NCERT Class 9 Science Chapter 3 Notes Atoms And Molecules- Download PDF Notes

Edited By Irshad Anwar | Updated on Apr 22, 2024 01:12 PM IST

Atoms and molecules is a very important chapter from the NCERT Book of the CBSE Board. The NCERT Class 9 Science Chapter 3 notes provide basic and detailed knowledge about atoms and molecules. The main topics covered in NCERT Class 11 Biology notes are definition, laws of chemical combination, such as the law of conservation of mass, the law of constant proportions, what is an atom, atomic mass, how do atoms exist, what is a molecule, what is an ion, writing chemical formulae, molecular mass, and mole concept. Download the CBSE Notes for Class 9 Science, Chapter 3, PDF to use offline anywhere. Students must go through each concept, including formulae and examples of atoms and molecules Class 9 Notes Science in the easiest and most effective way possible with the help of NCERT Notes for Class 9.

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NCERT Class 9 Chapter 3 Class Notes
Atoms and Molecules
Laws of Chemical Combination
What is a Molecule?
How to Write Chemical Formulae
Molecular Mass and Mole Concept

Class 9 Science chapter 3 notes also cover all the important concepts related to this chapter, which are the foundation for classes 11 and 12. Atoms and molecules NCERT Notes for Class 9 Science help you revise these major concepts given in the NCERT Book in no time during CBSE exam preparation. CBSE Class 9 Science Chapter 3 notes will help you with quick revision. The chapter Atoms and Molecules covers all headings of NCERT. CBSE Class 9 Science chapter 3 notes also contain important examples that have been frequently asked. Having revision notes and NCERT Solutions for Class 9 Science Chapter 3 handy is beneficial to save you time. The NCERT Class 9 notes PDF can be downloaded through the link given below.

Also, students can refer to:

NCERT Class 9 Chapter 3 Class Notes

Atoms and Molecules

Definition: Democritus explained that if we keep on dividing a matter, we will end up with its smallest unit, which won’t be divisible further, and he named it an atom.

Laws of Chemical Combination

Antoine L. Lavoisier is called the father of chemistry, for he gave two important laws of chemical combination which laid the foundation of chemical sciences, and the laws are as follows:

Law of Conservation of Mass

It states that “Mass is not created and not destroyed, in a chemical reaction”.

Law of Constant Proportions

“The elements are present in definite proportions by mass, in a chemical substance”.

Explanation of these laws as given by Dalton (Dalton’s Atomic Theory).

The postulates of the theory are given below:

“Matter is made of very small particles called atoms”.
“Atoms are not divisible particles, which are not created and not destroyed in a chemical reaction”.
“Atoms of the same element have identical mass and chemical properties”.
“The chemical properties of the atoms are different for different elements.”.
“Compounds are formed by combining atoms of different elements in ratios of small whole numbers”.

How big are atoms?

Relative size of different matters:

Radii (m)	Example
	Atoms of hydrogen
	Molecules of water
	Molecules of haemoglobin
	Grain of sand
	Ant
	Watermelon

What are the modern day symbols of atoms of different elements?

Some elements and their symbols:

Element	Symbol
Hydrogen	H
Helium	He
Lithium	Li
Sodium	Na
Gold	Au
Silver	Ag
Mercury	Hg
Iron	Fe
Chlorine	Cl

Atomic Number

It is the number of protons present in an element.

Atomic Mass

It is the number of protons and neutrons present in an element.

Carbon-12 isotope is the standard reference for calculating atomic masses of other elements. One atomic mass unit is a mass unit equal to one-twelfth (1/12th) the mass of one atom of carbon-12.

What is a Molecule?

It is a group of two or more elements held together in a chemical bond by attractive forces.

Atomicity

The number of atoms in a molecule is called the atomicity of the molecule.

For example:, argon and helium are monoatomic,
Oxygen and chlorine are diatomic.
Phosphorus is tetra-atomic and
Sulphur is polyatomic.

Ion

An ion is a charged particle; it can be positively charged or negatively charged. Positively charged ions is called ‘cation’ and negatively charged ion is called ‘anions’.

How to Write Chemical Formulae

It is a symbolic representation of a compounds and its composition.

Given below are the rules for writing a chemical formula:

The charges on the ions have to balance each other.
If the compound contains a metal and a nonmetal, the metal is written first.

For example: sodium chloride (NaCl), where chlorine is a non-metal, so written on the right, and sodium is a metal, so written on the left.

In compounds with polyatomic ions, the ion is enclosed in a bracket and then the number of the ion is indicated. If number of polyatomic ion is one, the bracket is not required.

For example: Formula for aluminium oxide is given by , as Al has 3+ charge and O has 2- charge.

Molecular Mass and Mole Concept

The molecular mass of a substance is equal to the sum of the atomic masses of all the atoms in that molecule.

If the substance contains ions as its constituents, then molecular mass is called Formula Unit Mass.

The number of particles present in 1 mole of any substance is given by Avogadro Number, which is 6.022 × 10²³.

Mole Concepts

Any quantity in number having a mass equal to its atomic or molecular mass in grammes is referred to as one mole of any species (atoms, molecules, ions, or particles). A single mole of any given substance comprises 6.022 × 1023 particles, which can be atoms, molecules, or ions. This is an experimentally obtained value. Named in honour of the Italian scientist Amedeo Avogadro, this number is also known as the Avogadro Constant or Avogadro Number (represented by N0).

