NCERT Solutions for Class 9 Science Chapter 4 - Structure of the Atom

Page No. 39

Q1. What are canal rays?
Ans: Canal rays, discovered by Goldstein in 1886, are positively charged radiation found in a gas discharge, which played a significant role in the discovery of protons, a subatomic particle.

Goldstein's experimental setup
Q2. If an atom contains one electron and one proton, will it carry any charge or not?
Ans: No, it will not carry any charge because positively charged particles are equal to negatively charged particles.

Page No. 41

Thomson's Model of an atom

Q2. On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?
Ans: On the basis of Rutherford's model, protons are present in the nucleus of an atom because the nucleus is positively charged. Since the whole mass of the atom is concentrated in the centre called the nucleus, therefore, neutrons are also present in the nucleus.

Rutherford's Model of an atom
Q3. Draw a sketch of Bohr's model of an atom with three shells.
Ans: 

Q4. 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?
Ans: If the metal taken is heavier than gold, then the deviation shown by α-particles will be more, and if the metal taken is lighter than gold, then the deviation shown will be less due to less repulsion between positively a-particles and nucleus.

Q5. Name the three sub-atomic particles of an atom.
Ans: The three sub-atomic particles of an atom are:
(i) Protons
(ii) Electrons
(iii) Neutrons

Q6. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Ans: Number of neutrons = Atomic mass - Number of protons
Therefore, the number of neutrons in the atom = 4 - 2 = 2

Page No. 42

Q1. Write the distribution of electrons in carbon and sodium atoms.
Ans: 
(a) Electronic distribution in carbon atom
Atomic number of carbon = 6 
Number of electrons = 6 
Distribution of electrons: K = 2, L= 4 
(b) Distribution of electrons in sodium atom
Atomic number of sodium atom =11 
Number of electrons in sodium atom =11 
Distribution of electrons in sodium atom: K = 2, L = 8, M = 1

Q2. If K and L shell of an atom are full, then what would be the total number of electrons in the atom?
Ans. The maximum capacity of the K shell is 2 electrons and the L shell can accommodate a maximum of 8 electrons in it. Therefore, there will be ten electrons in the atom.

Page No. 44

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

•  Cl (17) has 17 electrons. Its electronic configuration is 2, 8,-7. It can gain 1 electron to become stable like the noble gas argon, so its valency is equal to 1.
• S (16) has 16 electrons. Its electronic configuration is 2, 8, 6. It can gain 2 electrons to become stable like noble gas argon, so its valency is equal to 2.
• Mg (12) has 12 electrons. Its electronic configuration is 2, 8, 2. It can lose 2 electrons to become stable like noble gas neon, so its valency is equal to 2.

Q2. 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?
Ans:
(i) Atomic number = Number of protons = 8
(ii) Charge on atom is zero because the number of elections is equal to the number of protons.

Q3. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.

Ans: 
(a) To find the mass number of Oxygen:
Number of protons = 8
Number of neutrons = 8
Atomic number = 8
Atomic mass number = Number of protons + number of neutrons = 8 + 8 = 16
Therefore, mass number of oxygen = 16
(b) To find the mass number of Sulphur:
Number of protons = 16
Number of neutrons = 16
Atomic number = 16
Atomic mass number = Number of protons + number of neutrons = 16 + 16 = 32

Page No. 45

Q1. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
Ans: 

Q2. Write the electronic configuration of any one pair of isotopes and isobars.
Ans: 

• ¹²C₆ and ¹⁴C₆ are isotopes, have the same electronic configuration as (2, 4)

Electronic Configuration(2,4) of Carbon atom

• ²²Ne₁₀ and ²²Ne₁₁ are isobars. They have different electronic configurations as given below:
  ²²Ne₁₀ – 2, 8
  ²²Ne₁₁ – 2, 8, 1

Page No. 46, 47 & 48

Q1. Compare the properties of electrons, protons and neutrons.

Ans: 

Q2. What are the limitations of J.J. Thomson's model of the atom?
Ans: Limitations of JJ Thomson's model of an atom:
(i) The results of experiments carried out by other scientists could not be explained by this model
(ii) The positive charge is spread all over could not be justified.

Q3. What are the limitations of Rutherford's model of the atom?
Ans: Limitations of Rutherford's model of an atom
(i) According to classical electromagnetic theory, a moving charged particle, such as an electron, under the influence of attractive force, loses energy continuously in the form of radiations. As a result of this, the electron should lose energy and, therefore, should move in even smaller orbits, ultimately falling into the nucleus. But the collapse does not occur. There is no explanation for this behaviour.
(ii) According to Rutherford, the electrons revolve around the nucleus in fixed orbits. However, Rutherford did not specify the number of orbits and the number of electrons in each orbit.

Q4. Describe Bohr's model of the atom.
Ans: Bohr made a bold new suggestion that particles at the atomic level would behave differently from macroscopic (bigger) objects.
According to Bohr’s theory:
(i) Electrons revolve in certain orbits having well-defined energy.
(ii) Electrons do not emit radiations as long as they remain at the same energy level.
(iii) The supply of energy excites the electrons to a higher energy level.
(iv) Energy emission takes place when it jumps from a higher energy level to a lower energy level.

