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# NCERT Solutions for Class 9 Science Chapter 3 – Atoms and Molecules

NCERT Solutions for Class 9 Science Chapter 3 ‘Atoms and Molecules’ are designed to help you prepare for your science exam. The chapter ‘Atoms and Molecules’ deals with the fundamentals of the naming of molecules based on their formula. For each and every symbol and orientation there are particular rules which are required to be followed. This chapter of NCERT Solutions for Class 9 covers all such basic rules of naming compounds from the given formula and deriving formulas from the given names.

In this chapter, there are 4 exercises and 20 questions in total. There are different kinds of exercises like short answer questions, naming questions, formula derivations and a good number of numerical-based questions in this chapter. It is an advanced continuation of the previous chapters that you studied in class 9 NCERT book. It primarily deals with the laws of constant proportion, percentage composition, chemical formulas, the law of conservation of mass, etc.

## Important topics of NCERT Exemplar Class 9 Solution Chapter 1 – Atoms and Molecules

Dalton’s Atomic Theory

NCERT Solutions for Class 9 Science Chapter 3 ‘Atoms and Molecules’ will introduce you to Dalton’s Atomic Theory. This theory states that all matter around us is composed of indivisible and indestructible building blocks which are known as atoms. Dalton’s atomic theory has four postulates –

1. The first postulate states that all the forms of matter are made of very small particles which are known as atoms
2. The second postulate states that atoms can not be created by any means nor is it possible to form them.
3. For a single element, all its atoms will be identical to each other. Whereas in the case of different elements their atoms will differ from each other.
4. The atoms of any element combine together in the whole number ratio in order to produce a large number of atoms or a compound of a new substance.

Atomic mass unit

The concept of the atomic mass unit has been explained in the chapter. Atomic mass unit is equal to one-twelfth of the mass of one Carbon – 12 atom. This concept has further been used in solving the numerical of the chapter.

Chemical Formula

This chapter will also introduce you to the basics of symbolic representations of a chemical compound. This is one of the most essential concepts which will continue to be a part of your syllabus in all your senior classes. The chemical formula of a compound helps us in knowing the number and the kinds of an atom of the different elements which constitute the compound.

Molecular Mass

This chapter will enable you to find out the molecular masses of different elements and compounds based on their atomic mass.

Example –  Find out the molecular mass of C2H6

Answer – Molecular mass of C2H6 = 2 * atomic mass of C + 6 * atomic mass of H

=  2 * 12 + 6 * 1

= 30 u

Percentage composition of elements and compounds

Percentage composition refers to the percentage amount of the mass of each element in the total

composition of the compound. This concept is majorly used in the numerical problems of the chapter.

For example –

0.24 g sample of a compound of oxygen and boron was found by analysis to contain 0.096 g of boro and

0.144 g of oxygen. Calculate the percentage composition of the compound by weight

Mass of boron – 0.096 g

Mass of oxygen – 0.144 g

Mass of sample – o.24 g

Thus the percentage of boron by weight = (.096/.24) * 100

= 40 %

Percentage of oxygen by weight in the compound = (.144/.24) * 100

= 60%

## Exercise-wise Discussion of NCERT Class 9 Solutions Chapter 3 – Atoms and Molecules

• CBSE Class 9 Science NCERT Chapter 3 consists of a diverse set of questions. These include short answer questions, reasoning questions, description based questions and numerical problems. All these questions are based on the core concepts that have been tackled in the chapter.
• The exercises begin with analytical questions which require a thorough knowledge of the law of conservation of mass as well as the working required for the solution.
• Further, there are questions which involve the extraction of atomic mass from the given ratio of elements.
• The chapter also has direct theoretical questions which seek knowledge about the theories and basic definitions like Dalton’s atomic theory, the definition of chemical formula and atomic mass.
• There is a good number of numerical questions which involve the concepts of mass percentage, atomic weight and unit mass of a substance.

## About NCERT Solutions for Class 9 Chapter 3 by Instasolv

Our subject matter experts are well-versed with the problems that you might face in preparing this chapter. So they have prepared the NCERT Solutions for Class 9 Science Chapter 3 – ‘Atoms and Molecules’ accordingly for you. We have organized our solutions in a step by step manner so that you do not face any trouble in understanding them and all your doubts are cleared at one go. We have left no concept of the chapter untouched and have made sure that each and every NCERT exercise is covered by us.

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