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Chapter Analysis
Intermediate16 pages • EnglishQuick Summary
Chapter 12 of NCERT Physics Part 2 for Class 12 titled 'Atoms' focuses on the historical development and theoretical exploration of atomic models, particularly highlighting Bohr’s model. The chapter explores the electromagnetic spectrum and the quantization of angular momentum leading to the stability of atoms, where electrons revolve in discrete orbits without radiating energy. It explains the spectral lines of hydrogen and introduces concepts like the ground and excited states of atoms, de Broglie’s hypothesis, and the transition to modern quantum mechanics.
Key Topics
- •Bohr's model of the hydrogen atom
- •Quantization of angular momentum
- •Spectral line emission
- •Energy levels and transitions
- •Wave-particle duality
- •Limitations of Bohr’s model
- •Electric charge distribution in atoms
Learning Objectives
- ✓Understand the historical development and basic concepts of the Bohr model of the atom.
- ✓Describe how quantized electron orbits lead to discrete spectral lines.
- ✓Calculate energy changes during electron transitions in a hydrogen atom.
- ✓Appreciate the impact of de Broglie's hypothesis on atomic models.
- ✓Explain the limitations of classical physics concepts applied to atomic structure.
- ✓Analyze the importance of quantum mechanics in describing atomic structures beyond hydrogen.
Questions in Chapter
Choose the correct alternative from the clues given at the end of each statement: (a) The size of the atom in Thomson’s model is .......... the atomic size in Rutherford’s model. (b) In the ground state of .......... electrons are in stable equilibrium, while in .......... electrons always experience a net force. (c) A classical atom based on .......... is doomed to collapse. (d) An atom has a nearly continuous mass distribution in a .......... but has a highly non-uniform mass distribution in .......... (e) The positively charged part of the atom possesses most of the mass in ..........
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Suppose you are given a chance to repeat the alpha-particle scattering experiment using a thin sheet of solid hydrogen in place of the gold foil. What results do you expect?
Page 305
A difference of 2.3 eV separates two energy levels in an atom. What is the frequency of radiation emitted when the atom makes a transition from the upper level to the lower level?
Page 306
The ground state energy of hydrogen atom is –13.6 eV. What are the kinetic and potential energies of the electron in this state?
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A hydrogen atom initially in the ground level absorbs a photon, which excites it to the n = 4 level. Determine the wavelength and frequency of photon.
Page 306
Using the Bohr’s model calculate the speed of the electron in a hydrogen atom in the n = 1, 2, and 3 levels. Calculate the orbital period in each of these levels.
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The radius of the innermost electron orbit of a hydrogen atom is 5.3×10–11 m. What are the radii of the n = 2 and n = 3 orbits?
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A 12.5 eV electron beam is used to bombard gaseous hydrogen at room temperature. What series of wavelengths will be emitted?
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Additional Practice Questions
Explain why Bohr's model was not applicable to many-electron atoms.
mediumAnswer: Bohr's model could only accurately predict the behavior of hydrogen-like (single-electron) atoms because it did not take into account electron-electron interactions that occur in atoms with multiple electrons. Such interactions cause shifts in energy levels and spectra that the Bohr model could not predict.
Describe why atomic spectral lines are not continuous but appear as discrete lines.
easyAnswer: Atomic spectral lines are discrete because they arise from electrons transitioning between fixed energy levels in an atom. When an electron falls from a higher energy level to a lower one, it emits a photon with a frequency corresponding to the energy difference between those levels, producing spectral lines specific to each transition.
Discuss how the concept of wave-particle duality supports the idea of quantized electron orbits.
hardAnswer: Wave-particle duality, as proposed by de Broglie, suggests that particles such as electrons exhibit wave-like behavior. In an atom, standing wave patterns occur, leading to quantized orbits since only certain wavelengths fit into given orbital circumferences. This concept explains why electrons reside only in specific energy levels.
Calculate the energy required to ionize a hydrogen atom from its second excited state.
mediumAnswer: The energy levels for hydrogen are given by E_n = -13.6 eV/n^2. For n=3 (second excited state), E = -1.51 eV. The ionization energy from this level would be the energy required to bring the electron to 0 eV, i.e., 1.51 eV.
What is the significance of Bohr’s angular momentum quantization condition (L = nh/2π)?
mediumAnswer: Bohr’s quantization condition implies that the angular momentum of an electron in an atom is quantized and only specific set values are allowed. This condition is significant as it explains why electrons orbit the nucleus at certain discrete distances rather than in arbitrary paths, thereby introducing the concept of quantum states.