Tuesday, July 22, 2025

RBSE Class 9 Science Chapter 4 Structure of Atom Solutions

This post contains all the main and important topics which have complete and detailed solutions of Chapter 4 Structure of Atom. These solutions are based on the latest Rajasthan Board syllabus and prescribe NCERT book.


RBSE Class 9 Science Chapter 4 Structure of Atom Solutions


Board

RBSE

Subject

Science

Chapter

Structure of Atom

No. of Intext questions

15

No. of End exercise questions

19

Prescribed Book

 NCERT Science


 

Structure of Atom Questions - Answers

 

 

Answers of Questions on Page 39


Q1. What are canal rays?

Ans. These rays are positively charged radiations which are called canal rays.



Q2. If an atom contains one electron and one proton. Will it carry any charge or not?

Ans. The atom containing one electron and one proton will not carry any charge and it is neutral because one positive charge (Proton) balances one negative charge (electron)



Answers of Questions on Page 41


Q1. On the basis of Thomson’s model of an atom. Explain how the atom is neutral as whole.

Ans. According to Thomson’s atomic model – An atom consists of a positively charged sphere and electrons are embedded in it. The positive charges and negative charges are equal in magnitude so an atom is neutral.



Q2. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

Ans. Protons



Q3. Draw a sketch of Bhor’s model of an atom with three shells.

Ans.

 

Draw a sketch of Bhor’s model of an atom with three shells.

               

Q4. What do you think would be the observation if the  scattering experiment is carried out using a foil of a metal other than gold?

Ans.  If the  scattering experiment is carried out using a foil of a metal other than gold, the observation would be the same as all atoms have same structure.


 

Answers of Questions on Page 41


Q.1 Name the three sub-atomic particles of an atom.

Ans.      Electrons, protons and neutrons.


 

Q.2 Helium atom has an atomic mass of 4 μ and two protons in its nucleus. How many neutrons does it have?

Ans.     

 Atomic mass = Number of protons + Number of neutrons


4  =  2  +   Number of neutrons


Number of neutrons = 4 -2 = 2

 

Answers of Questions on Page 42


Q.1 Write the distribution of electrons in carbon and sodium atoms.

Ans.     

          

For Carbon

 

Atomic number   = 6

 

Number of protons =   6

 

Number of protons = Number of electrons

 

Electronic distribution =  K      L

                                         2      4

 

 

For Sodium

 

Atomic number   = 11

 

Number of protons =6

 

Number of protons = Number of electrons

 

Electronic distribution =     K      L  M

                                             2      8   1

 

 

Q.2 If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

Ans.      

K shell can hold 2 electrons

 While L shell can hold 8 electrons

 When both the shells are full, there will (8+2) = 10) electrons in the atom.


Answers of Questions on Page 44


Q1. How will you find the valency of chlorine, sulphur and magnesium?

 Ans.


Valency – The combining capacity of an atom is known as valancy.

For Chlorine

Atomic number = 17

Protons =17

Electrons = 17

Electrons distribution = KLM

287

Chlorine needs 1 electron to complete its outermost shell, so its valency is -1 (accepts one electron)


For Sulphur

Atomic number = 16

Protons =16

Electrons = 16

Electrons distribution = KLM

286

Sulphur  needs 2 electron to complete its outermost shell, so its valency is -2 (accepts two electron)

 

For Magnesium

Atomic number = 12

Protons =12

Electrons = 12

Electrons distribution = KLM

282

Magnesium   donates 2 electrons to complete its outermost shell, so its valency is +2 (donates two electron)


 

Answers of Questions on Page 44


Q.1 If the number of electrons in an atom is 8 and number of protons is also 8, then

(i)   What is the atomic number of the atom? and (ii) What is the charge on the atom?

Ans.     

Atomic number = Number of Protons =8


Number of electrons = number of protons , so atom will be neutral. There is no charge on the atom

 

Q.2 With the help of table given, find out the mass number of oxygen and sulphur  atom.

