NCERT Class 9 Science Solution, Notes, Atoms and molecules

Atoms and molecules

1.        Laws of chemical combination

  • Law of conservation of mass: Mass can neither be created nor destroyed in a chemical reaction
  • Law of constant proportion or Law of definite proportion:  In a chemical substance the elements are always present in definite proportions by mass

2.        Postulates of Dalton’s atomic theory

  • All matter is made of very tiny particles called atoms
  • Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction
  • Atoms of a given element are identical in mass and chemical properties
  • Atoms of different elements have different masses and chemical properties
  • Atoms combine in the ratio of small whole numbers to form compounds
  • The relative number and kinds of atoms are constant in a given compound

3.        Some Limitations of Dalton’s atomic theory

  • According to Dalton’s atomic theory atoms of a given element are identical in mass and chemical properties. Atoms of different elements have different masses and chemical properties. This is wrong because atoms of some elements vary in their mass and density. Such atoms of the same element having different masses are called isotopes. For example, chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.
  • According to Dalton’s atomic theory atoms of different elements have different masses and chemical properties. This has been proved wrong in certain cases like argon and calcium atoms, which have the same atomic mass of 40. Such atoms of different elements that have the same atomic mass are called isobar.
  • According to Dalton’s atomic theory atoms combine in the ratio of small whole numbers to form compounds. This is not seen in complex organic compounds like sugar C12H22O11. Ratio of C, H and O is 12:22:11 which is not simple.

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