CLASS: IX NCERT (CBSE)
Atoms and Molecules
SUMMARY
• The law of conservation of mass states that the masses of the reactants and products remain unchanged.
• The law of constant proportions states that all pure specimens of a compound always contain the same elements combined together in the same proportion by weight.
• According to Dalton’s atomic theory, all matter is made up of very small particles called atoms, which cannot be further divided and have independent existence.
• The smallest particle of an element or a compound, which can exist in the free state, is called a molecule.
• A chemical formula represents the constituent elements of a compound and the number of atoms of each combining element.
• A mole contains 6.023 x 1023 particles. This is called Avogadro number.
• Molar volume is one mole of any gas at STP having a volume of 22.4 L.
• Mass of one mole of substance is called its molar mass.