 

       Table : Composition of Atoms of the First Eighteen Elements with Electron Distribution in various Shells

     

Composition of Atoms of the First Eighteen Elements with Electron Distribution in various Shells

Ans.

For Oxygen

Number of protons = 8


Number of neutrons= 8


Atomic number = 8


 Mass number = P + N = 8 + 8= 16


For Sulphur

Number of protons = 16


Number of neutrons= 16


Atomic number = 16


 Mass number = P + N = 16 + 16= 32


Answers of Questions on Page 45


Q.1 For the symbol H, D and T tabulate three sub-atomic particles found in each of them.

Ans.     

For H ( 1H1)

 Atomic number =1

 Mass number =1

Number of protons =1

Number of electrons =1

 Number of neutrons = 1-1=0     


 For D ( 1H2)

Atomic number =1

 Mass number =2

 Number of protons =1

 Number of electrons =1

 Number of neutrons = 2-1=1

 For T ( 1H3)

Atomic number =1

Mass number =3

Number of protons = 1

Number of electrons= 1

 Number of neutrons= 3-1=2


Q.2 Write the electronic configuration of any one pair of isotopes and isobar.

Answer

Isotopes – Isotopes are atoms having same atomic number but different mass numbers.

Isotopes

6C12

6C14

Shells

K    L

K     L

No.of  electrons

2    4

2     4


Isobars – Isobars are atoms having same mass number but different atomic number.



 Answers of End Exercise Questions on Page 46-48


Q.1 Compare the properties of electrons, protons and neutrons.

Ans.      

Comparison between properties of Electron, Proton and Neutron


Compare the properties of electrons, protons and neutrons.

 Q2. What are the limitations of J.J. Thomson’s Model of the atom?

Ans.      

According to J.J. Thomson model of an atom, electrons are embedded all over in the positively charged sphere. But experiments show that protons are present only in the nucleus and electrons revolve around the nucleus

 


Q.3 What are the limitations of Rutherford’s model of the atom?

Ans.      

According to Rutherford’s model of an atom, electrons revolve in a circular orbit around the nucleus. But such particle would undergo acceleration and radiate energy and ultimately would loss its energy and fall into the nucleus. Now the atoms would become unstable, but atoms are quite stable.

This was not explained through Rutherford’s model of atom.



Q.4 Describe Bohr’s model of the atom.

Ans.      

Bohr’s model of an atom

 1. An atom has nucleus in the center.

 2. Electrons revolve in definite circular path of fixed energy around the nucleus.

 3.The electron can change its shell or energy levels by absorbing or releasing energy.

 4. Electron does not radiate energy while moving in discrete orbit.

Bohr’s model of an atom

Q.5 Compare all the proposed models of an silicon given in this chapter.

Ans.     


 

Q6. Summaries the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Ans. 

Following are the rules for writing of distribution of electrons in various shells for the first eighteen elements are-


The maximum number of electrons present in a shell is given by the formula- 2n2 .

n = orbit number- 1 ,2,3 etc.

Maximum numbers of electrons present in a shell are-

 

K shell (n =1) = 2n2 = 2 ×11 = 2 electrons

 

L shell (n = 2) = 2n2  = 2× 22 = 8 electrons

 

M shell (n= 3) = 2n2  = 2 ×32 = 18 electrons

 

N shell (n =4)  = 2n2 = 2 ×42 = 32 electrons

 

The maximum number of electrons that can be accommodated in the outermost orbit is 8.


Electrons are not accommodated in a given shell, unless the inner shells are filled in a step wise manner.



Q.7 Define valency by taking examples of silicon and oxygen.

Ans.      

Valency – The combining capacity of an atom is called valency. 


Silicon

 Atomic number = 14

Number of electorns = 14

Electronic configuration = K,L,M

                                            2,8,4

Its outermost shell has 4 electrons which it can share with other atoms to complete its orbit hence its valency is 4.

