An Overview of Ncert Books Class 12 Physics Chapter 12 Free Download
FAQs on Ncert Books Class 12 Physics Chapter 12 Free Download
1. What are the most important topics in the Atoms chapter for the CBSE Class 12 Physics board exam 2025-26?
For the CBSE Class 12 Physics exam, the most important topics in the Atoms chapter that students must focus on are: Rutherford's alpha-particle scattering experiment (observations and conclusions), Bohr's model of the hydrogen atom including its postulates, the derivation for the radius and energy of electron orbits, and the explanation of the hydrogen spectral series (Lyman, Balmer, Paschen, etc.). Questions based on the limitations of both Rutherford's and Bohr's models are also frequently asked.
2. State Bohr's three postulates for the model of a hydrogen atom. How are these typically tested in exams?
Bohr's three postulates are fundamental to understanding atomic structure and are a high-yield topic for exams. They are:
First Postulate (Stable Orbits): An electron in an atom can revolve in certain stable orbits without the emission of radiant energy. Each of these orbits has a definite energy associated with it.
Second Postulate (Quantization of Angular Momentum): The electron revolves around the nucleus only in those orbits for which the angular momentum is an integral multiple of h/2π, where h is Planck's constant. (mvr = nh/2π).
Third Postulate (Frequency Condition): An electron might make a transition from a higher energy orbit to a lower energy orbit. When it does so, a photon is emitted having energy equal to the energy difference between the initial and final states (E = E_i - E_f = hν).
In exams, this is often asked as a direct 2 or 3-mark question or is required as a basis for solving numerical problems on energy levels and spectral lines.
3. What were the key observations from Rutherford's alpha-particle scattering experiment, and what were the main conclusions drawn?
Rutherford's experiment is a cornerstone topic often asked for 3 marks. The key observations and conclusions are:
Observation 1: Most alpha particles passed straight through the gold foil undeflected. Conclusion: This implies that most of the space inside an atom is empty.
Observation 2: A small fraction of alpha particles were deflected by small angles. Conclusion: This suggests the presence of a positively charged mass within the atom that repels the positive alpha particles.
Observation 3: A very few alpha particles (about 1 in 8000) were deflected by large angles (more than 90°), and some even bounced back. Conclusion: The entire positive charge and most of the mass of the atom are concentrated in a very small volume, called the nucleus.
4. Why was Rutherford's atomic model ultimately considered unstable? How did Bohr's model address this key limitation?
Rutherford's model was considered unstable based on classical electromagnetic theory. According to this theory, an accelerating charged particle (like an electron orbiting a nucleus) must continuously radiate energy. This loss of energy would cause the electron to spiral inwards and eventually collapse into the nucleus, making the atom unstable. However, atoms are known to be stable. Bohr's model addressed this limitation through his first postulate, which stated that electrons can revolve in specific stationary or stable orbits without emitting radiation. Energy is only emitted or absorbed when an electron jumps between these allowed orbits, thus ensuring the stability of the atom.
5. What are some expected 5-mark questions from the Atoms chapter, often involving derivations?
For the 2025-26 board exam, a 5-mark question from this chapter is likely to be a derivation based on Bohr's model. The most important derivations to prepare are:
Derivation of the expression for the radius of the nth permissible orbit of an electron in a hydrogen atom.
Derivation of the expression for the total energy of an electron in the nth stationary orbit of a hydrogen atom.
These questions are often combined with a short conceptual part, such as explaining the negative sign of the total energy or calculating the energy for a specific transition.
6. How can we use Bohr's model to explain the different spectral series of the hydrogen atom? Which series falls in the visible region?
Bohr's third postulate is key to explaining the hydrogen spectrum. When an electron transitions from a higher energy level (n₂) to a lower energy level (n₁), it emits a photon of a specific wavelength, creating a spectral line. A series of such lines forms when the final state (n₁) is fixed:
Lyman Series: Transitions from n₂ = 2, 3, 4, ... to n₁ = 1. These lines are in the ultraviolet (UV) region.
Balmer Series: Transitions from n₂ = 3, 4, 5, ... to n₁ = 2. This is the only series that falls in the visible region of the electromagnetic spectrum.
Paschen Series: Transitions from n₂ = 4, 5, 6, ... to n₁ = 3. These lines are in the infrared (IR) region.
An important question can ask you to draw the energy level diagram and show these transitions.
7. What are the main limitations of Bohr's atomic model that students should know for their exams?
While successful for hydrogen, Bohr's model has several key limitations, which is a common 2 or 3-mark question. The main limitations are:
It is only applicable to hydrogen-like atoms (i.e., single-electron systems like He⁺, Li²⁺) and fails to explain the spectra of multi-electron atoms.
It could not explain the relative intensities of spectral lines.
It failed to explain the splitting of spectral lines in the presence of magnetic fields (Zeeman effect) and electric fields (Stark effect).
It violates the Heisenberg Uncertainty Principle, as it assumes definite paths (orbits) for electrons with known position and momentum.
8. Based on previous year board papers, which type of questions—derivations or conceptual questions—are more common from the Atoms chapter?
Analysis of previous year CBSE papers shows a balanced distribution of questions from the Atoms chapter. There is a high probability of getting one derivation-based question for 3 or 5 marks, typically related to Bohr's model (radius or energy). Alongside this, expect conceptual questions for 1, 2, or 3 marks testing your understanding of Rutherford's experiment, Bohr's postulates, spectral series, and the limitations of these models. It is crucial for students preparing for the 2025-26 exam to master both the derivations and the underlying concepts to score well.
