Class 9 Science Chapter 3 Atoms and Molecules

Class 9 Science Chapter 3 Atoms and Molecules

NCERT Notes for class 9 Science Chapter 3 Atoms and Molecules on this step-by-step answer guide . In some of State Boards and CBSE schools, students are taught thru NCERT books. As the chapter comes to an end, students are requested few questions in an exercising to evaluate their knowledge of the chapter. Students regularly want guidance dealing with those NCERT Notes. It’s most effective natural to get stuck withinside the exercises while solving them so that you could assist students rating better marks, we have provided grade by grade NCERT answers for all exercises of Class 9 Science Atoms And Molecules so you can be searching for assist from them. Students have to solve those exercises carefully as questions withinside the very last exams are requested from those so these exercises at once have an impact on students’ final score. Find all NCERT Notes for Class nine Science Science Chapter 3 Atoms And Molecules below and prepare for your tests easily.

Chapter 3: Atoms and Molecules

Ancient Indian and Greek philosophers

The idea of ​​divisibility of matter was considered long back in India, around 500 BC. 

An Indian philosopher Maharishi Kanad, postulated that if we go on dividing matter (padarth) we shall get smaller and smaller particles. 

Ultimately, a time will come when we shall come across the smallest particles beyond which further division will not be possible.  He named these particles Parmanu. 

Another Indian philosophe, Pakudha Katyayama, elaborated this doctrine and said that these particles normally exist in a combined form which gives us various forms of matter.

Same era, ancient Greek philosophers – Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further.

Democritus called these indivisible particles atoms (meaning indivisible). 

By the end of the eighteenth century, scientists recognized the difference between elements and compounds.

Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

Laws of Chemical Combination

The following two laws of chemical combination were established after much Experimentations by Lavoisier and L. Proust.  Joseph


Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction .

Law of constant proportions

which is also known as the law of definite proportions. 

In a chemical substance the elements are always present in definite proportions by mass.

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

The postulates of this theory.

  • All matter is made of very tiny particle called atoms.
  • Atoms are indivisible 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.

What is an Atom?

The name ‘atoms’ as given by the Greeks and said that the smallest particles of matter are atoms.

The building blocks of all matter are atoms.

How big are atoms?

Atoms are very small, they are smaller than anything that we can imagine or compare with.

Our entire world is made up of atoms.


Dalton was the first scientist to use the symbols for elements.

Berzilius: The symbols of elements be made from one or two letters of the name of the element.

In the beginning, The names of elements the were derived from the name of the place where they were found for the first time.

Now – a – days, IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.

Many of the symbols are the first one or two letters of the element’s name in English.

The first explain letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).

For example

(1) hydrogen, H

(ii) aluminum, Al and not AL

(iii) cobalt, Co and not CO.

Other symbols have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its  Latin name ferrum, sodium is Na from natrium, potassium is K from kalium.

Atomic mass

According to dalton each elements had a characteristic atomic mass.

Atomic masses were determined using the laws of chemical combination and the compounds formed.

Wxamplw of a compound carbons monoxide formed by carbion and was observged experimentally that 3g of carbionb comin3s with 4g of oxygen to form co.

In other words carbon combines with 4/3 times its mass of oxygen.

According to IUPAC atomic mass define or written as u’.

Scientists initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit.

This was considered relevant due to two reasons:

oxygen reacted with a large number of elements and formed compounds.

This atomic mass unit gave masses of most of the elements as whole numbers. 

In 1961 for a universally accepted atomic mass unit, the carbon – 12 isotope was chosen as the standard reference for measuring atomic masses.


Atoms of most elements are not able to exist independently.

These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.

What is a Molecule?

A molecule is in general a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces.

A molecule can be defined as the smallest particle of an element.

Atoms of the same element or of different elements can join together to form molecules.


The molecules of an element are constituted by the same type of atoms.

Molecules of many elements , such as argon ( Ar ), helium ( He ) etc. are made up of only one atom of that element.

A molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule o2.

The number of atoms constituting a molecule is known as its atomicity.


Atoms of different elements join together in definite proportions to form molecules of compounds.

What is an Ion?

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

An ion is a charged particle and can be negatively or positively charged.

A negatively charged ion is called an ‘anion’.

The positively charged ion, a ‘cation’. 

Example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl).

Ions may consist of a single charged atom or a group of atoms that have a net charge on them.

A group of atoms carrying a charge is known as a polyatomic ion.

Writing chemical formulae

Writing a chemical formula are as follows:

  • 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), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (Cu) etc.

where oxygen, chlorine, sulphur are non – metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.

  • In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

In case the number of polyatomic ion is one, the bracket is not required.  For example, NaOH.

Formulae of simple compounds

The simplest compounds, which are made up of two different elements are called binary compounds.

While writing the chemical formulas for compounds, we write the constituent elements and their valencies.

Then we must crossover the valencies of the combining atoms.

Molecular Mass and Mole Concept


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

It is therefore the relative mass of a molecule expressed in atomic mass units ( u ).


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

Formula unit mass is calculated in the same manner as we calculate the molecular mass.

Mole concept

The word “mole” was introduced around 1896 by Wilhelm Ostwald who derived the term from the Latin word moles meaning a ‘heap’ or ‘pile’. 

The unit mole was accepted in 1967 to provide a simple way of reporting a large number.

The mole, as per definition accepted internationally, is defined as:

A mole of a substance is that amount of the substance which contains the same number of entities

(atoms,molecules, ions, electrons or photons, etc.) as there are atoms  in exactly 12 grams of pure carbon – 12.

The number 6.022 x 1023 is called Avogadro’s number in the honor of Italian scientist Amedeo Avogadro.

Thus, one mole is a collection of 6.022 x 1023 ‘particles’, be they atoms, molecules, ions, electrons, photons or anything.

Avogadro number is also called Avogadro constant because it represents a specific (constant) number of items and does not depend on the items.

Benefits of NCERT Notes for class 9 Science Chapter 3 Atoms and Molecules

NCERT Notes for class 9 Science Chapter 3 Atoms and Molecules contains extremely important points, and for each chapter, each concept has been simplified to make it easier to remember and increase your chances of achieving excellent exam results. Exam Preparation References Here are some tips on how these solutions can help you prepare for the exam.

  1. This helps students solve many of the problems in each chapter and encourages them to make their concepts more meaningful.
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Tips & Strategies for Class 9 Exam Preparation

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  7. Take rest and a proper meal.  Don’t stress too much. 

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