1 mole (of anything) = 6.022 × 1023 in number

Chapter-Wise NCERT Class 9 Notes Science

NCERT Class 9th Science Chapter 1 Notes

NCERT Class 9th Science Chapter 2 Notes

NCERT Class 9th Science Chapter 3 Notes

NCERT Class 9th Science Chapter 4 Notes

NCERT Class 9th Science Chapter 5 Notes

NCERT Class 9th Science Chapter 6 Notes

NCERT Class 9th Science Chapter 7 Notes

NCERT Class 9th Science Chapter 8 Notes

NCERT Class 9th Science Chapter 9 Notes

NCERT Class 9th Science Chapter 10 Notes

NCERT Class 9th Science Chapter 11 Notes

NCERT Class 9th Science Chapter 12 Notes

NCERT Class 9th Science Chapter 13 Notes

NCERT Class 9th Science Chapter 14 Notes

NCERT Class 9th Science Chapter 15 Notes

Significance of NCERT Class 9 Science Chapter 3 Notes

Atoms and Molecules class 9th notes are helpful to revise the chapter and get an idea about the main topics covered in the chapter. Also, this NCERT class 9 science chapter 3 notes are useful to set base for competitive exams like VITEEE (Link), BITSAT (Link), JEE Main (Link), NEET (Link), etc. These Notes for Class 9 Science Chapter 3 are also not complicated, and they thoroughly explain step-by-step approaches to guarantee that students understand the concepts of this chapter, which is part of the CBSE Science syllabus for class 9. Class 9 science chapter 3 notes pdf download can be very helpful in reading through paper.

Subject Wise NCERT Exemplar Solutions

Subject wise NCERT Solutions

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Popular Questions

A block of mass 0.50 kg is moving with a speed of 2.00 ms^-1 on a smooth surface. It strikes another mass of 1.00 kg and then they move together as a single body. The energy loss during the collision is

Option 1) $0.34\; J$	Option 2) $0.16\; J$
Option 3) $1.00\; J$	Option 4) $0.67\; J$

A person trying to lose weight by burning fat lifts a mass of 10 kg upto a height of 1 m 1000 times. Assume that the potential energy lost each time he lowers the mass is dissipated. How much fat will he use up considering the work done only when the weight is lifted up ? Fat supplies 3.8×10⁷J of energy per kg which is converted to mechanical energy with a 20% efficiency rate. Take g = 9.8 ms⁻² :

Option 1)

2.45×10⁻³ kg

Option 2)

6.45×10⁻³ kg

Option 3)

9.89×10⁻³ kg

Option 4)

12.89×10⁻³ kg

An athlete in the olympic games covers a distance of 100 m in 10 s. His kinetic energy can be estimated to be in the range

Option 1) $2,000 \; J - 5,000\; J$	Option 2) $200 \, \, J - 500 \, \, J$
Option 3) $2\times 10^{5}J-3\times 10^{5}J$	Option 4) $20,000 \, \, J - 50,000 \, \, J$

A particle is projected at 60⁰ to the horizontal with a kinetic energy $K$ . The kinetic energy at the highest point

Option 1) $K/2\,$	Option 2) $\; K\;$
Option 3) $zero\;$	Option 4) $K/4$

In the reaction,

$2Al_{(s)}+6HCL_{(aq)}\rightarrow 2Al^{3+}\, _{(aq)}+6Cl^{-}\, _{(aq)}+3H_{2(g)}$

Option 1) $11.2\, L\, H_{2(g)}$ at STP is produced for every mole $HCL_{(aq)}$ consumed	Option 2) $6L\, HCl_{(aq)}$ is consumed for ever $3L\, H_{2(g)}$ produced
Option 3) $33.6 L\, H_{2(g)}$ is produced regardless of temperature and pressure for every mole Al that reacts	Option 4) $67.2\, L\, H_{2(g)}$ at STP is produced for every mole Al that reacts .

How many moles of magnesium phosphate, $Mg_{3}(PO_{4})_{2}$ will contain 0.25 mole of oxygen atoms?

Option 1) 0.02	Option 2) 3.125 × 10^-2
Option 3) 1.25 × 10^-2	Option 4) 2.5 × 10^-2

If we consider that 1/6, in place of 1/12, mass of carbon atom is taken to be the relative atomic mass unit, the mass of one mole of a substance will

Option 1) decrease twice	Option 2) increase two fold
Option 3) remain unchanged	Option 4) be a function of the molecular mass of the substance.

With increase of temperature, which of these changes?

Option 1) Molality	Option 2) Weight fraction of solute
Option 3) Fraction of solute present in water	Option 4) Mole fraction.

Number of atoms in 558.5 gram Fe (at. wt.of Fe = 55.85 g mol^-1) is

Option 1) twice that in 60 g carbon	Option 2) 6.023 × 10²²
Option 3) half that in 8 g He	Option 4) 558.5 × 6.023 × 10²³

A pulley of radius 2 m is rotated about its axis by a force F = (20t - 5t²) newton (where t is measured in seconds) applied tangentially. If the moment of inertia of the pulley about its axis of rotation is 10 kg m² , the number of rotations made by the pulley before its direction of motion if reversed, is