CBSE – 9th Standard Science

Class 9 chapter 3 – Atoms & Molecules



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


  • The relative atomic mass of the atom of an element is defined as the average mass of the atom.
  • The number of atoms constituting a molecule is known as its atomicity.


  • A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.
  • Atoms of the same element or of different elements can join together to form molecules.


Compounds composed of metals and non-metals contain charged species. The charged species are known as ions





  • It is therefore the relative mass of a molecule expressed in atomic mass units (u).
  • The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 x 1023. This number is called the Avogadro Constant or Avogadro Number.
  • Avogadro Number is named in honour of the Italian scientist, Amedeo Avogadro.
  • The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams.
  • Molar mass of atoms is also known as gram atomic mass.





The atoms of same element, having same atomic number but different mass numbers. e.g. hydrogen 11H, deuterium 1 2H and tritium 1 3H

CBSE 9th Standard Science

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CBSE 10th Standard Science Syllabus and Revision Notes

  • Chapter 1 – Matter in our surroundings
  • Chapter 2 – Is matter around us pure
  • Class 9 chapter 3 – Atoms & Molecules
  • Class 9 Chapter 4 – Structure of the Atom
  • Class 9 Chapter 5 – The fundamental unit of life
  • Class 9 Chapter 6 – Tissues
  • Class 9 Chapter 7 – Diversity in living organisms
  • Class 9 Chapter 8 – Motion
  • Class 9 Chapter 9 – Force and laws of motion
  • Class 9 Chapter 10 – Gravitation
  • Class 9 Chapter 11 – Work & Energy
  • Class 9 Chapter 12 – Sound
  • Class 9 Chapter 13 – Why do we fall ill
  • Class 9 Chapter 14 – Natural Resources