IN-TEXT ANSWERS Structure of Atom

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✔ IN-TEXT QUESTION–ANSWERS

4.1 Charged Particles in Matter

Q1. What are canal rays?

Answer: Canal rays are positively charged radiations discovered by E. Goldstein. They led to the discovery of protons.

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

Answer: No. The atom will be electrically neutral, because one negative charge (electron) cancels one positive charge (proton).

4.2 The Structure of an Atom

Q1. On the basis of Thomson’s model, explain how the atom is neutral as a whole.

Answer: In Thomson’s model, positive charge is spread throughout the atom and electrons are embedded in it. The positive and negative charges are equal in magnitude, so the atom is overall neutral.

Q2. According to Rutherford’s model, which sub-atomic particle is present in the nucleus of an atom?

Answer: The proton (and later neutron, discovered by Chadwick) is present in the nucleus.

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

Answer:
K–L–M shells with electrons revolving around a central nucleus.

(As text description)

  • Nucleus at the center.
  • First shell (K) nearest to nucleus.
  • Second shell (L) further out.
  • Third shell (M) outermost.
    Each shell contains electrons in circular paths.

Q4. What would be the observation if ฮฑ-particle scattering experiment is carried out with a metal foil other than gold?

Answer: The same general observations would be obtained because all atoms have:

  • large empty space
  • small positively charged nucleus
    Only the degree of scattering may vary slightly due to different nuclear mass/charge.

4.2.4 Neutrons

Q1. Name the three sub-atomic particles of an atom.

Answer:

  • Electron
  • Proton
  • Neutron

Q2. Helium has atomic mass 4 u and 2 protons. How many neutrons does it have?

Answer:
Mass number = protons + neutrons
4 = 2 + n
Neutrons = 2

4.3 Distribution of Electrons

Q1. Write the electron distribution in carbon and sodium atoms.

Carbon (Z = 6): 2, 4
Sodium (Z = 11): 2, 8, 1

Q2. If K and L shells of an atom are full, what is the total number of electrons?

K shell = 2
L shell = 8
Total = 10 electrons

4.4 Valency

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

Answer:
Valency = electrons lost or gained to complete octet

  • Chlorine (2,8,7): gains 1 → valency = 1
  • Sulphur (2,8,6): gains 2 → valency = 2
  • Magnesium (2,8,2): loses 2 → valency = 2

4.5 Atomic Number & Mass Number

Q1. If number of electrons = 8 and protons = 8:

(i) Atomic number = 8
(ii) Charge = neutral (equal + and − charges)

Q2. Using Table 4.1, find mass number of oxygen and sulphur.

Oxygen: 8 protons + 8 neutrons = 16 u
Sulphur: 16 protons + 16 neutrons = 32 u

4.6 Isotopes

Q1. For symbols H, D and T, tabulate the three sub-atomic particles.

Atom

Protons

Neutrons

Electrons

H (¹H)

1

0

1

D (²H)

1

1

1

T (³H)

1

2

1

 

 

 

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

Example:

  • Isotopes: ¹²C → 2,4 ; ¹⁴C → 2,4 (same electrons because same atomic number)
  • Isobars: ⁴⁰Ar (2,8,8) and ⁴⁰Ca (2,8,8,2)

  

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