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NCERT Solution for Class 9 Science Chapter 4 : Structure of the Atoms

Structure of the Atoms
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Chapter 4 also falls under Chemistry and it is on the “Structure of Atoms”. It is a very interesting chapter if you understand it properly. If you are planning to pursue Science for your higher studies, then what you learn here will benefit you in the long run. You will get NCERT Solutions for Class 9 Science by experts on Swiflearn for this chapter too. This chapter involves some basic mathematical calculations and different theories about the structure of atoms which is why it is very scoring. If you can grasp this chapter well, you will be able to score better in the science exam. Also, keep practicing the atomic structure frequently, or else you will forget the theories related to the atomic structure. This chapter is an important chapter for you from an exam perspective also as it includes many important concepts such as Neutrons, Protons, Isotopes, Valency, etc.

NCERT Solutions for Class 9 Science Chapter 4 includes Intext Exercise 1, 2, 3, 4, 5, 6, 7, and an NCERT Exercise. It contains a total of 19 questions in the NCERT exercise. This chapter includes application and memory-based questions to test your understanding of the chapter.

Ncert solutions

NCERT Solution for Class 9 Science Chapter 4 : Structure of the Atoms Ex 4.1

 

 

Exercise 4.1

Question 1.
What are canal rays?

 

Solution:
Canal ray or anode ray a beam of positive ions that is created by certain types of gas-discharge tubes.

 

Question 2.
If an atom contains one electron and one proton, will it carry any charge or
not?

 

Solution:
Electron and proton contain negative and positive charges and equal in magnitude. Hence the atom does not carry any charge.

 

Exercise 4.2.3

Question 1.
On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

 

Solution:
Thomson proposed that:
(i) An atom consists of a positively charged sphere and the electrons are embedded in it.
(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

 

Question 2.
On the basis of Rutherford’s model of an atom, which subatomic particle is
present in the nucleus of an atom?

 

Solution:
Protons (positively charged particles) are present in the nucleus.

 

Question 3.
Draw a sketch of Bohr’s model of an atom with three shells.

 

Solution: Refer pdf.

 

Question 4.
What do you think would be the observation if the α -particle scattering
experiment is carried out using a foil of a metal other than gold?

 

Solution:
In the α-scattering experiment, a gold foil was taken because gold is highly malleable. It would be very difficult to produce such thin sheets of other metal.
So, if carried out with other metal’s sheets, there would be no change.

 

Exercise 4.2.4

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

 

Solution:
Electrons, protons, neutrons

 

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

 

Solution: Refer pdf.

Exercise 4.3

Question1.
Write the distribution of electrons in carbon and sodium atoms.

 

Solution: Refer pdf.

 

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

 

Solution:  Refer pdf.

 

Exercise 4.4

Question1.
How will you find the valency of chlorine, sulphur and magnesium?

 

Solution: Refer pdf.

Exercise 4.5

Question 1.
If 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?

 

Solution: Refer pdf.

 

Question 2.
Find out the mass number of oxygen and sulphur atom.

 

Solution:-  Refer pdf.

 

Exercise 4.6

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

Solution: Refer pdf.

 

Question 2.
Write the electronic configuration of any one pair of isotopes and isobars.

 

Solution: Refer pdf.

 

Exercise 1

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

 

Solution: Refer pdf.

 

Question 2.
What are the limitations of J.J. Thomson’s model of the atom?

 

Solution:
Thomson suggested that the positive charge in the atom is spread all over like while the electrons are studded in the positively charged sphere but later it was discovered that positive charge is present in the nucleus only and electrons revolve around it.

 

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

 

Solution:
It could not explain the stability of the atom as the the revolving electron would lose energy and finally fall into the nucleus. But the atom is stable.

