CBSE Class 9 Science Revision Notes Chapter 3

CBSE Class 9 Science Revision Notes Chapter 3 – Atoms and Molecules

There are many atoms, and molecules involved in chemical reactions. An atom is a tiny piece of matter that possesses an element’s characteristics. Some atoms create ions and molecules to support their non-independent existence. When one or more atoms combine through chemical bonds, molecules are created. The smallest component of an element is the one that displays its characteristics. A chemical bond between two or more atoms can result in the formation of molecules. 

The Class 9 Science Chapter 3 Notes thoroughly explain this topic of atoms and molecules. Students can easily access Class 9 Science Chapter 3 Notes from the Extramarks platform to help them effectively cover the entire curriculum and perform better in exams.

CBSE Class 9 Science Chapter 3 – Atoms and Molecules Revision Notes – Free Download

Law of Conservation of Mass:

According to the rule of conservation of mass, no new mass may be formed during a chemical process. 

This law states that regardless of the physical or chemical change, the total mass of the products and reactants remain equal.

Law of Constant Proportion:

A chemical compound always consists of the same elements mixed in the same proportion by mass, according to another French chemist named Joseph Proust.

Law of Multiple Proportions:

The Law of Multiple Proportions by John Dalton (1803) states that “The mass of the element that combines with the fixed mass of the other bears a straightforward whole-number ratio when two elements combine to form two or more compounds”.

Dalton’s Atomic Theory:

Dalton’s atomic theory explains – “All matter, whether an element, a compound, or a mixture, is composed of tiny units called atoms”. 

The following are the postulates of this theory:

  • Incredibly small atoms, which are tiny particles that participate in chemical reactions, make up all matter.
  • Atoms are indestructible, indivisible particles that cannot be created or destroyed during a chemical reaction.
  • Atoms of a particular element have the same mass and chemical properties.
  • The masses and chemical properties of atoms from different elements vary.
  • When atoms combine in a ratio of small whole numbers, compounds are created.
  • The number and types of atoms in a compound are constant.


  • An element’s distinctive structure, which cannot be destroyed chemically, is its atom.
  • The three particles that make up an atom are the electron, proton, and neutron.
  • All of an atom’s mass is contained in its nucleus.
  • The electrons in an atom are organised into shells or orbitals.

Three components make up the atomic symbol:

The standard element symbol “X”;

The atomic symbol that represents the number of protons is “A”;

The total number of protons and neutrons in an element is indicated by the mass number “Z”.

  • Nanometers are the units used to measure an atom’s radius.

Atomic Mass:

  • Dalton put forth the atomic hypothesis of atomic mass.
  • The sum of the masses of the electrons, neutrons, and protons determines the average mass of an atom or group of atoms.
  • The atomic mass is the weight of an atomic particle.
  • According to the international agreement, this is frequently expressed in terms of a single atomic mass unit (AMU).
  • The average mass of one atom of an element, as compared to 1/12th the mass of one carbon-12 atom, is called atomic mass.


  • Valence electrons are the ones located in the atom’s outermost orbit.
  • The ability of an atom to gain, lose, or share valence electrons ‌to complete its octet is what determines an atom’s valency.


  • The molecular mass of a molecule is the sum of the masses of all the elements it contains.
  • The mass of a molecule is calculated by adding the masses of all of its constituent elements after multiplying each element’s atomic mass by the number of atoms it contains.
  • The atomicity of an element refers to how many atoms there are in a single molecule of that element.
  • As an illustration, because each molecule of hydrogen, nitrogen, oxygen, chlorine, iodine, and bromine contains two atoms, they all have two atomicities.


  • A compound is formed when two or more elements combine chemically in a specific mass ratio.
  • Compounds are substances made up of two or more distinct atom types in a predetermined ratio.


  • A molecule or atom that has a net positive or negative charge as a result of the gain or loss of one or more of its valence electrons is said to have an ion.
  • A cation is a positively charged particle, whereas an anion is negatively charged.
  • Chemical compounds called ionic compounds include ions that are held together by ionic bonds, a specific type of bond.
  • In an ionic substance, the positive and negative charges are always distributed equally.

Molecular Mass:

  • The combined masses of all the elements that make up a molecule make up its molecular mass.
  • After dividing each element’s atomic mass by the number of atoms it contains, the masses of all the elements in the molecule are summed together.

Mole and Avogadro Number:

  • Mole units are used to express how many atoms, molecules, and ions make up a substance. 
  • A mole is 6.022 × 1023 molecules of any substance.
  • One of the most helpful ways to specify the number of reactants and products in a reaction is the concept of mole.
  • The value of Avogadro’s number is around 6.022 × 1023.
  • The number of particles that make up a mole (or mol) of a substance is calculated using the formula referred to as Avogadro’s number.

Some Important formulae:

No. of Molecules = Given MassMolar Mass Avogadro Number



  • 1 Mole = Mass of a substanceGram atomic mass
  • Mass of a substance = Molar mass of a substanceNo. of Moles
  • Percentage composition of an element = Total weight of element in a moleculeGram Molecular Weight100
  • RMM = Mass of one molecule of the substance112 Mass of the atom  of Carbon C12
  • Gram Molecular Volume = Gram Molecular WeightWeight / Volume of gas at STP

CBSE Class 9 Science Notes Chapter 3 Atoms And Molecules

What is an atom as described in Chapter 3 Science Class 9 Notes?

Atoms, the smallest unit of an element, cannot be further broken down by a chemical reaction. The atomic number abbreviated “A,” of an element indicates how many protons it has. The symbol “Z” stands for an element’s mass number, which is equal to the total of its neutron and proton counts. Specific symbols are used to represent an element’s mass number and atomic number.

What does the Daltonic theory define in Class 9 Science Note Chapter 3?

Here are the details of the Daltonic theory as mentioned in Class 9 Science Notes of Chapter 3

  • All matter is made up of incredibly tiny particles called atoms.
  • Atoms cannot be created or destroyed.
  • All atoms in a given element have the same physical and chemical characteristics.
  • Different elements’ atoms have different physical and chemical characteristics.
  • Atoms from different elements combine through the process of chemical combination to create compounds.

What are atomic mass and atomic mass units in Class 9 Science Notes Chapter 3?

Atomic mass is the total mass of a single atom. It is approximately equivalent to the number of neutrons and protons present. The atomic mass unit serves as a representation for the measurement of atomic mass (AMU). It is also known as Daltons, D.

FAQs (Frequently Asked Questions)

1. What is a chemical reaction as explained in Class 9 Science Chapter 3 Notes?

One or more reactants or substances are transformed into one or more products during a chemical reaction. These substances can either be chemical elements or chemical compounds. In a chemical reaction, different substances are rearranged to form the atoms of the reactants.It has been demonstrated that a chemical reaction can involve one or two molecules interacting with one another to create a new compound. While the newly formed compounds are typically referred to as products, this compound is known as a reactant.Additionally, a physical change that denotes a chemical change can be seen during a chemical reaction. Changes in temperature, colour, or precipitation are just a few examples of these changes.

2. What are the basics of atoms and molecules for Class 9?

Atoms are regarded as the fundamental units of matter. It is the smallest part of matter and is composed of particular chemical components. Atoms are produced from molecules and ions instead, as they cannot survive separately. We can feel, see, and touch objects because of the many compounds that are made when these atoms are combined. Most molecules are made up of one or more atoms connected by specific chemical bonds. The smallest units of matter that can be divided while still being a part of the same substance are called molecules.