most of his life teaching and researching, eventually arriving in Manchester to teach mathematics, physics, and chemistry. It was there that he developed his revolutionary ideas about atoms.

Dalton revived the ancient Greek idea of indivisible particles called 'atoms' (meaning indivisible), which had previously been confined to philosophical discussions. His genius lay in connecting this philosophical concept to the experimentally observed laws of chemical combination. Dalton's atomic theory successfully provided a coherent explanation for both the Law of Conservation of Mass and the Law of Constant Proportions.

Key Postulates of Dalton's Atomic Theory:

Dalton's atomic theory can be summarized through its fundamental postulates:

1. All matter is made of very tiny particles called atoms. These atoms are the smallest units that participate in chemical reactions.
2. Atoms are indivisible particles. According to Dalton, atoms cannot be created or destroyed in a chemical reaction. This postulate explains the Law of Conservation of Mass, as mass is simply conserved by the rearrangement of indestructible atoms.
3. Atoms of a given element are identical in mass and chemical properties. This means all atoms of, say, oxygen, are exactly alike in every way.
4. Atoms of different elements have different masses and chemical properties. For instance, an atom of oxygen is distinct from an atom of carbon in both mass and how it behaves chemically.
5. Atoms combine in the ratio of small whole numbers to form compounds. This postulate elegantly explains the Law of Constant Proportions. It states that when elements combine to form a specific compound, they always do so in simple, fixed numerical ratios of atoms, leading to definite proportions by mass.
6. The relative number and kinds of atoms are constant in a given compound. This reinforces the idea that each compound has a unique and unchanging atomic composition.

While modern science has since discovered that atoms themselves are made of still smaller particles (subatomic particles), Dalton's atomic theory provided the essential conceptual framework that allowed chemistry to develop into a quantitative science. It remains a cornerstone of chemical understanding, explaining how elements combine to form the vast array of substances in our world.

Fun Fact: John Dalton also made significant contributions to meteorology and was the first to study color blindness, a condition from which he himself suffered.

Question for You: Which postulate of Dalton's atomic theory directly explains why the ratio of hydrogen to oxygen by mass in water is always 1:8?