ROUTERA

Atoms

Class 12th Physics Chapter Assertion and Reason

Assertion and Reason Questions Chapter-12 Atoms

Assertion (A) and other labelled Reason (R). Select the correct answer to these questions from the options as given below.

A. Both A and R are true, and R is the correct explanation of A.

B. Both A and R are true, but R is not the correct explanation of A.

C. A is true, but R is false.

D. A is false, but R is true.

1. Assertion (A): An atom consists of a nucleus surrounded by electrons.

Reason (R): The nucleus contains protons and neutrons that are held together by strong nuclear forces.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The structure of an atom indeed consists of a nucleus with protons and neutrons, and the electrons are arranged around it, which is explained by the strong nuclear forces acting within the nucleus.

2. Assertion (A): The number of protons in an atom determines its atomic number.

Reason (R): Atomic number is defined as the number of neutrons present in the nucleus.

  • (b) A is true, but R is false.
  • Answer: (b)
    Explanation: Atomic number is defined as the number of protons in an atom, making R false since it incorrectly defines atomic number.

3. Assertion (A): Isotopes of an element have the same number of protons but different numbers of neutrons.

Reason (R): Isotopes exhibit different chemical properties due to the difference in neutron number.

  • (b) A is true, but R is false.
  • Answer: (b)
    Explanation: Isotopes do have the same number of protons, but their chemical properties are primarily determined by electron configuration, making R false.

4. Assertion (A): Electrons can occupy the same energy level in an atom.

Reason (R): The Pauli exclusion principle states that no two electrons can have the same set of quantum numbers.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Electrons can share the same energy level but must differ in their quantum states as per the Pauli exclusion principle.

5. Assertion (A): An electron in a hydrogen atom can exist in various energy levels.

Reason (R): The energy levels in an atom are quantized, allowing only specific energies.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The quantized nature of energy levels in an atom permits electrons to occupy distinct energy states.

6. Assertion (A): The emission spectrum of hydrogen shows discrete lines.

Reason (R): Electrons transition between fixed energy levels when they emit energy.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The discrete lines in the emission spectrum result from electron transitions between quantized energy levels in hydrogen.

7. Assertion (A): The ground state of an atom is its lowest energy state.

Reason (R): Atoms in the ground state have electrons in the highest energy levels.

  • (b) A is true, but R is false.
  • Answer: (b)
    Explanation: The ground state is indeed the lowest energy state, but electrons are in the lowest energy levels, making R false.

8. Assertion (A): The ionization energy of an atom is the energy required to remove an electron.

Reason (R): Ionization energy increases as the atomic number increases.

  • (b) A is true, but R is false.
  • Answer: (b)
    Explanation: While the assertion about ionization energy is true, the trend in ionization energy varies; it generally increases across a period but can decrease in some cases down a group, making R false.

9. Assertion (A): The Bohr model of the atom explains the quantized energy levels of electrons.

Reason (R): According to the Bohr model, electrons travel in circular orbits around the nucleus.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The Bohr model effectively explains quantized energy levels, with electrons in fixed circular orbits around the nucleus.

10. Assertion (A): The mass number of an element is the sum of protons and neutrons in the nucleus.

Reason (R): Electrons contribute significantly to the mass of the atom.

  • (b) A is true, but R is false.
  • Answer: (b)
    Explanation: The mass number is indeed the sum of protons and neutrons, but electrons contribute negligibly to atomic mass, making R false.

11. Assertion (A): Atoms can lose or gain electrons to form ions.

Reason (R): Ions are formed to achieve a stable electron configuration, similar to noble gases.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Atoms form ions by losing or gaining electrons to attain stable configurations, often resembling noble gases.

12. Assertion (A): The radius of an atom increases with the addition of electrons in the same energy level.

Reason (R): Increased electron-electron repulsion in the same energy level leads to expansion.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Additional electrons in the same energy level cause repulsion, leading to an increase in atomic radius.

13. Assertion (A): The Rutherford model proposed that most of the atom's volume is empty space.

Reason (R): Most of the atom's mass is concentrated in the nucleus.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The Rutherford model indicates that a large part of the atom's volume is empty, with the mass concentrated in a tiny nucleus.

14. Assertion (A): The energy required to remove an electron from an atom is called electron affinity.

Reason (R): Electron affinity is typically positive for all elements.

