131.

How Dalton's atomic theory was modified on the basis of modern reseraches? or Give various postulates of modern atomic theory.

On the basis of new experimental facts, Dalton’s atomic theory has been modified as:

1. Atom is no longer indivisible. Atom is composed of still smaller particles which are called electrons, protons and neutrons.

2. Atoms of same element may not have same atomic mass. For example, some atoms of chlorine have atomic mass 35 amu while others have atomic mass 37 amu. Such atoms of same element having different atomic masses are called isotopes.

3. Atoms of different elements may have same atomic masses. For example, atomic mass of calcium and argon atoms are 40 amu. Such atoms of different elements having same atomic mass are called isobars.

4. There are no compound atoms but these are called molecules. Atom is the smallest particle that takes part in a chemical reaction.

5. The ratio in which atoms of different elements combine may not be simple e.g. in a molecule of sugar (C₁₂H₂₂O₁₁), the ratio of C,H and O atoms is 12 : 22 : 11 which is not simple.

6. Atoms of one element can be transmuted into atoms of other elements (Discovery of artificial transmutation).

7. Atom is no longer considered indestructible. In nuclear reaction, during the conversion of atoms of one substance into the atoms of other substance, some of the mass is lost and converted into energy according to the relation E = mc² (Einstein equation) where E = energy, m = mass and c is the, velocity of light (3×10⁸ ms^–1). Hence, in nuclear reaction atoms get destroyed and their mass changes into energy.