Class 9 Science Chapter 4 – Structure of the Atom Exercise Questions and Answers | NCERT Solutions

In this post, you will get the complete NCERT Class 9 Science Chapter 4 – Structure of the Atom Exercise Questions and Answers. All answers are explained in simple and easy language to help students understand the concepts clearly. These solutions cover important topics like electrons, protons, neutrons, atomic number, mass number, isotopes and isobars. This post will help you in revising the chapter quickly and scoring good marks in exams.

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STRUCTURE OF THE ATOMS

EXERCISE QUESTION-ANSWER

Ques 1 :- Compare the properties of electrons, protons and neutrons.

Ans :-

Ques 2 :- What are the limitations of J.J. Thomson’s model of the atom?

 

Ans :- According to J.J. Thomson's model of an atom, the electrons are embedded all over in the positively charged spheres. But experiments done by other scientists showed that protons are present only in the Centre of the atom and electrons are distributed around it.

Ques 3 :- What are the limitations of Rutherford’s model of the atom?

Ans:- The major drawback Of Rutherford's model of an atom is that it does not explain the stability of atom. When charged bodies move in circular orbits, they emit radiation. This emission of radiation would cause a reduction in the energy of the electrons. The electrons should come closer and closer to the nucleus, as they lose energy and finally fall into the nucleus. Rutherford could not explain why this did not seem to be happening.

Ques 4:- Describe Bohr’s model of the atom.

Ans:- Basic postulates of the Bohr's atomic model are :-

--- Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

--- While revolving in discrete orbits the electrons do not radiate energy. These orbits or shells are called energy levels.

Energy levels in an atom are shown in Fig.

Fig :- A few energy levels in an atom. 

(iv) These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

Ques 5:- Compare all the proposed models of an atom given in this chapter.

 

Ques 6:- Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Ans :- Bohr and Bury scheme for the distribution of electrons in an atom is based on the following rules :-

(i) The maximum number of electrons present in a shell is given by the formula 2n^² , where ‘n’ = orbit number or energy level , n = 1,2,3,….

Hence the maximum number of electrons in different shells are as follows :

first orbit or K-shell  

(n=1) =2n^²= 2 (1)^²= 2,

second orbit or L-shell   

(n=2) = 2n^² = 2 (2)^² = 2x 4 =8,

third orbit or M-shell     

(n=2) = 2n^² = 2 (3)^² = 2x 9 =18

fourth orbit or N-shell   

(n=2) = 2n^² = 2 (4)^² = 2x 16 = 32, and so on…….

(ii) The outermost shell (valence shell) can have a maximum of 8 electrons.

(iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner 

So the highest element has K=2 , L =8 , M=8 distribution of electrons.

Ques 7:- Define valency by taking examples of silicon and oxygen.

Ans :-  The combining capacity of an atom of an element known as its valency.

The number of electrons gained , lost or shared so as to make the octet of electrons in the outermost shell of an atom of an element gives us directly the combining capacity of the element.

Valency of silicon is 4 because it can loose , gain or share 4 electrons to complete its octet. Valency of oxygen is 2 because it can complete its octet by gaining 2 electrons.

Ques 8:- Explain with examples 

(i) Atomic number, (ii) Mass number,

(iii) Isotopes and iv) Isobars. 

Give any two uses of isotopes.

 

Ans:- (i) The number of protons present in the nucleus of an atom is called its atomic number. It is denoted by Z.

e.g.:- Oxygen has 8 protons hence atomic number(z) = 8.

Carbon has 6 protons hence atomic number(z) = 6.

 

(ii) The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. It is denoted by ‘A’.

Eg. :- mass of carbon is 12 u because it has 6 protons and 6 neutrons, 

6 u + 6 u = 12 u. 

Similarly, the mass of aluminium is 27 u (13 protons+14 neutrons).

(iii) Isotopes:- Atoms of the same element, having the same atomic number, but different mass numbers are called isotopes of that element.

Isotopes may also be defined as follows :-

"The atoms of an element having the same number of protons but different number of neutrons are called isotopes of that element".

Example :- 

(i) There are three isotopes of hydrogen (H)

(ii) There are two isotopes of chlorine (Cl)

(iii) Isobars:- Atoms of the same element, having the same mass number, but different atomic  numbers are called isobars of that element.

