Class 9 Atoms and Molecules Notes

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Atoms and Molecules Notes

Laws of chemical combination:-

  • Law of conservation of mass:- It states that mass can neither be created nor be destroyed in a chemical reaction.
    It states that the total mass of reactants is equal to total mass of products in a chemical reaction.
  • Law of constant proportion:- It states that in a chemical substance the elements are always in constant proportion by mass.

Dalton’s atomic theory:-

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

Postulates of Dalton’s atomic theory:-

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

One atomic mass unit:- One atomic mass unit is the mass unit equal to exactly one twelfth (1/12th) the mass of one atom of carbon-12.

Relative atomic mass:- The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.


A molecule can be defined as the smallest particle of an element or compound that is capable of independent existence and shows all the properties of the substance.

Molecules of element:- Molecules of an element are constituted by the same type of atoms. Ex:- He, H2, Cl2, O2, P4 etc.

Molecules of compound:- The molecules of compound are constituted by atoms of different elements which are joined together in definite proportion. Ex:- H2O, CO2 etc.


The number of atoms present in a molecule is known as atomicity.



An atom or a group of atoms having some charge is called an ion.
Ex:- Na+, OH, Mg2+ etc.

Polyatomic ion:- A group of atoms having some charge is called polyatomic ion. Ex:- SO42-, NH4+ etc.

Cation:- A positively charged ion is called cation. Ex:- H+, Mg2+, etc.

Anion:- A negatively charged ion is called anion. Ex:- Cl, OH, etc.

Valence electron:- The number of electrons present in the outermost shell of an element is known as its valence electron.

Valency:- The combining capacity of an element is known as its valency.

Binary compounds:- The simplest compounds which are made up of two different elements are called binary compounds. Ex:- H2O, CO2 etc.

Chemical formula:-

The symbolic representation of the composition of a compound is known as its chemical formula. Ex:- H2O, CO2 etc.

Rules to write chemical formula:-

  • The valencies or charges on the ion must balance.
  • When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO).
  • In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

Molecular mass:- The sum of the atomic masses of all the atoms in a molecule of the substance is known as its molecular mass.

Formula unit mass:- The formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit of a compound is known as its formula unit mass.

  • Formula unit mass is calculated in the same manner as we calculate the molecular mass.
  • The only difference is that we use the word formula unit for those substances whose constituent particles are ions.

Mole concept:-

  • One mole:- One mole of any species (particles) is that quantity in number having a mass equal to its atomic or, molecular mass in grams.
  • 1 mole = 6.022 x 1023 number of particles = Relative atomic/molecular mass in grams
  • Avogadro number (No) = 6.022 x 1023
  • Number of moles = Given mass/Molar mass
    n = m/M
  • Number of moles = Given number of particles/Avogadro number
    n = N/No
  • m/M = N/No



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