Atoms and Molecules

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03 Dalton’s Atomic Theory

Atomic theory: The theory that ‘all matter is made up of very tiny indivisible particles called atoms’ is known as atomic theory of matter.

Postulates of Dalton’s atomic theory:

  • All the matter is made up of very small particles called ‘atoms’.
  • Atoms are indivisible particles.
  • Atoms can neither be created nor destroyed.
  • Atoms are of various kinds. There are as many kinds of atoms as are elements.
  • In a given element, all the atoms are identical in every respect. In fact, these atoms possess same mass, size and chemical properties.
  • In different elements, atoms differ in in mass, size and chemical properties.
  • Atoms of different elements join together to form molecules of compounds during the chemical reaction of two or more elements.
  • In any given compound, the ‘number’ and ‘kind’ of atoms are fixed.
  • Atoms of different elements combine in small whole numbers to form compounds during a  chemical combination
  • Atoms of same element can combine in more than one ratio to form more than one compound.

Drawbacks of Dalton’s Atomic Theory:

  • One of the major drawbacks of Dalton’s atomic theory of matter is that atoms were thought to be indivisible i.e. they cannot be divided. However, we now know that under special circumstances, atoms can be further divided into even smaller particles called electrons, protons and neutrons. We can say that, atoms are themselves made up of three particles: electrons, protons and neutrons.
  • According to Dalton’s atomic theory, all the atoms of an element have exactly the same mass. It is, however, now known that atoms of same element can have slightly different masses.
  • Dalton’s atomic theory says that atoms of different elements have different masses. But it is, now known that even atoms of different elements can have same mass.

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