Dalton's Atomic Theory Class 11 Notes | EduRev

Chemistry Class 11

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DALTON'S ATOMIC THEORY

Matter has been one of the most important subjects of research for the science enthusiasts. Scientists and philosophers have always tried to simplify things and so was the case with the matter. They wanted to know about the fundamental particles that make matter, their properties, structure etc. This led to the formulation of a number of atomic theories.

Democritus was the first man who proposed that matter is made up of particles. He named these particles, ‘atomos’ meaning indivisible. This was the Democritus Atomic Theory. Due to the lack of technological setup back then, scientists had very limited information on this theory.

Dalton`s Atomic Theory Class 11 Notes | EduRevFig: John DaltonAlmost after two thousand years, the works on the simplifying matter was materialized by scientist, John Dalton. In 1808, John Dalton postulated the famous Dalton’s Atomic Theory. He published this theory in a paper titled “A New Chemical Philosophy”; indeed the philosophy was new for that era. Let us now look at the postulates of this theory.

Postulates of Dalton’s Atomic Theory

  • The matter is made up of indivisible particles known as atoms.
  • The properties of all the atoms of a given element are same including mass. This can also be stated as all the atoms of an element have identical mass while the atoms of different elements have different masses.
  • Atoms of different elements combine in fixed ratios to form compounds.
  • Atoms are neither created nor destroyed. This implies that during chemical reactions, no atoms are created nor destroyed.
  • The formation of new products (compounds) results from the rearrangement of existing atoms (reactants).
  • Atoms of an element are identical in mass, size and many other chemical or physical properties, but atoms of two-different elements differ in mass, size and many other chemical or physical properties.
    However, this theory is not entirely free of limitations. Let us now look at the drawbacks of this theory.Dalton`s Atomic Theory Class 11 Notes | EduRevFig. Dalton's Atomic theoryDrawbacks of Dalton’s Atomic Theory of Matter
  • It was proved that an atom is not indivisible. As an atom can be subdivided into electrons, protons and neutrons. But remember that atom is the tiniest particle that takes part in a chemical reaction.
  • According to Dalton Atomic Theory, atoms of an element are identical in mass, size and many other chemical or physical properties. But, practically we observe that atoms of several elements differ in their densities and masses. These atoms with the different masses are known as isotopes. For example, Chlorine (Cl) has 2 isotopes with the mass numbers of 35 and 37.
  • Also, according to Dalton Atomic Theory, atoms of two-different elements differ in mass, size and many other chemical or physical properties. However, this is not correct for all situations. For example, Argon (Ar) and Calcium (Ca) atoms, each have an atomic mass of 40 amu. These atoms with similar atomic masses are isobars.
  • According to Dalton Atomic Theory, when atoms of different elements (atoms of two or more elements) combine in simple whole number ratios, we get chemical compounds. But this is not true in case of complex organic compounds.
  • Dalton Atomic Theory fails to explain the existence of allotropes. This implies that the Dalton atomic theory fails to explain the differences in properties of charcoal, graphite and diamond (allotropes of carbon).
  • Dalton’s Atomic Theory also suggested that an atom is the smallest part of an atom that can take part in a chemical reaction. Some postulates of this theory remain valid even in today’s modern chemical thoughts. The atomic structure model proposed by indeed proves to be a significant, stepping stone in chemistry. It forms the base for modern atomic theories and quantum mechanics.

Solved Examples
Q: Give the supporting laws for Dalton’s Atomic Theory.
Ans: John Dalton based his theory on two laws. They are explained below:

  • Law of Conservation of Mass:
    According to the law of conservation of mass, the matter is neither created nor destroyed. This means, in a chemical reaction, amount of elements remains same in starting when only reactants there and at the completion of the reaction when product formed. We always use the “Law of conservation of mass” when we balance chemical equations.

Dalton`s Atomic Theory Class 11 Notes | EduRevFig. Law of conservation of Mass

  • Law of Constant Composition:
    According to the law of constant composition, a pure compound will always have the same proportion of the same elements. For example, table salt with the molecular formula of NaCl holds the same proportions of the elements Na (sodium) and Cl (chlorine). This composition doesn’t depend on where the salt came from and how much salt one should have.

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