CBSE Class 9 Answered

John Dalton put his theory for atoms in the year 1808. His findings were based on experiments and also from laws of chemical combination.

Main assumptions or postulates of Dalton

• All matter consists of indivisible particles called atoms.

• Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements.

• Atoms cannot be created or destroyed.

• Atoms of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms.

• Atoms of same element can combine in more than one ratio to form two or more compounds.

• Atoms are the smallest unit of matter that can take part in a chemical reaction.

Shortcomings of Dalton's atomic theory

• The indivisibility of an atom was proved wrong, for, an atom can be further subdivided into protons, neutrons and electrons. However an atom is the smallest particle, which takes part in chemical reactions.

• According to Dalton, the atoms of same element are similar in all respects. 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.

• Dalton also said atoms of different elements are different in all respects. 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.