Atoms — Class 12 MCQs with Answers

Atoms — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Rutherford's α-particle experiment showed:

   AAtom has a small dense nucleusBAtom is solidCNo nucleusDAll electrons in centre
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   Answer: Atom has a small dense nucleus

   Explanation: Most α-particles passed straight through, but a few rebounded through large angles, meaning positive charge and mass are concentrated in a tiny, dense nucleus, not spread through the atom.

2. Q2 of 6Easy

   Bohr's model: angular momentum is:

   AZeroBContinuousCQuantised in units of ℏDInfinite
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   Answer: Quantised in units of ℏ

   Explanation: Bohr postulated angular momentum is quantised: L = nh/2π = nℏ, allowing only discrete orbits (n=1,2,3…) — not a continuous range as classical mechanics would allow.

3. Q3 of 6Medium

   Distance of closest approach for α-particle of KE K to nucleus of charge Ze:

   A2kZe²/KBkZe²/KC4kZe²/KDkZe/K
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   Answer: 2kZe²/K

   Explanation: At closest approach all kinetic energy converts to electrostatic potential energy: K = k(2e)(Ze)/r₀, so r₀ = 2kZe²/K — note the factor 2 from the α-particle's own +2e charge.

4. Q4 of 6Medium

   Assertion (A): Bohr's model explains the relative intensities of the lines in the hydrogen spectrum. Reason (R): Bohr's model correctly predicts the frequencies of the light emitted by hydrogen.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is false but R is true

   Explanation: A is false: in the hydrogen spectrum some lines are strong and others weak, and Bohr's model cannot account for these intensity differences. R is true; the frequencies follow from hν = Eᵢ − E_f with the Bohr energy levels.

5. Q5 of 6Hard

   Limitation of Rutherford model:

   ANo limitationBPredicts unstable atom (electrons should radiate and spiral in)CPredicts strong forceDWrong nucleus charge
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   Answer: Predicts unstable atom (electrons should radiate and spiral in)

   Explanation: Classical electromagnetism says an accelerating orbiting electron must radiate energy continuously and spiral into the nucleus — a fatal flaw Bohr's quantisation postulate fixed.

6. Q6 of 6Hard

   Assertion (A): As a hydrogen atom is raised to higher excited states, the minimum energy needed to free its electron decreases. Reason (R): The total energy of the electron, −13.6/n² eV, becomes less negative as n increases.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true: freeing the electron needs about 13.6 eV from n = 1 but only 3.40 eV from n = 2 and 1.51 eV from n = 3. R is true, and since the energy needed to free the electron is just the gap between Eₙ and 0 eV, a less negative Eₙ means a smaller ionisation energy; so R explains A.

Atoms notes: key concepts (Class 12 Physics)

How the structure of the atom was established: Rutherford's alpha-scattering experiment and the nuclear model it forced, the impact parameter and distance of closest approach, why classical physics could not keep the atom stable, Bohr's postulates with the resulting radii and energy levels, and the hydrogen spectral series.

Thomson's model

The atom pictured as positive charge with electrons embedded like plums in a pudding. It could not explain the large-angle scattering Rutherford observed.

Rutherford's alpha-scattering

Alpha particles fired at gold foil mostly passed straight through, but roughly one in eight thousand rebounded through large angles.

Nuclear model of the atom

The rare large-angle deflections meant the positive charge and nearly all the mass sit in a tiny central nucleus, with electrons orbiting far outside.

Size of the nucleus

About 10⁻¹⁵ m against an atomic size of 10⁻¹⁰ m — the nucleus is roughly a hundred thousand times smaller, so the atom is mostly empty space.

Impact parameter

The perpendicular distance between the alpha particle's initial path and the nucleus. A smaller impact parameter produces a larger scattering angle.

Distance of closest approach

The point at which an alpha particle fired head-on stops, its kinetic energy having converted entirely into electrostatic potential energy.

Failure of the classical model

An orbiting electron accelerates, so classically it must radiate continuously and spiral into the nucleus within nanoseconds. Atoms are demonstrably stable.

Bohr's postulates

Electrons occupy certain stationary orbits without radiating; angular momentum is quantised in units of h/2π; and radiation is emitted only on jumping between orbits.

Quantisation of angular momentum

Only orbits whose angular momentum is an integral multiple of h/2π are allowed, which is what selects the discrete radii and energies.

Bohr radius

The radius of hydrogen's ground-state orbit, 0.53 Å. Higher orbits grow as the square of the principal quantum number.

Energy levels of hydrogen

Given by −13.6/n² eV. The energies are NEGATIVE because the electron is bound, and they crowd together as n rises.

Ionisation energy

The energy needed to remove the electron completely from the ground state, 13.6 eV for hydrogen. It is the depth of the ground-state energy well.

Excitation and emission

Absorbing exactly the right energy lifts an electron to a higher level; falling back emits a photon whose energy equals the level difference.

Hydrogen spectral series

Lyman lines end at n = 1 and lie in the ultraviolet, Balmer lines end at n = 2 and are visible, and Paschen lines end at n = 3 in the infrared.

Limitations of Bohr's model

It works only for single-electron atoms, cannot explain spectral line intensities or fine structure, and mixes classical orbits with quantum rules arbitrarily.