# NCERT Solutions for Class 9 Science Chapter 4 Structure of The Atom

NCERT solutions for class 9 science chapter 4 Structure of The Atom: After going through chapter 3 Atoms and Molecules, you must be wondering that; what makes the atom of one element different from another element? Are atoms really indivisible, or are there smaller constituents inside the atom? With the assistance of CBSE NCERT solutions for class 9 science chapter 4 Structure of the Atom,  you will find out the answers to these questions. Through solutions for NCERT class 9 science chapter 4 Structure of the Atom, you will also learn about sub-atomic particles and the various models that have been introduced to describe how these particles are arranged within the atom.

The NCERT solutions contain topic-wise solutions and exercise solutions that are created by the experts. Chapter 4 Structure of the Atom includes important topics and theories such as Thomson's model of an atom, Rutherford's model of an atom, Bohr's model of an atom, neutrons, etc. All these theories are very important for the exams.

Here are the important Topics of NCERT for Class 9 Science Chapter 4 - Structure of The Atom mentioned below:

4.1 Charged Particles in Matter

4.2 the Structure of an Atom

4.2.1 Thomson’s Model of an Atom

4.2.2 Rutherford’s Model of an Atom

4.2.3 Bohr’s Model of Atom

4.2.4 Neutrons

4.3 How Are Electrons Distributed in Different Orbits (shells)?

4.4 Valency

4.5 Atomic Number and Mass Number

4.5.1 Atomic Number

4.5.2 Mass Number

4.6 Isotopes

4.6.1 Isobars

If you have any problem in answering the questions or you are not able to understand then don't worry, our experts are there for you to help.

## Topic 4.1 Charged particles in the matter?

Q. 1.  What are canal rays?

Canal rays are the positively charged radiations which consist of positively charged particles of atoms. they can pass through the perforated ( pierced ) cathode and then travel towards another cathode in a gas discharge tube.

They were given the name Canal rays by E. Goldstein in 1866 who discovered these radiations.

The proton is a positively charged particle and the electron is a negatively charged particle. their magnitude is equal and hence net charge in an atom is zero.

## NCERT free solutions for class 9 science chapter 4 Structure of the Atom

Topic 4.2 The structure of an atom

According to Thomson’s model of an atom, an atom consists of a sphere of a positive charge.

The positive charge in the atom is spread all over like the red edible part of a watermelon, while the electrons are studded in the positively charged sphere, just like the seeds in the watermelon.

As negative and positive charges are equal in magnitude, they balance each other and thus the atom becomes electrically neutral as a whole.

On the basis of Rutherford’s model of an atom, the subatomic particle which is present in the nucleus of an atom is Proton which is a positively charged particle.

The sketch of Bohr’s model of an atom with three shells:

If a foil of a heavy metal like platinum is used, then the observations in the alpha-particle scattering experiment would be the same as that in the gold foil experiment.

If a foil of a light metal like lithium is used, then the observations in the alpha-particle scattering experiment would not be the same because these metal are not so malleable so the thin foil is difficult to obtain.

The problem with not using thin foil is that the number of the alpha particle will bounce back from the thick foil and the location of positive mass would be difficult to find.

## CBSE NCERT solutions for class 9 science chapter 4 Structure of the Atom

Topic 2.4.2 Neutron

The three sub-atomic particles of an atom are :

1. Electron: a negatively charged particle

2. Proton: a positively charged particle

3. Neutron: a neutral particle

The atomic mass of Helium = 4 u
No. of protons = 2

As atomic mass = no. of protons + no. of neutrons
No. of neutrons  = At. mass - no. of protons
= 4 - 2

= 2

Hence Heium atom has 2 neutrons.

## Solutions for NCERT class 9 science chapter 4 Structure of the Atom

Topic 4.3 How are electrons distributed in different orbits(shells)?

Number of electrons in carbon atom = 6

Number of electrons in sodium atom =  11

Electron Distribution:

 Element First Orbit or K -shell Second Orbit or L-shell Third Orbit or M-shell Carbon 2 4 0 Sodium 2 8 1

Maximum Number of electrons in K-shell = 2

Maximum Number of electrons in L-shell = 8

The total no. of electrons in the atom = 2 + 8

= 10

If $\inline K$ and $\inline L$ shells of an atom are full, then the total number of electrons in the atom will be 10.

## NCERT solutions for class 9 science chapter 4 Structure of the Atom

Topic 4.4 Valency

Valancy is basically the minimum number of the electron we have to add or remove such that every shell in the atom is completely filled.

Mathematically,

when the outermost shell of an atom contains 4 or less than 4 electrons, its valency is equal to the number of valence electrons in the outermost shell and when the outermost shell contains more than 4 electrons, the valency of the atom is equal to 8 - no. of valence electrons in the atom.

