CHEMISTRY - Dalton's Atomic Theory

To describe the structure of matter which could explain the experimental fact's known at that time about elements , compounds and mixtures and also the law of chemical combination, John Dalton in 1808 put a forward a theory known as Dalton's atomic Theory. The main points of this theory are as follows :-

1. Matter is made up of extremely small indivisible particle called atom.
2. Atoms of the same element are identical in all respects i.e size, shape, and mass.
3. Atoms of different elements have different masses, sizes and also pissess different chemical properties.
4. Atoms of same different element combined together to form compound atom.
5. When atoms combine with one another to form compound atoms, they do so in simple whole number ratios, such as 1:1 , 2:1 , 2:3 and so on.
6. Atoms of two elementsmay combine in different ratios to form more than one compound.
7. An atom is the smallest particles that takes part in chemical reactions.
8. An atom can neither be created nor desyroyed.

:- Law of Chemical Combination by Dalton's Atomic Theory :-

1. Law of conservation of mass. Matter is made up of atoms which can neither be created nor destroyed. Hence, matter can neither be created nor destroyed.
2. Law of constant composition. It follows directly from postulate 5.
3. Law of multiple proportion. It follows directly from postulate 6.
4. Law of reciprocal proportons. As atoms combine with each other in simple ratio, therefore all the ratio involved are simple which may be same or some simple multiple of each other.

Limitation of Dalton's Atomic Theory :-
1. It could explain the laws of chemical combinationby mass but failed to explain the law of gaseous volume.
2. It could not explain why atoms of different elements have differents masses, sizes, valencies etc.
3. Why do atoms of the same or different element combine at all to form molecule ?