NCERT Questions for Class 12 Physics Chapter 12 – Atoms

Important Questions with Solutions of Class 12 Physics Chapter 12 – Atoms

Short Answer Questions

1) Describe Bohr’s atomic model’s drawbacks.

Ans – The following are Bohr’s atomic model’s drawbacks:

• It provides no information about the distribution and configuration of electrons within an atom.
• It was unable to explain how electrons are waves.

2) If Bohr’s quantisation hypothesis (angular momentum = nh/2π) is a fundamental law of nature, it should also apply to planetary motion. Why do we not discuss the quantisation of planetary orbits around the sun?

Ans – The angular momentum related to planetary motion is predominantly dependent upon Planck’s constant (h), so we do not discuss the quantisation of planetary orbits around the Sun. The angular momentum of the Earth in its orbit is approximately 1070 h. This results in an exceedingly high quantum level n, approximately 1070. For substantial values of n, consecutive energies and angular momenta are observed to be comparatively minimal. Therefore, the quantum levels for planetary motion are consistently regarded as continuous.

3) Assume you had the opportunity to perform the alpha-particle scattering experiment without the gold foil by utilising a thin sheet of solid hydrogen. (When the temperature is lower than 14K, hydrogen is solid.) What outcomes are you hoping for?

Ans – The scattering angle in the alpha-particle scattering experiment would be insufficient if a thin sheet of solid hydrogen were substituted for the gold foil. The reason for this is that the mass of incident α-particles (6.64 × 10⁻²⁷ kg) is more than that of hydrogen (1.67 × 10⁻²⁷ kg). As a result, the target nucleus (hydrogen) is less massive than the scattering particle. Therefore, if solid hydrogen is employed in the α-particle scattering experiment, the α-particles would not bounce back.

3 Mark Questions

1) Two energy levels in an atom are separated by a difference of 2.3 eV. What is the frequency of radiation emitted during the transition of an atom from the higher energy level to the lower energy level?

Ans – Given

E = 2.3 eV = 2.3 × 1.6 × 10⁻¹⁹ = 3.68 × 10⁻¹⁹ J

Let v represent the radiation frequency that is released as the atom moves from the upper to the lower level. The relationship for energy is as follows:

E = hv

h = 6.62 × 10⁻⁴ Js

2) Determine the quantum number that describes the earth’s rotation around the sun in an orbit with a radius of 1.5 x 10¹¹ m and an orbital speed of 3 x 10⁴ m/s using Bohr’s model (earth mass = 6.0 x 10²⁴ kg).

Ans – Given,

The Earth’s orbital radius around the Sun is r = 1.5×1011m.

Earth’s orbital speed, v = 3×104 m/s

Earth’s mass, m = 6.0×1024 kg

h = Planck’s constant = 6.62 × 10⁻³⁴ Js

n = Quantum number

Bohr’s model states that angular momentum is quantised and can be expressed as follows:

5 Marks Questions

1) A hydrogen atom in the ground state absorbs a photon, elevating it to the n=4 energy level. Find the photon’s frequency and wavelength.

Ans – At ground level, n₁ = 1

Assign E₁ as the energy corresponding to this level. E₁ is associated with n₁ as follows:

The atom is elevated to an excited state at n₂ = 4.

Assign E₂ as the energy corresponding to this level.

The energy absorbed by the photon is expressed as:

The energy expression for a photon with wavelength λ is defined as:

h = Planck’s constant = 6.6 × 10^-34 Js

c = Speed of light = 3 × 10⁸ m/s

The frequency of a photon is determined by the following connection.

Therefore, the photon possesses a wavelength of 97 nm and a frequency of 3.1 × 10¹⁵ Hz.