# NCERT Solutions for Class 9 Science Chapter 3

## NCERT Solutions for Class 9 Science Chapter 3 - Atoms and Molecules There is good news for all students struggling with the NCERT problems of Class 9 Atoms and Molecules chapter. The NCERT Solutions for Class 9 Science Chapter 3 PDF file prepared by Vedantu experts are now available for free download. Our experts have followed a very easy approach in preparing these CBSE Solutions for Class 9 Science Atoms and Molecules chapter. Referring to the Class 9 Science Chapter 3 PDF will help you solve and practice all the exercise questions precisely. So, download the PDF and refer to the solutions for effective exam preparation. Maths Students who are looking for the better solutions ,they can download Class 9 Maths NCERT Solutions to help you to revise complete syllabus and score more marks in your examinations.

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NCERT Solutions for Class 9 Science

The NCERT Solutions for Class 9 Science Chapter 3 on Vedantu can be accessed by students online and can also be downloaded for free. Following these NCERT solutions will help you get an insight into all the concepts covered in this chapter.

## 3.1 Law of Chemical Combination

There are two laws of chemical combination- the law of conservation of mass, and the law of constant proportion. Both these laws are discussed in detail in chapter 3, science class 9.

There is one important question related to the chemical reaction. Is there any change in the total mass of the system when a chemical reaction takes place? According to the law of conservation of mass, mass can neither be created nor be destroyed. The same logic stands true for chemical reactions as well.  Several experiments carried out by Lavoisier standardized this law. This law is discussed in NCERT Solutions for Class 9 Science Chemistry Chapter 3.

The law of chemical combination is based on another law- the law of constant proportion. This law was also the result of experiments conducted by Lavoisier and Joseph L. Proust. According to the law of constant proportion, any compound is made by mixing two or more elements in a constant proportion, and this proportion does not change irrespective of the source of the compounds. For example, in water, the ratio of the mass of Hydrogen to the mass of Oxygen is 1:8. The ratio of the mass is the same for water from different sources. This law is also known as the law of definite proportion. The law of constant proportion is explained thoroughly in Class 9 Science Chapter 3.

## 3.2 What is an Atom?

The issue that the scientist faced with the law of constant proportion is to provide a possible explanation for this. Eminent scientist John Dalton tried to provide an explanation and introduced the concept of atoms. An atom is the smallest unit of matter. They are so small that thousands of them are stacked in your hair. The atom of one element is different from that of the other elements. Therefore, each of these atoms is depicted by different symbols.

Dalton used different combinations of straight lines and circles for denoting various atoms. However, the modern-day symbols consider the first or the first two letters of the element's name to denote an atom of the element. For example, Hydrogen is denoted by H and Oxygen by O. In some cases, and the Latin names are also considered. For example, the Latin name of Potassium is Kalium, and it is denoted by K. A list of names of elements is shown in Atom and Molecules Class 9.

Every atom has a definite mass, called the atomic mass unit. The atomic mass is the characteristic of the atom. The concept of atomic mass supports the law of constant proportion and the law of conservation of mass. A detailed analysis of atomic mass is provided in the NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules.

However, atoms cannot exist freely in nature. It interacts with the same or different atoms to form molecules. They can also form ions. These ions or molecules aggregate to form elements of compounds that we can see. These concepts are covered in Ch 3 Science Class 9.

## 3.3 What is Molecule?

A group of two or more atoms is joined together to form a molecule. A molecule can exist freely in nature. For example, Hydrogen, Oxygen, Calcium molecules can exist freely in nature. Molecules can be created from the same atoms or different atoms. If the molecules are made from the same kind of atoms, they are called molecules of elements. For example, Sodium and Potassium elements. Examples of molecules of elements are shown in Atoms and Molecules Class 9 NCERT Solutions.

Molecules can also be made from dissimilar atoms. When different atoms come together, they form a compound. The molecules made of different atoms are called molecules of compounds. Atoms and Molecules Class 9 NCERT PDF contains more examples of molecules of compounds.

Sometimes, the atoms can be charged. Charged atoms are called ions. An ion can always be charged positively or negatively charged. Positively charged ones are known as cations, while negative ones are known as anions. Sometimes, a molecule can contain more than one ion. Such molecules make a polyatomic ion. The individual charges on each atom determine the net charge of such ions. To learn more about polyatomic ions, go through Class 9 Science Chapter 3.

## 3.4 Writing Chemical Formulae

The symbolic representation of the compounds is said to be their chemical formulae. The chemical formula considers the net charge of individual ions in the formula. The charge of individual ions determines the valency of the radical. The valency of the individual ions determines how many ions of a particular kind will take part in the interaction.

For the proper representation of the chemical formula, the valency of individual ions must be balanced. The metal names should come before any non-metal name. In the case of polyatomic ions, the individual elements must be kept in a bracket if their overall valency is more than one. Otherwise, there is no need to use a bracket—for example, NaOH. You will learn more about writing chemical formulas when you go through the NCERT Solutions for Class 9 Science Chapter 3.

## 3.5 Molecular Mass and Mole Concept

The molecular mass of a compound is the sum of all the atomic masses. Therefore, the molecular mass is represented in atomic mass units. Read NCERT solutions for class 9 science chapter atoms and molecules to learn more about molecular mass.

Formula unit mass is calculated similarly to molecular mass. However, the ions’ mass is considered in calculating the formula unit mass of a compound. Learn more about formula mass units in ch 3 science class 9.

Mole concept is another important concept related to atoms and molecules. It defines the total number of molecules taking part in a chemical reaction. You can learn the mole concept in detail in Atoms and Molecules Class 9.

Exercise 3.5 total Solutions: 11 Questions (6 short question and 5 Long questions).

### Key Features of NCERT Solutions for Class 9 Science Chapter 3

NCERT Solutions for Class 9 Science Chapter 3 has the following features.

• Chapter 3 Science Class 9, is presented comprehensively.

• All the concepts covered in the Atom and Molecules Class 9 chapter are explained with the help of relevant examples in these NCERT Solutions.

• The practice exercise of NCERT Class 9 Science Chapter 3 is solved as per the CBSE guidelines so that students can score good marks in the examination.

• The pointwise approach of chemistry ch 3 class 9 NCERT solutions help students revise the chapter before the examination.

• The topics covered in ch 3 science class 9 become easily understandable when students go through the atoms and molecules class 9 NCERT solutions.

Q1.What is an Atom?

A1. When you study ch 3 science class 9, you will have a clear concept about atoms. Atoms are the smallest component of any matter that can exist independently in nature. However, some atoms cannot exist freely and form molecules with the same type of atoms or form compounds with different atoms. Go through the class 9th science chapter 3 for a detailed explanation of the concepts related to atoms and molecules.

Q2.What is a Molecule?

A2. When atoms combine, they form a molecule. A minimum of two atoms can form a molecule. A molecule can be made of the same or different atoms. To completely understand the concept of molecules, refer to the atoms and molecules class 9 solutions. SHARE TWEET SHARE SUBSCRIBE