 

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

 

Solution:
Neils Bohr put forward the following postulates about the model of an atom:
(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom. (ii) While revolving in discrete orbits the electrons do not radiate energy.
The orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

The following rules are followed for writing the number of electrons in different energy levels or shells:
(i) The maximum number of electrons present in a shell is given by formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3.
(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.
(iii) Electrons are not accommodated in a given shell, unless the inner shells are filled.

 

Question 5.
Compare all the proposed models of an atom given in this chapter.

 

Solution: Refer pdf.

 

Question 6.
Summaries the rules for writing of distribution of electrons in various shells for the first eighteen elements.

 

Solution:
The following rules are followed for writing the number of electrons in different energy levels or shells:
(i) The maximum number of electrons present in a shell is given by formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3,….
Hence the maximum number of electrons in different shells are as follows: first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18,
(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.
(iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. Electrons in different shells are as follows: first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18, fourth orbit or N-shell will be = 2 × 42 = 32, and so on.

 

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

 

Solution:
Valency is combing capacity of of an element.
Let X = No of electrons in the outermost shell
If X<=4 then valency = X else valency = 8-X
Electronic configuration of Oxygen: 2,8,6
Hence, valency = 8-6 = 2
Electronic configuration of Silicon: 2,8,4
Hence, valency = 4

 

Question 8.
Explain with examples
(i) Atomic number (ii) Mass number (iii) Isotopes and iv) Isobars. Give any
two uses of isotopes.

 

Solution:
(i)The atomic number is defined as the total number of protons present in the nucleus of an atom.
Ex. The atomic number of oxygen is 8 means an atom of oxygen has 8 protons.
(ii) Mass number: 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’
Ex. Nitrogen has 7 protons and 7 neutrons. Therefore its mass number is 14.
(iii) Isotopes: isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers.
Ex. Hydrogen atom has three atomic species, namely protium, deuterium and tritium. The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively.
Uses:
(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii) An isotope of cobalt is used in the treatment of cancer
(iv) Isobars:
Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.
Eg. Calcium, atomic number 20, and argon, atomic number 18. The number of protons in these atoms is different, but the mass number of both these elements is 40.

 

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

 

Solution:
The electronic configuration of Na: 2,8,1
The electronic configuration of Na+: 2,8,
K and L are filled with 2 and 8 electrons respectively which are the maximum number of electrons present.

 

Question 10.
If bromine atom is available in the form of, say, two isotopes

(49.7%) and

(50.3%), calculate the average atomic mass of bromine atom.

 

Solution:
Average atomic mass 80.006 u 

 

Question 11.
The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes and in the sample?

 

Solution: Refer pdf.

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

 

Solution:
Electronic configuration for Z = 3 is 2,1.
Outmost shell contains 1 electron. Hence, valency = 1
Element with Z = 3 is Lithium.

 

Question 13.
Composition 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 numbers of X and Y. What is the relation between the two
species?

 

Solution:
Since X and Y have same atomic number but have different number of neutrons. Hence they are isotopes.

 

Question 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.
(b) A neutron is formed by an electron and a proton combining together.
Therefore, it is neutral.
(c) The mass of an electron is about (1/2000) times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

 

Solution:
(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.
TRUE

 

Choose the correct option for questions 15, 16, 17.

 

Question 15.
Rutherford’s alpha-particle scattering experiment was responsible for the
discovery of
(a) Atomic Nucleus (b) Electron (c) Proton (d) Neutron

 

Solution:
(a) Atomic Nucleus

 

Question 16.
Isotopes of an element have

(a) The same physical properties (b) different chemical properties (c) different number of neutrons (d) different atomic numbers.

 

Solution:
(c) Different number of neutrons

 

Question 17.
Number of valence electrons in Cl–
ion are:
(a) 16 (b) 8 (c) 17 (d) 18

 

Solution:
Electronic configuration of Cl- ion: 2,8,8
Valence electrons = 8

 

Question 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.

 

Solution:
(d) 2,8,1

 

Question 19.
Complete the following table.

 

Solution: Refer pdf.

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