Dalton's Atomic Theory

Dalton's atomic theory, proposed by the English physicist and chemist John Dalton in the early 19th century, is a fundamental concept in chemistry that revolutionized the understanding of matter.
In 1808, Dalton published ‘A New System of Chemical Philosophy’, in which he proposed the following postulates, to explain the laws of chemical combination:
  1. Dalton proposed that all matter is composed of tiny, indivisible particles called atoms. According to this postulate, atoms are the fundamental building blocks of matter and cannot be further divided into smaller parts while retaining their chemical properties.
  2. Dalton suggested that all atoms of a particular element are identical in size, mass and chemical properties. This implies that atoms of different elements have different properties and behave differently in chemical reactions.
  3. Dalton proposed that chemical compounds are formed when atoms of different elements combine in fixed, simple, whole-number ratios to form compounds. This postulate is consistent with the Law of Definite Proportions, which states that compounds always contain the same elements in the same proportion by mass.
  4. According to Dalton, atoms can be rearranged, combined, or separated in chemical reactions.
  5. Dalton postulated that atoms are neither created nor destroyed in chemical reactions, but only rearranged to form new substances. This principle is consistent with the Law of Conservation of Mass, which states that the total mass of the reactants must equal the total mass of the products in a chemical reaction.
Note- Dalton’s theory could explain the laws of chemical combination. However, it could not explain the laws of gaseous volumes. It could not provide the reason for combining of atoms, which was answered later by other scientists.

Limitations of Dalton’s Atomic Theory

  • It does not account for subatomic particles: Dalton’s atomic theory stated that atoms were indivisible. However, the discovery of subatomic particles (such as electrons, protons and neutrons) disproved this postulate.
  • Elements do not need to combine in simple, whole-number ratios to form compounds. Some complex organic compounds do not have simple ratios of component atoms. e.g., Sugar/sucrose (C₁₂H₂₂O₁₁).
  • It does not account for isotopes: As per Dalton’s atomic theory, all atoms of an element have identical masses and densities. However, different isotopes of elements have different atomic masses (e.g., hydrogen, deuterium, and tritium).
  • It does not account for isobars: This theory states that the masses of the atoms of two different elements must differ. However, it is possible for two different elements to share the same mass number. Such atoms are called isobars (e.g., Argon and calcium, with atomic numbers of 18 and 20, respectively, share the same mass number of 40).
  • The theory does not account for allotropes: The differences in the properties of diamond and graphite, both of which contain only carbon, cannot be explained by Dalton’s atomic theory.

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