Daltons Atomic Theory Some Basic Concepts Of Chemistry Class 11th Chapter 1 Science

Dalton's Atomic Theory: Postulates, Explanations, and Later Revisions

John Dalton's atomic theory, published in 1808 under the title "A New System of Chemical Philosophy," was the first comprehensive scientific theory of matter based on experimental evidence rather than philosophical speculation. It transformed chemistry from a collection of empirical observations into a coherent science with a unifying theoretical framework. Dalton's theory successfully explained all the known laws of chemical combination — the law of conservation of mass, the law of constant proportions, and the law of multiple proportions — and provided a clear mental picture of what atoms are and how they combine to form compounds.

Dalton's atomic theory consists of five main postulates. (1) All matter is made up of extremely small, indivisible particles called atoms. This was a revival of the ancient Greek concept of "atomos" (uncuttable) by Democritus (400 BCE), but Dalton supported it with experimental evidence from his studies of gas pressures and chemical reactions. (2) Atoms of the same element are identical in all respects — they have the same mass, size, and chemical properties. Atoms of different elements have different masses and different chemical properties. (3) Atoms cannot be created, divided into smaller particles, or destroyed in a chemical reaction — chemical reactions simply rearrange atoms from one combination to another. (4) Atoms of different elements combine in simple whole-number ratios (1:1, 1:2, 2:3, etc.) to form compounds. (5) The smallest particle of a compound is called a molecule, which is formed by the combination of atoms of two or more elements. Dalton also developed the first table of relative atomic masses, assigning hydrogen an atomic mass of 1 as the reference, and determining the masses of other elements relative to hydrogen.

Dalton's theory provided elegant explanations for the laws of chemical combination. It explains the law of conservation of mass because atoms are indestructible in chemical reactions — they merely rearrange, so total mass remains constant. It explains the law of constant proportions because compounds are formed by atoms combining in fixed, specific ratios — water is always H₂O because two hydrogen atoms always combine with one oxygen atom, never a different ratio. It explains the law of multiple proportions by showing that when two elements form more than one compound, the different possible whole-number ratios of atoms lead to different compounds with masses related by simple whole-number ratios, such as CO and CO₂ (1:1 vs 1:2 carbon:oxygen atom ratio). Despite its monumental success, Dalton's theory was later modified in several important ways. The postulate that atoms are indivisible was disproved by J.J. Thomson's discovery of the electron (1897), Rutherford's discovery of the nucleus (1911), and Chadwick's discovery of the neutron (1932) — atoms are actually composed of subatomic particles. The postulate that atoms of the same element are identical is not entirely correct — isotopes (discovered by Soddy, 1913) are atoms of the same element with different numbers of neutrons and hence different masses, such as protium ¹H, deuterium ²H, and tritium ³H. The postulate that atoms combine only in simple whole-number ratios is true for molecular compounds but not for ionic compounds (formula units like NaCl, Al₂O₃) in the same sense. Also, Dalton's atomic mass values were inaccurate because he assumed water was HO (instead of H₂O) — his atomic mass of oxygen was 8 instead of 16. Nevertheless, the core principles of Dalton's theory remain the foundation of modern chemistry: matter is made of atoms, atoms are rearranged but not created or destroyed in chemical reactions, and compounds form by atoms combining in specific, characteristic ratios.

• Dalton's five postulates: (1) Matter is made of indivisible atoms. (2) Atoms of each element are identical. (3) Atoms cannot be created or destroyed. (4) Atoms combine in simple whole-number ratios. (5) The smallest particle of a compound is a molecule.
• Explains law of conservation of mass (atoms are indestructible), law of constant proportions (fixed combining ratios), and law of multiple proportions (different whole-number ratios give different compounds).
• Later revisions: atoms are divisible into subatomic particles (electrons, protons, neutrons); isotopes exist (same element, different masses); Dalton's own atomic mass values were sometimes incorrect.
• Modern view: atoms are divisible, but the principle that atoms rearrange in chemical reactions without being destroyed is still valid.
• Dalton's theory shifted chemistry from philosophical speculation to experimental science based on measurable quantities and testable hypotheses.