Isotopes	Atomic Number	Mass Number
Argon	18	40
Calcium	20	40

(b) Some application of isotopes are :-

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

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

(iii) An isotope of Iodine is used in the treatment of goiter.

Ques 9 :- Na+ has completely filled K and L shells. Explain.

Ans :- The atomic number of Sodium atom (Na) = 11

Number of protons = 11

Number of electrons = 11

Electronic configuration of Na =

 

K	L	M
2	8	1

Sodium atom (Na) looses 1 electron to become stable and form Na⁺ ion.

Electronic configuration of Na⁺ =

K	L
2	8

As the maximum capacity of K shell is 2 and the maximum capacity of L shell is 8.

 Hence Na⁺ has completely filled K and L shells.

Ques 10: - If bromine atom is available in the form of, say, two isotopes (₃₅⁷⁹) Br, (49.7%) and (₃₅⁸¹) Br (50.3%), calculate the average atomic mass of bromine atom.

Ans :- Isotope of bromine with atomic mass 79 u = 49.7%

Therefore, Contribution of (₃₅⁷⁹)Br to atomic mass = 79 x 49.7 / 100 = 39.26 u

Isotope of bromine with atomic mass 81 u = 50.3%

Contribution of (₃₅⁸¹) Br to the atomic mass = 81 x 50.3/100= 40.64 u

The average atomic mass of the bromine atom = 39.26 + 40.64 u = 79.9u

Hence the average atomic mass of the bromine atom = 79.9 u

Ques 11: - The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes (₈¹⁶) X and (₈¹⁸) X in the sample?

Ans :- Let the percentage of (₈¹⁶) X in the sample be x

Then the percentage of (₈¹⁸) X in the sample is (I00—x).

Now , Atomic mass of X = 16 x  X/100 + 18 x 100- X /100 .

162/10 x 100 = 16 x - 18 x + 1800

162 x 10 = - 2x + 1800

1620 = - 2x + 1800

2x = 1800 -1620

2x=180

X = 180/2

X = 90

percentage of (₈¹⁶) X in the sample= 90 %

percentage of (₈¹⁸) X in the sample = I00 % - 90% = 10%

percentage of (₈¹⁸) X in the sample = 10 %

Ques 12:- If Z = 3, what would be the valency of the element? Also, name the element.

Ans :- Atomic number, Z= 3 

Distribution of electrons =

K	L
2	1

Valency = 1 (electrons on outer orbit)

The element is Lithium.

Ques 13:- Composition of the nuclei of two atomic species X and Y are given as under

	X	Y
Proton	6	6
Neutrons	6	8

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

Ans :- Atomic Mass Number = Number of protons + Number of neutrons

Mass number of X = 6 + 6 = 12

Mass number of Y = 6 + 8 = 14

Atomic number = Number of protons

Atomic number of X = 6

Atomic number of Y = 6

Since X and Y have the same atomic number but different mass number.

So both X and Y are isotopes of same element.

Ques 14: - For the following statements, write T for True and F for False.

(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. (False)

(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. (False)

(c) The mass of an electron is about 1/2000 times that of proton. (True)

(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (False)

Ques 15:- Rutherford’s alpha-particle scattering experiment was responsible for the discovery of

(a) Atomic Nucleus    

(b) Electron

(c ) Proton    

(d) Neutron

Ques 16 :- Isotopes of an element have :-

(a)  the same physical properties          

(b) different chemical properties

(c) different number of neutrons 

(d)  different atomic numbers.

 

Ques 17 :- Number of valence electrons in Cl– ion are:

(a) 16     (b) 8       (c ) 17        (d) 18

Ques 18:- Which one of the following is a correct electronic configuration of sodium?

(a) 2,8               (b) 8,2,1            

(c) 2,1,8            (d) 2,8,1

Ques 19:- Complete the following table.

Ans :-

In this chapter, we learned how atoms are structured and how different models like Dalton’s theory, Thomson’s model, and Rutherford’s experiment helped scientists understand the arrangement of particles inside an atom. We also understood the concepts of electrons, protons, neutrons, atomic number, mass number, isotopes, and isobars. The exercise questions help strengthen these basic ideas. Understanding the structure of the atom is important because it forms the foundation for future topics in chemistry such as chemical reactions, bonding, and molecular structure.