  • (b) A is false, but R is true.
  • Answer: (b)
    Explanation: The correct term is ionization energy, not electron affinity. Electron affinity can be either positive or negative, making A false.

15. Assertion (A): Electrons in the same subshell have the same energy.

Reason (R): Electrons are indistinguishable in an atom.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Electrons within the same subshell share energy levels, and their indistinguishable nature reinforces this idea.

16. Assertion (A): An atom with an equal number of protons and electrons is electrically neutral.

Reason (R): Protons are positively charged, while electrons are negatively charged.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The equal number of positive protons and negative electrons results in a neutral charge.

17. Assertion (A): The stability of an atom is related to its electron configuration.

Reason (R): Atoms with filled electron shells are more stable than those with partially filled shells.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Atoms with filled shells exhibit greater stability due to lower energy states.

18. Assertion (A): The principal quantum number indicates the size of an electron cloud.

Reason (R): A higher principal quantum number corresponds to a larger electron cloud.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: As the principal quantum number increases, the average distance of the electron cloud from the nucleus increases, indicating larger size.

19. Assertion (A): The photoelectric effect is evidence of the particle nature of light.

Reason (R): Photons must have sufficient energy to eject electrons from a material.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The necessity for photons to have specific energy to cause photoelectric emission supports the particle theory of light.

20. Assertion (A): The atomic mass of an element is not a whole number.

Reason (R): Atomic mass is a weighted average of the masses of all isotopes.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Atomic mass is averaged over all isotopes, often resulting in a non-integer value.

21. Assertion (A): Light can behave as both a wave and a particle.

Reason (R): The dual nature of light is fundamental to quantum mechanics.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Quantum mechanics describes light as having both wave-like and particle-like properties, supporting A.

22. Assertion (A): The uncertainty principle applies to all particles.

Reason (R): The principle states that it is impossible to know both position and momentum of a particle simultaneously.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The uncertainty principle is a fundamental aspect of quantum mechanics affecting all particles.

23. Assertion (A): Quantum numbers are used to describe the state of electrons in an atom.

Reason (R): Each electron in an atom can be described by a unique set of four quantum numbers.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Quantum numbers provide a complete description of an electron's state, confirming R as the explanation for A.

24. Assertion (A): Atomic orbitals are regions of space where electrons are likely to be found.

Reason (R): Orbitals are defined by mathematical functions that describe electron behavior.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Orbitals represent probabilities of finding electrons, as defined by their wave functions.

25. Assertion (A): The emission of photons from an atom occurs when an electron drops to a lower energy level.

Reason (R): Photons carry energy away from the atom during this transition.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: When electrons transition to lower energy levels, they emit photons that correspond to the energy difference.

26. Assertion (A): The principal quantum number can have values of 0 and above.

Reason (R): The principal quantum number indicates the shell in which an electron resides.

  • (b) A is false, but R is true.
  • Answer: (b)
    Explanation: The principal quantum number starts from 1, making A false, while R is true.

27. Assertion (A): The half-life of a radioactive isotope is constant.

Reason (R): The half-life of an isotope depends on its chemical environment.

  • (b) A is true, but R is false.
  • Answer: (b)
    Explanation: The half-life is indeed a constant property of a radioactive isotope and does not vary with chemical environment, making R false.

28. Assertion (A): A neutron is more massive than a proton.

Reason (R): Neutrons contribute to the atomic mass more than protons.

  • (b) A is false, but R is true.
  • Answer: (b)
    Explanation: While the neutron is indeed slightly more massive than the proton, the assertion is misleading, as neutrons and protons are very close in mass.

29. Assertion (A): The kinetic energy of an electron in an atom is quantized.

Reason (R): Electrons can only occupy certain allowed energy levels in an atom.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The quantization of energy levels results in discrete kinetic energy values for electrons.

30. Assertion (A): Electromagnetic radiation consists of oscillating electric and magnetic fields.

Reason (R): Electromagnetic waves do not require a medium to propagate.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Electromagnetic radiation features oscillating fields that can propagate through a vacuum, supporting both A and R.

31. Assertion (A): Heisenberg's uncertainty principle is a fundamental concept in quantum mechanics.

Reason (R): The principle has practical implications for measuring atomic properties.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The uncertainty principle is indeed central to quantum mechanics and impacts measurements at the atomic level.

