CBSE Class 9 Chemistry Notes for Chapter 3 Atoms and Molecules

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          CHAPTER 3 ATOMS AND MOLECULES

 

LAWS OF CHEMICAL COMBINATION

There are two laws of chemical combination:

  1. Laws of conservation of mass: This law states that during a chemical reaction, mass can neither be created nor destroyed.
  2. Laws of constant proportions: This law states that in a chemical substance, elements are always present in definite proportions by mass.

 

DALTON’S ATOMIC THEORY

The postulates of this theory are:

  • Matter is made of very tiny particles called atoms.
  • Atoms are indivisible. They cannot be created or destroyed during a chemical reaction.
  • All atoms of an element are identical. They all have the same physical and chemical properties.
  • Atoms combine in the ratio of whole numbers to form compounds.

 

ATOM

An atom is the building block of all matter. The size of an atom is very, very small. In fact, the radius of an atom is around 1 nm i.e. 10-9 m.

Atomic Mass: One atomic mass unit(amu) is equal to 1/12th the mass of one atom of carbon 12.

 

MOLECULE

A molecule is a group of two or more atoms held together by attractive forces. For e.g. CO2 is a molecule of carbon dioxide.

 

 

ION

The charged particles in substances are called ions. A positively charged ion is called a cation and a negatively charged ion is called anion. For e.g. HCl contains H+ ions and Cl ions.

 

CHEMICAL FORMULA

The chemical formula of a compound is determined by the valency of each element.

For e.g. The valency of H is 1 and that of S is 2. The valencies are exchanged and the formula becomes H2S.

 

MOLECULAR MASS

The molecular mass is the sum of the atomic masses of all the atoms in the molecule.

 

FORMULA UNIT MASS

The formula unit mass is the sum of the atomic masses of all atoms in a formula unit of the compound.

 

MOLE CONCEPT

One mole of a substance is that quantity which has mass equal to its atomic or molecular mass in grams.

The number of particles present in 1 mole of any substance is constant i.e. 6.022 x 1023. This value is known as Avogadro’s Constant (N0).

 

MOLAR MASS

The mass of 1 mole of a substance is called molar mass.

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