Chlorine :

Atomic No. of Cl = 17

Its electronic configuration = 2, 8, 7

Valency of Cl = 8 - 7 = 1

Sulphur :

Atomic no. of S = 16

Its electronic configuration = 2, 8, 6

Valency of S = 8 - 6 = 2

Magnesium:

Atomic  no. of Mg = 12

Its electronic configuration = 2, 8, 2

Valency of Mg = 2

## NCERT textbook solutions for class 9 science chapter 4 Structure of the Atom

### Topic 4.5 Atomic number and Mass number

(i) what is the atomic number of the atom?

Given,

Number of electrons in the atom = 8

Number of proton in the atom = 8

The atomic number of an atom is equal to the number of proton in that atom. hence the atomic number of the given atom is 8.

(ii) what is the charge on the atom?

In the given atom, the total number of positive charges is equal to the total number of negative charge.

Number of Protons (8) = Number of electrons (8)

They both will neutralize each other. So, the atom will not possess any charge.

For Oxygen:

Number of protons = 8

Number of electrons = 8

Mass number = Number of Protons + Number of neutrons

= 8 + 8

= 16

Hence mass number for Oxygen is 16.

For Sulphur:

Number of protons = 16

Number of electrons = 16

Mass number = Number of Protons + Number of neutrons

= 16 + 16

= 32

Hence Mass number for Sulphur is 32.

## CBSE NCERT solutions for class 9 science chapter 4 Structure of the Atom

Topic 4.6 Isotopes

H, D, and T are the three isotopes of hydrogen with the same atomic number and different mass numbers of 1, 2 and 3 respectively.

 Element Symbol Number of Electrons Number of Protons Number of Neutrons Hydrogen H 1 1 0 Deuterium D 1 1 1 Tritium T 1 1 2

Isotopes :

Isotopes are the atoms with the same number of proton and different atomic mass. The difference in atomic mass arises due to the different number of neutrons present in the atom.

Some Examples of Isotopes are :

1.$^{12}C_6$  and $^{14}C_6$

2.$^{35}Cl_{17}$  and $^{37}Cl_{17}$.

Isobar:

Isobars are the atom with the same atomic mass and different atomic number.

Some example of Isobars are :

1. $^{40}Ca_{20}$ and $^{40}Ar_{18}$

2.$^{22}Ne_{10}$ and $^{22}Na_{11}$

## NCERT solutions for class 9 science chapter 4 Structure of the Atom:

Exercise solutions

The Comparison of Properties between Electron, Proton, and Neutron:

 Properties Electrons Protons Neutrons Charge Negatively charged Positively charged No charge Weight Negligible 1 a.m.u 1 a.m.u Location in atom Outside the nucleus Inside the nucleus Inside the nucleus Reaction with a charged particle Attracts positive charge Attracts negative charge gives no reaction to any charge

The limitations of J.J. Thomson’s model of the atom are:

1. Thomson's model of the atom could not explain the results of alpha particle scattering experiment carried out by Rutherford. this model failed to depict why most of the alpha particle passes through gold foil and why some of them got diverted in different angles and some of them rebounded and returned back to their paths.

2. It was solely based on the imagination and did not have any experimental evidence.

The limitations of Rutherford's model of the atom is that It does not explain the stability of the atom. As we know now, when charged bodies move in a circular motion, they emit radiations.

This means that the electrons revolving around the nucleus (as suggested by Rutherford) would lose energy and come closer and closer to the nucleus, and a stage will come when they would finally merge into the nucleus.

This makes the atom unstable, which is clearly not the case. The electrons do not fall into the nucleus, atoms are very stable and do not collapse on their own.

In order to overcome the objections raised against Rutherford's model of the atom, Neils Bohr put forward his model of the atom. According to Bohr's model of the atom,
1. An atom holds the nucleus in the center. the whole mass of the atom is concentrated at the nucleus.

2. The negatively charged particle revolves around the nucleus in definite circular paths known as orbits or which are designated as K, L, M, N, etc. or numbered as n = 1, 2, 3, 4, etc. (outward from the nucleus).

3.  While revolving in discrete orbits, the electrons do not radiate energy. But when an electron jumps from one energy level to another, the energy of the atom changes.

.

