Atoms is a conceptual and scoring chapter in Class 12 Physics that introduces the structure of atoms and atomic models. This chapter covers key topics such as Thomson’s atomic model, Rutherford’s scattering experiment, Bohr’s model of the hydrogen atom, energy levels, spectral lines, and atomic spectra, which are frequently asked in CBSE board exams and competitive exams like JEE and NEET.
NCERT Solutions for Class 12 Physics Chapter 12 – Atoms are prepared strictly according to the latest CBSE syllabus and exam pattern. The solutions are explained in simple, step-by-step language with clear derivations, diagrams, and solved numericals, helping students understand atomic structure clearly and score well in board examinations.
NCERT Solutions for Class 12 Physics Chapter 12 – Atoms
Q.
Choose the correct alternative from the clues given at the end of the each statement:
(a) The size of the atom in Thomson’s model is …… the atomic size in Rutherford’s model.
(much greater than/no different from /much less than.)
(b) In the ground state of …….. electrons are in stable equilibrium, while in ……. electrons always experience a net force.
(Thomson’s model/Rutherford’s model.)
(c) A classical atom based on……. is doomed to collapse.
(Thomson’s model / Rutherford’s model.)
(d) An atom has a nearly continuous mass distribution in a……. but has a highly non-uniform mass distribution in ……………..
(Thomson’s model /Rutherford’s model.)
(e) The positively charged part of the atom possesses most of the mass in …………
(Rutherford’s model/both the models.)
Q.
In accordance with the Bohr’s model, find the quantum number that characterizes the earth’s revolution around the sun in an orbit of radius 1.5 × 1011 m with orbital speed 3 × 104 ms-1. (Mass of earth 6.0 × 1024 kg)
Q.
If Bohr’s quantization postulate
is a basic law of nature, it should be equally valid for the case of planetary motion also. Why then do we never speak of quantization of orbits of planets around the sun ?
Q.
The total energy of an electron in the first excited state of the hydrogen atom is about – 3.4 eV.
(a) What is the kinetic energy of the electron in this state?
(b) What is the potential energy of the electron in this state?
(c) Which of the answers above would change if the choice of the zero of potential energy is changed?
Q.
Classically, an electron can be in any orbit around the nucleus of an atom. Then what determines the typical atomic size? Why is an atom not, say, thousand times bigger than its typical size? The question had greatly puzzled Bohr before he arrived at his famous model of the atom that you have learnt in the text. To simulate what he might well have done before his discovery, let us play as follows with the basic constants of nature and see if we can get a quantity with the dimensions of length that is roughly equal to the known size of an atom (~ 10
-10 m).
(a) Construct a quantity with the dimensions of length from the fundamental constants e, m
e and c. Determine its numerical value.
(b) You will find that the length obtained in (a) is many orders of magnitude smaller than the atomic dimensions. Further, it involves c. But energies of atoms are mostly in non-relativistic domain where c is not expected to play any role. This is what may have suggested Bohr to discard c and look for ‘something else’ to get the right atomic size. Now, the Planck’s constant h had already made its appearance elsewhere. Bohr’s great insight lay in recognising that h, m
e and e will yield the right atomic size. Construct a quantity with the dimension of length from h, m
e and e and confirm that its numerical value has indeed the
correct order of magnitude.
Q.
Obtain an expression for the frequency of radiation emitted when a hydrogen atom de-excites from level n to level (n-1) . For large n, show that this frequency equals the classical frequency of revolution of the electron in the orbit.
Q.
The gravitational attraction between electron and proton in a hydrogen atom is weaker than the coulomb attraction by a factor of about 10-40. An alternative way of looking at this fact is to estimate the radius of the first Bohr orbit of a hydrogen atom if the electron and proton were bound by gravitational attraction. You will find the answer interesting.
Q.
Answer the following questions, which help you understand the difference between Thomson’s model and Rutherford’s model better.
(a) Is the average angle of deflection of α-particles by a thin gold foil predicted by Thomson’s model much less, about the same, or much greater than that predicted by Rutherford’s model?
(b) Is the probability of backward scattering (i.e. scattering of α -particles at angles greater than 90°) predicted by Thomson’s model much less, about the same or much greater than that predicted by Rutherford’s model?
(c) Keeping other factors fixed, it is found experimentally that for small thickness t, the number of α-particles scattered at moderate angles is proportional to t. What clue does this linear dependence on t provide?
(d) In which model is it completely wrong to ignore multiple scattering for the calculation of average angle of scattering of α -particle by a thin foil?
Q.
An electron beam with energy 12.5 eV is used to bombard on hydrogen atom at room temperature. What series of wavelengths will be emitted?
Q.
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. (Hydrogen is a solid at temperatures below 14 K) What results do you expect?
Q.
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?
Q.
(a) Using the Bohr’s model, calculate the speed of the electron in a hydrogen atom in the n = 1, 2 and 3 levels.
(b) Calculate the orbital period in each of these levels.
Q.
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.
Q.
The ground state energy of hydrogen atom is -13.6 eV. What are the kinetic and potential energies of the electron in this state?
Q.
A difference of 2.3 eV separates two energy levels in an atom. What is the frequency of radiation emitted when the atom transits from the upper level to the lower level?
Q.
What is the shortest wavelength present in the Paschen series of spectral lines?
Q.
Obtain the first Bohr’s radius and the ground state energy of a muonic hydrogen atom (i.e, an atom in which a negatively charged muon (μ-) of mass about 207 me orbits around a proton).
Q. 1) 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.
(much greater than / no different from / much less than)
(b) In the ground state of …….. electrons are in stable equilibrium, while in ……. electrons always experience a net force.
(Thomson’s model / Rutherford’s model)
(c) A classical atom based on……. is doomed to collapse.
(Thomson’s model / Rutherford’s model)
(d) An atom has a nearly continuous mass distribution in a……. but has a highly non-uniform mass distribution in …………..
(Thomson’s model / Rutherford’s model)
(e) The positively charged part of the atom possesses most of the mass in …………
(Rutherford’s model / both the models)
Ans:
(a) no different from
(b) Thomson’s model, Rutherford’s model
(c) Rutherford’s model
(d) Thomson’s model, Rutherford’s model
(e) both the models
Q. 2) 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?
Ans: The scattering angle would be very small because the mass of hydrogen nucleus is very small as compared to the alpha particle. Hence the repulsive force is weak and backward scattering is negligible.
Q. 3) Answer the following questions:
(a) Average angle of deflection predicted by Thomson’s model is about the same as Rutherford’s model.
(b) Probability of backward scattering is much less in Thomson’s model.
(c) Linear dependence on thickness suggests scattering is due to single collision.
(d) Multiple scattering cannot be ignored in Thomson’s model.
Note: Q&A containing MathML or Latex or Katex code cannot be rendered in pdf document.
FAQs: Class 12 Physics Chapter 12 – Atoms
Q1. Is Atoms an important chapter for exams?
Yes, it is a high-weightage chapter for CBSE, JEE, and NEET.
Q2. Which topics are most important in this chapter?
Bohr’s atomic model, energy levels, and hydrogen spectrum.
Q3. Are numericals asked from this chapter?
Yes, Bohr model and energy-level numericals are common.
Q4. Are derivations important here?
Yes, derivations related to radius, velocity, and energy of electron in Bohr’s model are frequently asked.
Q5. How do NCERT Solutions help?
They provide NCERT-aligned, exam-ready explanations with solved numericals and diagrams.