Bohr's Model of an atom

Q5. Compare all the proposed models of an atom given in this chapter.
Ans: 

Q6. Summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Ans: The distribution of electrons in various shells of an atom is called electronic configuration.
In order to write the electronic configuration of an element, one should proceed as follows:
(i) With the help of an atomic number of the element, find out the total number of electrons present in the atom.
(ii) Allot these electrons in the various shells one by one in the order of increasing energy according to 2n² rule.
(iii) The shell with the least energy should be filled first, and then the other shells with increasing energies should be filled.
(iv) Write the name and number of electrons in various shells, starting with the lowest. Each number of electrons is separated from the other shell by a comma. For example, sodium (atomic number 11) has 11 electrons in its atom. According to 2n² rule, K shell should be filled with 2 electrons and then L shell will be filled with 8 electrons and in the end, one left electron will go to N shell.
Example: Sodium (atomic number 11)
Electronic configuration of Na : K = 2, L = 8, M = 1

Q7. Define valency by taking examples of silicon and oxygen.
Ans: The valency of an atom is the number of hydrogen atoms which combine with one atom of an element.
For example, 

• Silicon (atomic number 14) has the following electronic distribution:
  K = 2, L = 8, M = 4.
  In the outermost shell, there are 4 electrons; hence, one atom of silicon will combine with 4 atoms of hydrogen. So, the valency of silicon is 4.

• In the case of oxygen (atomic number 8), the electronic distribution in various shells is given below:
  K = 2, L = 6
  There are six valence electrons in the atom of oxygen. So, one atom of oxygen will combine with 2 atoms of hydrogen to make a noble gas configuration. Hence, the valency of oxygen is 2.

Q8. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
Ans: 
(i) Atomic number: The atomic number of an atom is equal to the number of protons in the nucleus of an atom.
Atomic number ~ Number of protons.
(ii) Mass number: The total mass of the atom is due to the sum of the masses of protons and neutrons present in the nucleus. The mass number of an atom is equal to the sum of the number of protons and neutrons in the nucleus.
Mass number = Number of protons + Number of neutrons.
(iii) Isotopes: Isotopes are the atoms of the same element having the same atomic number but different mass numbers. For example, hydrogen has three isotopes. They are called protium
(iv) Isobars: Isobars are the atoms of the different elements having the same mass number and different atomic numbers.
Uses of isotopes:
(i) Cobalt-60 is a radioactive isotope of cobalt. It is used in radiotherapy for cancer.
(ii) ¹⁴C is a radioactive isotope of carbon. It is used in carbon dating.

Q9. Na⁺  has completely filled K and L shells. Explain.
Ans: Sodium has the atomic number 11. It has 11 electrons in its orbitals, wherein the number of protons is equal to the number of electrons. Hence, its electronic configuration is K-2, L-8, and M-1. The one electron in the M shell is lost, and it obtains a positive charge since it has one more proton than electrons and obtains a positive charge, Na⁺. Na⁺ is formed when one electron is lost by a sodium atom, as given below:

The new electronic configuration is K-1; L-8 which is the filled state. Hence it is very difficult to eliminate the electron from a filled state as it is very stable.

Q10. If bromine atom is available in the form of, say, two isotopes  ⁷⁹Br₃₅ (49.7%) and ⁸¹Br₃₅ (50.3%) calculate the average atomic mass of bromine atom.
Ans: Average atomic mass of Br
= ( (79 x 49.7) + (81 x 50.3) ) / 100
= (3926.3 + 4074.3) / 100
= 8000.6 / 100
= 80.006 u

Q11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes  in the sample?
Ans: It is given that the average atomic mass of the sample of element X is 16.2 u.
Let the percentage of isotope 18 / 8 X be y%. Thus, the percentage of isotope 16 / 8 X will be (100 - y) %.
Therefore,
18 x (y/100) + 16 x ((100-y)/100) = 16.2
18y/100 + ( 16(100 - y) / 100 ) = 16.2
(18y+1600-16y )/100 = 16.2
18y + 1600 - 16y = 1620
2y + 1600 = 1620
2y = 1620 - 1600
y = 10
Therefore, the percentage of isotope 18 / 8 X is 10%.
And, the percentage of isotope 16 / 8 X is (100 - 10) % = 90%.

Q12. If Z = 3, what would be the valency of the element? Also, name the element.
Ans. Z = 3
Electronic configuration is 2, 1.
Valence electron is equal to 1.
Valency is also equal to 1.
The name of the element is lithium.

Q13. Composition of the nuclei of two atomic species X and Y are given as under

Give the mass numbers of X and Y. What is the relation between the two species?
Ans: Mass number of X = 6 + 6 = 12 
Mass number of Y = 6 + 8 = 14
Atomic number of X = 6 
Atomic number of Y = 6
X and Y are isotopes since both the elements have same atomic number but different mass number.

Q14. 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.
Ans. (a) False (b) False (c) True (d) False

Q15. 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
An atomic nucleus is the dense central core of an atom, composed of protons and neutrons held together by the strong nuclear force.

Q16. Isotopes 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

Q17. Number of valence electrons in Cl ^-ion are:
(a) 16
(b) 8
(c) 17
(d) 18
Ans: (b) 8
Cl^- has 8 valence electrons. Cl^- has 17 + 1 = 18 electrons. Its electronic configuration is 2, 8, 8.

Q18. 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
Ans: (d)  2 ,8 ,1 
2 ,8 ,1 is the correct electronic configuration of sodium.

Q19. Complete the following table.

Ans: 
Atomic number(Z) =Number of protons
Mass number = Number of neutrons + atomic number
=> Mass number(A) = Number of neutrons + number of neutrons