NIOS Class 12 Chemistry Important Questions: Chapter 2 – Atomic Structure

Q: Compare the masses of a proton and an electron.
Answer: m_p = 1.672 × 10⁻²⁷ kg; m_e = 9.11 × 10⁻³¹ kg.
Therefore, a proton is approximately 1040 times heavier than an electron.

Q:What are the fundamental particles of an atom?
Answer: The fundamental particles of an atom are electrons, protons, and neutrons. A proton carries a unit positive charge, an electron carries a unit negative charge, whereas a neutron carries no charge.

Q: What was the purpose of Rutherford’s alpha (α) particle scattering experiment?
Answer: The objective of Rutherford’s experiment was to investigate Thomson’s “Plum-Pudding” model.

Q: Explain Rutherford’s model.
Answer: According to Rutherford’s model:
① The nucleus is the positively charged part of the atom that contains almost all of its mass.
② The remaining part of the atom is empty space. Electrons reside in this region.

Q: On what grounds was Rutherford’s model rejected?
Answer: Rutherford’s model failed to explain the stability of the atom.

Q:Explain Bohr’s model, and describe its limitations.
Answer:Bohr’s model explains the stability of the atom as well as the line spectrum of hydrogen.
Limitations of Bohr’s Model:
① Bohr failed to explain the line spectra of other elements.
② It could not explain Heisenberg’s Uncertainty Principle.

Q: What is the difference between a quantum and a photon?
Answer: A single quantum of visible light is called a photon. The energy of a quantum is directly proportional to the frequency of the radiation.

Q: State the main postulates of Bohr’s model.
Answer:The main postulates of Bohr’s model are as follows: ① Electrons revolve around the nucleus, situated at the center of the atom, in specific circular paths. These are called stationary orbits.
When an electron moves within a specific orbit, its energy remains unchanged.
(iii) An electron absorbs or emits a photon of energy; thus, E = E2 – E1.
(iv) The angular momentum of an electron with mass *me* is equal to the product of its mass, its velocity *v*, and the radius of its orbit, *r*.
Therefore, for an electron in a Bohr atom:
*mevr* = *nh*
where *n* = 1, 2, 3, 4, 5…
The energy of an electron…
Em = – *me* *Z²* *e⁴* / (8 *h²* *ε₀²*)
where *me* = mass of the electron, *Z* = nuclear charge, and *e* = magnitude of the electron’s charge