 

Oxygen

Atomic number = 8

Number of electorns = 8

Electronic configuration = K L

                                            2 6

Its outermost shell has 6 electrons. It will gain 2 electrons to complete its octet. Hence its valency is -2 



Q.8 Explain with examples (i) Atomic number (ii) Mass number (iii) Isotopes and (iv) Isobars.

Ans.      

(i). Atomic number – the atomic number of an element is equal to the number of protons in the nucleus of its atom.eg.-  carbon has 6 protons so its atomic number is – 6


(ii). Mass number – The mass number is equal to the number of protons and neutrons in its nucleus. It is denoted by ‘A’.. eg. – Carbon has 6 protons and 6 neutrons, so its mass number is 6 +6 = 12


(iii). Isotopes - Isotopes are atoms having same atomic number but different mass numbers. eg- 6C14   6C14


(iv) Isobars - Isobars are atoms having same mass number but different atomic number.eg- -  18Ar40 ,  20Ca40  



Q.9 Na+ has completely filled K and L shells. Explain.

Ans.      

Atomic number of sodium is 11 and its electronic configuration is 2, 8, 1. Sodium ion (Na+) is formed by the loss of one electron from sodium atom.

  Na      → Na+   + e-

  2,8,1       2,8

 KLM        KL


K and L shells of sodium ion have 2 and 8 electrons respectively and are completely filled.        


Q.10If bromine atom is available in the form of, say two isotopes    35Br79 (49.7%) and    35Br81(50.3%). Calculate the average atomic mass of bromine atom.

Ans.      

Percentage of Br isotope with mass number 79 = 49.7


Percentage of Br isotope with mass number 81 = 50.3 

   

The average atomic mass of bromine atoms = `\frac{49.7 ×79 +50.3×81}{100}`


=`\frac{392666.3 + 4074.3}{100}`

     

 = `39.263 + 40.743 = 80.006u`                                                                                                                                      


Q.11 The average atomic mass of a sample of an element X is 16.2 μ. What are percentages of isotope  and  in the sample?

Ans.      

Let the percentage of the isotope  be x. Hence, the percentage of the isotope  is (100- x).                                 


                                                                                           



Q.12 If Z = 3, what would be the valency of the element?

Ans.     

For Z =3, the element is lithium (Li)

 Electronic configuration of lithium

                           Z = 3 = K    L

                                         2    1

 As the outermost shell has 1 electron. Its valency is 1.  



Q.13 Compositions of the nuclei of two atomic species X and Y are given as under:

 

X        Y

Protons 

6         6

Neutrons

 

 6         8

Give the mass number of X and Y, what is the relation between the two species?

Ans.      

Mass number of      

           X = 6 + 6 = 12

           Y = 6 + 8 = 14

 

Both X and Y contain equal number of protons but different number of neutrons. Hence, X and Y are isotopes.


Q.14  For the following statements write “T” for true and “F” for false:

 (a)  J. J. Thomson proposed that the nucleus of an atom contain only nucleus.

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

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

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

Ans.      

(a) False (b) False (c) True (d) False



Q.15 Rutherford’s alpha-particle scattering experiment was responsible for the discovery of

 (a) Atomic nucleus (b) Electron (c) Proton (d) Neutron.

Ans.      (a) Atomic nucleus.   

                                                                                              

Q.16Isotopes of an element have:

  (a)  The same physical properties

  (b)  Different chemical properties

  (c)  Different number of neutrons

  (d)  Different atomic numbers.

Ans.  (c)  Different number of neutrons.    


 

Q.17 Number of valence electrons is Cl- ion are:

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

Ans.      

As the electronic configuration of chlorine Cl (Z = 17) is                                                                                             



Q.18  Which one of the following is a correct configuration of sodium?

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

Ans.   (d)  Atomic number of sodium = 11

   Its electronic configuration 




Q.19     Complete the following:

Answer 





                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     




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