Class 9th Science Chapter-4 In-Text Questions

Structure of Atom

In-Text Question Answers

Page 39 – Questions & Answers

Question 1. What are canal rays?

Answer:
Canal rays are streams of positively charged particles produced in a gas discharge tube.
They were discovered by E. Goldstein and are also called positive rays.

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

Answer:
No, the atom will not carry any charge.

Reason:

• Electron has a charge of –1

• Proton has a charge of +1

• These charges are equal and opposite, so they cancel each other

Hence, the atom is electrically neutral.

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Page 41 – Set 1 (After Bohr’s Model of Atom)

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

Answer:
According to Thomson’s model, an atom consists of a positively charged sphere with negatively charged electrons embedded in it.
The total positive charge is equal to the total negative charge of electrons, therefore the atom as a whole is electrically neutral.

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

Answer:
According to Rutherford’s model, the nucleus contains positively charged particles called protons.

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

Answer (description for exam):
Bohr’s model shows:

• A central nucleus

• Three circular shells around it named K, L, and M

• Electrons revolving in these fixed shells

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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?

Answer:
Similar observations would be obtained, but the extent of deflection may vary because:

• Gold is extremely thin and malleable

• Other metals are less malleable, so fewer α-particles may pass through without deflection

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Page 41 – Set 2 (After Neutrons)

Question 1.

Name the three sub-atomic particles of an atom.

Answer:
The three sub-atomic particles are:

1. Electrons

2. Protons

3. Neutrons

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Question 2.

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

Answer:

Mass number = Protons + Neutrons

$$4=2+\text{Neutrons}$$
$$\text{Neutrons}=2$$

Helium atom has 2 neutrons.

Page 42 – Questions & Answers

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

Carbon (Atomic number = 6):

• K shell = 2 electrons

• L shell = 4 electrons

Electronic configuration of carbon = 2,4

Sodium (Atomic number = 11):

• K shell = 2 electrons

• L shell = 8 electrons

• M shell = 1 electron

Electronic configuration of sodium = 2,8,1

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

• Maximum electrons in K shell = 2

• Maximum electrons in L shell = 8

$$\text{Total electrons}=2+8=\boxed{{\displaystyle 10}}$$

Total number of electrons = 10

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Page 44 – Second Set of Questions

Question 1.

If the number of electrons in an atom is 8 and the number of protons is also 8, then
(i) what is the atomic number of the atom?
(ii) what is the charge on the atom?

Answer:

(i) Atomic number = number of protons = 8

(ii) Charge on the atom = 0
Reason: Number of electrons (–8) = number of protons (+8), so charges cancel out.

The atom is electrically neutral.

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Question 2.

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

Answer:

• Oxygen atom
  Protons = 8, Neutrons = 8

  $$\text{Mass number}=8+8=\boxed{{\displaystyle 16}}$$

• Sulphur atom
  Protons = 16, Neutrons = 16

  $$\text{Mass number}=16+16=\boxed{{\displaystyle 32}}$$
  

Page 45 – Questions & Answers

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

Answer:

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

(a) Isotopes (example: Chlorine)

Isotopes have the same atomic number, so their electronic configuration is the same.

• Chlorine-35 (³⁵₁₇Cl): 2, 8, 7

• Chlorine-37 (³⁷₁₇Cl): 2, 8, 7

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(b) Isobars (example: Calcium and Argon)

Isobars have the same mass number but different atomic numbers, so their electronic configuration is different.

• Calcium-40 (⁴⁰₂₀Ca): 2, 8, 8, 2

• Argon-40 (⁴⁰₁₈Ar): 2, 8, 8