32. Assertion (A): The mass of an electron is negligible compared to that of a proton or neutron.

Reason (R): This is why electrons are often ignored in atomic mass calculations.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The small mass of electrons justifies their exclusion in atomic mass computations.

33. Assertion (A): The atomic structure of elements determines their reactivity.

Reason (R): Elements with similar electron configurations exhibit similar chemical properties.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The atomic structure indeed influences reactivity, as similar configurations lead to comparable behaviors in chemical reactions.

34. Assertion (A): The mass defect in a nucleus results from the binding energy.

Reason (R): Binding energy is the energy required to separate nucleons in a nucleus.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The mass defect correlates to the binding energy necessary to overcome the forces holding the nucleons together.

35. Assertion (A): Alpha particles are more massive than beta particles.

Reason (R): Alpha particles consist of two protons and two neutrons.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The definition of alpha particles as helium nuclei accounts for their greater mass compared to beta particles.

36. Assertion (A): The quantum mechanical model of the atom replaces the Bohr model.

Reason (R): The quantum mechanical model incorporates wave-particle duality.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The quantum mechanical model's incorporation of wave properties indeed supersedes the Bohr model.

37. Assertion (A): Chemical bonds form due to interactions between electrons.

Reason (R): Electrons determine the overall charge and reactivity of an atom.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The behavior of electrons facilitates the formation of chemical bonds and governs atom reactivity.

38. Assertion (A): The principal quantum number affects the energy of an electron.

Reason (R): Higher principal quantum numbers correspond to higher energy levels.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Increased principal quantum numbers correlate directly with higher energy states for electrons.

39. Assertion (A): Photons carry momentum as well as energy.

Reason (R): The momentum of a photon is related to its wavelength.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: Photons indeed carry momentum, with their momentum being inversely related to wavelength.

40. Assertion (A): The energy levels of electrons in an atom are quantized.

Reason (R): Electrons can only occupy specific energy levels without existing in between.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The quantization of electron energy levels allows for discrete energy states without intermediates.

41. Assertion (A): Nuclear fission releases a significant amount of energy.

Reason (R): The binding energy per nucleon is higher in the products than in the reactants.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The release of energy during fission is due to the increased binding energy per nucleon in the resulting nucleus.

42. Assertion (A): The electron configuration of an atom determines its position in the periodic table.

Reason (R): Elements with similar configurations belong to the same group.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The periodic table's organization reflects electron configurations, placing similar elements in the same groups.

43. Assertion (A): Ionization energy generally increases across a period in the periodic table.

Reason (R): Increased nuclear charge leads to stronger attraction between electrons and the nucleus.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: As nuclear charge increases across a period, the effective nuclear attraction grows, necessitating more energy for ionization.

44. Assertion (A): Quantum entanglement is a phenomenon observed in particles.

Reason (R): Entangled particles exhibit correlations that cannot be explained classically.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The unique behavior of entangled particles supports the assertion by illustrating non-classical correlations.

45. Assertion (A): The strong nuclear force binds protons and neutrons in the nucleus.

Reason (R): The strong force is stronger than the electromagnetic force at very short distances.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The strong nuclear force effectively overcomes electromagnetic repulsion at close ranges, thus binding nucleons together.

46. Assertion (A): The de Broglie wavelength of a particle is inversely proportional to its momentum.

Reason (R): A higher momentum results in a shorter wavelength.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The de Broglie wavelength's inverse relationship with momentum supports the assertion.

47. Assertion (A): Isotopes of an element have the same number of protons.

Reason (R): Isotopes differ only in their neutron count.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The definition of isotopes confirms that they share the same proton number, differing only in neutrons.

48. Assertion (A): X-rays are a form of electromagnetic radiation.

Reason (R): X-rays have higher energy and shorter wavelengths than visible light.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The classification of X-rays as electromagnetic radiation is supported by their energy and wavelength characteristics.

49. Assertion (A): Atomic mass is not always a whole number.

Reason (R): Atomic mass accounts for the average mass of an element's isotopes.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The averaging of isotopic masses results in non-integer atomic masses.

50. Assertion (A): The photoelectric effect supports the particle theory of light.

Reason (R): The effect demonstrates that light can eject electrons from a metal surface.

  • (a) Both A and R are true, and R is the correct explanation of A.
  • Answer: (a)
    Explanation: The photoelectric effect's dependence on light frequency supports the idea of light behaving as particles (photons).