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

Page No. 39

Q1. What are canal rays?
Ans: Canal rays are a type of positively charged radiation discovered by E. Goldstein in 1886. They are produced in a gas discharge and are significant for the following reasons:

• They helped in the identification of protons, a key subatomic particle.
• Canal rays consist of positive ions that move towards the cathode in a discharge tube.
• Their discovery contributed to the understanding of atomic structure.

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 the number of protons (positively charged) is equal to the number of electrons (negatively charged).

Page No. 41

• The atom is neutral because it contains an equal number of electrons and positively charged particles.
• The positive charge of the sphere balances the negative charge of the electrons.
• Thus, the overall charge of the atom is zero, making it electrically neutral.

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: According to Rutherford's model of the atom:

• Protons are present in the nucleus, giving it a positive charge.
• The nucleus contains most of the atom's mass.
• Neutrons are also found in the nucleus, contributing to the overall mass.

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: On using any metal foil, the observations of the a-particle scattering experiment would remain the same as all atoms would have same structure.

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 
For a helium atom:

• Atomic mass = 4 u
• Number of protons = 2
• Therefore, number of neutrons = 4 - 2 = 2

Page No. 42

Q1. Write the distribution of electrons in carbon and sodium atoms.
Ans: 

 (a) Electronic distribution in a carbon atom:

• Atomic number: 6
• Number of electrons: 6
• Distribution: K = 2, L = 4

(b) Distribution of electrons in a sodium atom:

• Atomic number: 11
• Number of electrons: 11
• Distribution: 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 number of electrons in an atom with full K and L shells is:

• K shell: 2 electrons
• L shell: 8 electrons

Therefore, the total number of electrons in the atom is:

• Total: 10 electrons

Page No. 44

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

Chlorine (Cl):

• Atomic number: 17
• Electrons: 2, 8, 7
• Can gain 1 electron to achieve stability (like argon).
• Valency: 1

Sulphur (S):

• Atomic number: 16
• Electrons: 2, 8, 6
• Can gain 2 electrons to achieve stability (like argon).
• Valency: 2

Magnesium (Mg):

• Atomic number: 12
• Electrons: 2, 8, 2
• Can lose 2 electrons to achieve stability (like neon).
• Valency: 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) The atomic number is equal to the number of protons in an atom. In this case:
Atomic number = Number of protons = 8
(ii)  The charge of an atom is determined by the balance between protons and electrons.

• Since the number of electrons (8) is equal to the number of protons (8), the atom has no overall charge.
• Thus, the charge on the atom is zero.

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: 

Hydrogen (H): The most common isotope, consisting of a single proton and no neutrons.

Deuterium (D): A heavier isotope of hydrogen, with one neutron in addition to the proton.

Tritium (T): A radioactive isotope of hydrogen, containing two neutrons and one proton.

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

Isotopes : Isotopes have the same electronic configuration because they have the same number of electrons and protons, differing only in the number of neutrons.
Example : ¹²C₆ and ¹⁴C₆ are isotopes, have the same electronic configuration as (2, 4)

Electronic Configuration(2,4) of Carbon atom

Isobars: Isobars have the same mass number but differ in atomic number, leading to different electronic configurations.

Example :  ²²Ne₁₀ and ²²Na₁₁ are isobars. They have different atomic number but mass number is same. 

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: 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.

Q3. What are the limitations of Rutherford's model of the atom?
Ans: According to Rutherford’s model of an atom the electrons are revolving in a circular orbit around the nucleus. Any such particle that revolves would undergo acceleration and radiate energy. The revolving electron would lose its energy and finally fall into the nucleus, the atom would be highly unstable. But we know that atoms are quite stable.

Q4. Describe Bohr's model of the atom.
Ans: Bohr’s model of the atom
(1) Atom has nucleus in the centre.
(2) Electrons revolve around the nucleus.
(3) Certain special orbits known as discrete orbits of electrons are allowed inside the atom.
(4) While revolving in discrete orbits the electrons do not radiate energy.
(5) These orbits or shells are called energy levels.
(6) These orbits or shells are represented by the letters K, L, M, N or the numbers n = 1, 2, 3, 4

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 rules for writing of distribution of electrons in various shells for the first eighteen elements are:
(i) The maximum number of electrons present in a shell is given by the formula-2 n²
∵ n = orbit number i.e., 1, 2, 3
∵ Maximum number of electrons in different shells are:
K shell n = 1 2n² => 2(1)² = 2
L shell n = 2 2n² => 2(2)² = 8
M shell n = 3 2n² => 2(3)² = 18
N shell n = 4 2n² => 2(4)² = 32
(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. (Shells are filled step-wise).

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 element is equal to the number of protons in the nucleus of its atom. e.g., Oxygen has 6 protons hence atomic no. = 6.
(ii) Mass number: The mass number of an atom is equal to the number of protons and neutrons in its nucleus.
Nucleons = number of protons + number of neutrons Example: Protons + Neutrons = Nucleus = Mass number  6 + 6 = 12

(iii) Isotopes: Isotopes are atoms of the same element which have different mass number but same atomic number.
(iv) Isobars: Isobars are atoms having the same mass number but different atomic numbers.
Both calcium and argon have same mass number but different atomic number.
Two uses of isotopes are:
(i) An isotope of iodine is used in the treatment of goitre.
(ii) An isotope of uranium is used as a fuel in nuclear reactors.

Q9. Na⁺  has completely filled K and L shells. Explain.
Ans: Sodium atom (Na), has atomic number =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. Hence it has completely filled K and L shells.

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.
Answer:

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: 

and 

Q12. If Z = 3, what would be the valency of the element? Also, name the element.
Ans. Z = 3, (i.e, atomic number —> z)
∴ Electronic configuration = 2, 1
Valency = 1
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 = Protons + Neutrons
= 6 + 6 = 12
Mass number of Y = Protons + Neutrons = 6 + 8 = 14
As the atomic number is same i.e., = 6.
[atomic number = number of protons].
Both X and Y are isotopes of same element.

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