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Chemistry - Atoms and Molecules

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Laws of Chemical Combination:-

Antoine L. Lavoisier , known as ‘Father of Modern Chemistry’ laid the foundation of chemical science.

Law of conservation of mass

It states that ‘ mass can neither be created nor destroyed in a chemical reaction’.

Total mass of reactants must be equal to total mass of products i.e., why all chemical reactions are balanced.

E.g.    C  + O2 → CO2

12 + 32   =  12 + 2 x 16

Mass reactants = Mass of products

Law  of constant proportion  or  Law of Definite proportion

It states ‘In a pure chemical substance (compound) , the elements are always present in definite proportions of mass’.

Compound prepared by any method contains the same elements in the fixed ratio by mass, e.g., H2O, the ratio of mass of hydrogen to mass of oxygen is always 1:8, whatever may be the source.

2 H2O → 2H2  +  O2

If we decompose 9g of H2O , we shall get 1 g of hydrogen and 8 g of oxygen.

2H2          +         O2       →     2H2O
2  x  2       +      2  x  16 
=4                   =32

4g of H2 reacts with 32 g of Oxygen; 1g of H2 will react with 8 g of O2.

If we react 5g of hydrogen with 8 g of oxygen, 1 g of hydrogen will react with 8 g of oxygen to form 9 g of water and 4 g of hydrogen will remain unreacted.

Similarly in ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass, whatever the method or the source from which it is obtained.

All matter whether element, compound or mixture is composed of small particles called ‘atoms’.

Postulates of Dalton’s atomic Theory:

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

Advantages of Dalton’s Atomic theory

  1. This theory could explain the existing laws at that time.
  2. It could predict the law of multiple proportion.
  3. It proposed concept of atomic mass.

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Prepared By: Mrs. Kritika Bhola