Comparison of different proposed Model:

 Feature Thomson's Model Rutherford's Model Bohr's Model Positive Charge The Positive charge is distributed in Sphere The positive charge is concentrated at the core of the atom, which is called the nucleus The positive charge is present in the core of the atom, called nucleus. Negative Charge The electrons are embedded in the positively charged sphere of an atom, like the seeds in a watermelon. The nucleus is surrounded by electrons, and the electrons and the nucleus are held together by the electrostatic force of attraction The electrons move in discrete orbits, and each orbit is associated with a definite amount of energy. Limitation This model could not explain the results of an alpha particle scattering experiment This model could not explain the stability of the atom. This model perfectly explains the stability of an atom Diagrammatic representation

The Bohr and Bury scheme for the distribution of electrons in an atom is based on the following rules :
1. The maximum number of electrons that a shell can have is represented by $2n^2$, where n is the quantum number of that particular energy shell. Thus, the maximum number of electrons in the first four shells are :
1st (K) shell 2 x $1^2$ = 2
2nd (L) shell 2 x $2^2$ = 8
3rd (M) shell 2 x $3^2$ = 18
4th (N) shell 2 x $4^2$ = 32

2. The outermost shell, which is also called valence shell, can have a maximum of 8 electrons.

3. If permitted by rule 1, The shell inner to the outermost shell (the second last shell ) can accommodate a maximum of 18 electrons.

4. Electrons are not taken in unless the inner shells are filled, i.e., the shells are filled in a step-wise manner.

The definite combining capacity of an atom of an element, in which electrons are lost, gained or shared with other atoms to complete the octave in the outermost shell is defined as valency.

In other words,

Valancy is basically the minimum number of the electron we have to add or remove from or in the outermost shell such that every shell in the atom is completely filled.

And Mathematically,

when the outermost shell of an atom contains 4 or less than 4 electrons, its valency is equal to the number of valence electrons in the outermost shell and when the outermost shell contains more than 4 electrons, the valency of the atom is equal to 8 - no. of valence electrons in the atom.

The valency of Silicon:

Atomic number = 14

Distribution of electron :

K = 2

L = 8

M = 4

Number of electrons in outermost shell = 4

Valency = 8 - 4 = 4.

The valency of Oxygen:

Atomic number = 8

Distribution of electrons:

K = 2

L = 6

The number of electron in outermost shell = 6

Valency = 8 - 6 = 2

Q. 8.(i)  Explain with examples

(i)   Atomic number,

Atomic Number :

An atomic number of an atom is the total number of protons present within the nucleus of an atom is known as the atomic number. it is denoted by symbol Z.

Example: As the Oxygen atom has 8 protons in its nucleus, its atomic number is 8.

Q. 8.(ii)   Explain with examples

(ii)    Mass number,

Mass Number :

The mass number of an atom is the sum total of the masses of all the nucleons present in the nucleus of an atom, i.e.,
Mass Number = No. of Protons + No. of Neutrons

It is denoted by A.

Example: As a sodium atom has 11 protons and 12 neutrons in its nucleus,

So, it's mass number = 11 + 12 = 23.

Q. 8.(iii)     Explain with examples

(iii)     Isotopes

Isotopes:

Isotopes are the atoms of the same element having the same atomic number but a different mass number.

Example: Carbon molecule exists as  $^{12}C_6$  and $^{14}C_6$.

Q. 8.(iv)   Explain with examples

(iv)  Isobars.

Isobar:

Isobars are the atoms of different elements having the same mass number but different atomic numbers.

Example: $^{40}Ca_{20}$ and $^{40}Ar_{18}$  . Mass numbers of calcium and argon atoms have different atomic numbers (20 and 18 ) but the same mass number 40.

Two uses of isotopes are:

(i) An isotope of uranium is used as fuel in nuclear reactors.

(ii) An isotope of cobalt is used in the treatment of cancer.

The atomic number of Na = 11

No. of electrons in Na atom = 11

In $Na^{+}$ ,the positive charge is obtained due to the loss of one electron from the M shell of Na atom.

So, No. of electrons in Na+ ion = 11 -1 = 10

Hence, electronic configuration of Na+ = 2, 8

In Na+, K and L shells are completely filled since K shell can have a maximum of 2 electrons and L shell can have a maximum of 8 electrons.

Given,  two isotopes  $_{35}^{79}\textrm{Br}(49.7^{o}/_{o})$ and  $_{35}^{81}\textrm{Br}(50.3^{o}/_{o})$.

Average atomic mass:

$=79\times\frac{49.7}{100}+81\times\frac{50.3}{100}$

$=\frac{3926.3}{100}+\frac{4074.3}{100}$

$=\frac{8000.6}{100}$

$=80.006u$.

Given, the average atomic mass of a sample of an element $X$ is $16.2\; u$.

Two isotopes of element =  $_{8}^{16}\textrm{X}$ and $_{8}^{18}\textrm{X}$

Now, Let's percent of isotope $_{8}^{16}\textrm{X}$ be x and percent of $_{8}^{18}\textrm{X}$ be 100 - x

So, According to the question,

Average Atomic Mass :

$16.2=16\times\frac{x}{100}+18\times\frac{100-x}{100}$

$16.2=\frac{16x}{100}+18-\frac{18x}{100}$

$-1.8=-\frac{2x}{100}$

$2x=180$

$x=90$

Hence the percentage of isotope $_{8}^{16}\textrm{X}$  is 90 % and thepercentage  isotope $_{8}^{16}\textrm{X}$ is 10%.

