Answer: To describe the structure of atom and laws of chemical combination, Dalton’s Atomic Theory was put forward. According to this theory, matter is made up of small particles called atoms which are indivisible and indestructible. Atoms of same element are identical in shape, size, mass and chemical properties but the properties are different for atoms of different elements. They react with each other in different ratios (in simple whole number ratios) to form compounds or molecules.This theory has some limitations like
1) It can explain the laws of chemical combination by mass but not by gaseous volumes.
2) it cannot explain why atoms of different elements have different properties.
3) Why atoms of different elements combine to form molecules?
4) What is the nature of binding forces that exist between atoms and molecules to form three states i.e. solid, liquid and gases?
To explain all above points modern atomic theory was introduced. According to this theory-
1) Atom is not indivisible as it is not the smallest particle. It is made up of electrons, protons and neutrons.
2) Atoms of different elements may have same atomic masses. Such atoms are called isobars.
3) Atoms of same element may have different atomic number. These atoms are called isotopes.
4) Atoms of different elements may react with each other in integral ratios also. It is not necessary to react in simple whole number ratios to form compounds or molecules.
5) Atoms are not indestructible. Atom of one element can be changed into another.
Because of all these explanations that modern atomic theory can give, it is better than Dalton's Atomic theory.