Given

the Atomic number, Z = 3

Distribution of electrons :

K = 2,

L = 1
So, Valency = 1 .

The element with atomic number 3 is lithium.

X            Y

Protons  =     6             6

Neutrons  =   6              8

Give the mass numbers of $X$ and $Y$. What is the relation between the two species?

As we know,

the mass number of an atom = No. of protons + No. of Neutrons

So,

The mass number of X = No. of protons of X  + No. of Neutrons of X

= 6 + 6

= 12

The mass number of Y = No. of protons of Y  + No. of Neutrons of Y

= 6 + 8

= 14

As both X and Y have the same atomic number (6) but different numbers (i.e., 12 and 14 respectively), they are isotopes.

(a) Atomic Nucleus

(b) Electron

(c) Proton

(d) Neutron

Rutherford’s alpha-particle scattering experiment was responsible for the discovery of the Atomic Nucleus.

Hence, option (a) is the correct answer.

(a) the same physical properties

(b) different chemical properties

(c) different number of neutrons

(d) different atomic numbers

Isotopes of an element have a different number of neutrons.

Hence option (c) is correct.

(a) 16

(b) 8

(c) 17

(d) 18

The Electronic configuration of  $Cl^{-}$ ion is :

K = 2

L = 8

M = 8

Hence Number of valance electron in  $Cl^{-}$ ion = 8.

Hence, option (b) is the correct answer.

(a) 2,8

(b) 8,2,1

(c) 2,1,8

(d) 2,8,1.

The atomic number of sodium = 11

The electronic configuration of the sodium :

K = 2

L = 8

M = 1

Hence, option (d) is correct.

First row:

atomic number =  9

so, the element is Fluorine.

Atomic no. = No. of protons = no. of electrons = 9

Mass number = no. of protons + no. of neutrons = 9 + 10 = 19

Second row:

Since atomic no. is 16 so, no. of protons = no. of electrons = 16

No. of neutrons = Mass no. - no. of protons = 32 - 16 = 16

Third row:

No. of protons = Atomic no. = 12

So, the element is Magnesium.

No. of electrons = no. of protons = 12

No. of neutrons = Mass no. - no. of protons = 24 - 12 = 12

Fourth row:

No. of protons = Atomic no. = 1

So, the element is Deuterium.

No. of electrons = no. of protons = 1

No. of neutrons = Mass no. - no. of protons = 2 - 1 = 1

Fifth row:

No. of protons = Atomic no. = 1

The element is Protium since the mass number is 1.

So the Table becomes,

 Atomic number mass number Number of neutrons Number of protons Number of electrons Name of the element 9 19 10 9 9 Fluorine 16 32 16 16 16 Sulfur 12 24 12 12 12 Magnesium 1 2 1 1 1 Deuterium 1 1 1 1 0 Hydrogen ion

## NCERT Solutions for Class 9 Science- Chapter Wise

 Chapter No. Chapter Name Chapter 1 NCERT solutions for class 9 science chapter 1 Matter in Our Surroundings Chapter 2 NCERT textbook solutions for class 9 science chapter 2 Is Matter Around Us Pure Chapter 3 Solutions for NCERT class 9 science chapter 3 Atoms and Molecules Chapter 4 NCERT solutions for class 9 science chapter 4 Structure of the Atom Chapter 5 CBSE NCERT solutions for class 9 science chapter 5 The Fundamental Unit of Life Chapter 6 NCERT textbook solutions for class 9 science chapter 6 Tissues Chapter 7 NCERT free solutions for class 9 science chapter 7 Diversity in Living Organisms Chapter 8 NCERT solutions for class 9 science chapter 8 Motion Chapter 9 CBSE NCERT solutions for class 9 science chapter 9 Force and Laws of Motion Chapter 10 NCERT textbook solutions for class 9 science chapter 10 Gravitation Chapter 11 NCERT free solutions for class 9 science chapter 11 Work and Energy Chapter 12 Solutions for NCERT class 9 science chapter 12 Sound Chapter 13 NCERT textbook solutions for class 9 science chapter 13 Why Do We Fall ill? Chapter 14 CBSE NCERT solutions for class 9 science chapter 14 Natural Resources Chapter 15 NCERT solutions for class 9 science chapter 15 Improvement in Food Resources

## How NCERT solutions for class 9 science chapter 4 Structure of the Atom will help you to get a good score in your exam?

• As you know these solutions are created by the experts, so the solutions are error-free.
• The CBSE NCERT solutions for class 9 science chapter 4 Structure of the Atom are explained in very simple language.
• NCERT textbook class 9 chapter 4 has a good weightage so, a better understanding of all the concepts covered in this chapter will help you achieve up to 10 marks from this chapter alone.