NCERT Solutions Class 12 Physics Chapter 12
VNCERT Solutions Class 12 Physics Chapter 12- Atoms
Extramarks provides comprehensive NCERT Solutions Class 12 Physics Chapter 12 based on the latest CBSE syllabus. The notes are prepared lucidly with stepwise explanations to help students boost confidence and gain complete knowledge about all concepts. They contain answers using coloured illustrations for all the questions present in the NCERT textbook. The experts at Extramarks have analyzed many CBSE previous year question papers and sample papers prior to preparing the notes to ensure that all the important topics are covered in the Solutions. Students can take full benefit of NCERT Solutions Class 12 Physics Chapter 12 provided by Extramarks to excel in their Class 12 exams.
To gain a complete understanding of Chapter 12, students must recall the concepts introduced in the previous standard using the NCERT Solutions Class 11. In this chapter, advanced concepts of atoms are included. One will learn about Rutherford’s model, the Paschen series of spectral lines, the Bohr Model Of The Hydrogen Atom, etc. There are many questions asked from this chapter in the CBSE exams and entrance examinations like NEET and JEE.
If you are looking for perfect study material for Relations and Function, you may refer to Extramarks NCERT Solutions Class 12 Mathematics Chapter 12. As a student, you can also view chapter-wise NCERT Solutions from Class 1 to Class 12. Using these reference notes will help students to revise and attain perfect scores.
Key Topics Covered In NCERT Solutions Class 12 Physics Chapter 12
The Extramarks NCERT Solutions Class 12 Physics Chapter 12 provides apt information and knowledge about important concepts of Atomic Physics. With this chapter, students will get a hold of the fundamental concepts and properties of atoms. These concepts would go a long way in many science and engineering-oriented curriculum.
Based on the latest syllabus, the main topics covered in NCERT Solution for Class 12 Physics Chapter 12 are given below:
Alpha-Particle Scattering And Rutherford’s Nuclear Model Of Atom
Bohr Model Of The Hydrogen Atom
The Line Spectra Of The Hydrogen Atom
De Broglie’s Explanation Of Bohr’s Second Postulate Of Quantisation
A brief of the main topics covered in NCERT Solution for Class 12 Physics Chapter 12 is below.
Chapter 12 Physics class 12 is based on Thomson’s, Rutherford’s, and Bohr’s atomic models and application of Mosley’s law. The chapter includes concepts of Alpha-Particle Trajectory, Electron Orbits, and the genesis of spectra. The NCERT Solutions Class 12 Physics Chapter 12 also describes fundamentals and principles associated with the structure of atoms, H2 atoms, and X-rays. Students also gain an insight into the advanced concepts of atoms like emission and absorption of energy, Coolidge Tubes, and applications in machines in our everyday lives.
The main features of the chapter are
- The Rutherford Atomic Model
- Equations by Bohr
- Line spectrum
- Energy levels of different atoms
- Production of X-rays
- De Broglie’s Hypothesis
12.2 Alpha-Particle Scattering And Rutherford’s Nuclear Model Of Atom
In this section of Chapter 12, students are introduced to various fundamentals of atoms like electron orbit, Kinetic, potential, total energy, etc. The structure of the atom using the Dalton’s atomic theory with its pros, cons, and limitations is explained in the NCERT Solutions Class 12 Physics Chapter 12. Furthermore, the Thomson model of the atom, also known as the watermelon model, is described using diagrams and coloured illustrations. Students can also find a detailed description of Rutherford’s nuclear model of the atom and its drawbacks.
There are various formulae included in this section. Students are advised to solve sums based on these formulae included in the NCERT Solutions Class 12 Physics Chapter 12.
12.3 Atomic Spectra
In the NCERT Solutions Class 12 Physics Chapter 12, information regarding the atomic spectra is given in detail using diagrams and graphs. Within this section, students learn about the three types of atomic spectra, namely emission spectra, absorption spectra, and continuous spectra. Students will also gain knowledge about Hydrogen spectra under the topic of spectral series. Explanation of different types of spectral series like Balmar series, Lyman series, Paschen Series, and Pfund series with their formulae is mentioned in the NCERT Solutions.
In the CBSE exams, various numerical questions are asked based on the formulas from this section. Extramarks NCERT Solutions Class 12 Physics Chapter 12 covers all the essential formulae for reference.
12.4 Bohr Model Of The Hydrogen Atom
This section of NCERT Solutions Class 12 Physics Chapter 12 summarises the drawbacks of Rutherford’s atomic model to the Hydrogen model of Bohr. Bohr’s three postulates explain stationary states, stable orbits, and quantization of moving electrons. Topics like angular momentum, Bohr’s radius, and velocity of electrons are explained in detail in this section. With help from NCERT Solutions Class 12 Physics Chapter 12, students can also gain an in-depth understanding of discrete energy levels in Bohr’s model. The drawbacks of the Bohr model are also included.
12.5 The Line Spectra Of The Hydrogen Atom
Within this section, detailed information about fundamentals like Excitation Energy, Ionisation Energy, Excitation Potential, and Ionisation Potential is included.
12.6 De Broglie’s Explanation Of Bohr’s Second Postulate Of Quantisation
The NCERT Solutions Class 12 Physics Chapter 12 will be very helpful in order to study De Broglie’s equation which gives the wave-particle nature of matter. Using the De Broglie relation, students will be able to solve numerical questions and find out the energy and wavelength of a photon.
The NCERT Solutions Class 12 Physics Chapter 12 includes all answers, formulas, derivations, and solved examples in one place. Students can refer to these notes for a quick revision before the exam. Students may click on the respective topics to access the NCERT Solutions Class 12 Physics Chapter 12.
List of NCERT Solutions Class 12 Physics Chapter 12 Exercise & Answer Solutions
To attain good scores and excel in exams, students should study with the help of the NCERT Solutions Class 12 Physics Chapter 12. Extramarks provides these NCERT Solutions, which are beneficial for understanding atomic Physics. These solutions offer all detailed explanations, numerical questions, and answers. It offers in-depth knowledge of all concepts included in the atoms class 12 NCERT solutions. Students may click here to access the NCERT Solutions Class 12 Physics Chapter 12.
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NCERT Exemplar Class 12 Physics
The NCERT Exemplar Class 12 Physics makes learning easy and simplified. The Extramarks online platform provides 100% reliability to enable a proper understanding of all complex concepts. With NCERT Solutions Class 12 Physics Chapter 12, students can prepare for CBSE school tests as well as other competitive examinations. The notes prepared by subject elites aim to strengthen the foundation with its easy-to-read and effective solutions.
Chapter 12 comes under unit 8- Atomsand Nuclei. Unit 7 and Unit 8 together carry 12 marks in the CBSE board exam. This chapter has good weightage in the boards as well as in JEE and NEET. Students may study using NCERT textbooks, CBSE reference books, NCERT Solutions Class 12 Physics Chapter 12, and other study materials provided by Extramarks.
Key Features of NCERT Solutions Class 12 Physics Chapter 12
The key features of Atoms class 12 NCERT solutions offered by Extramarks are as follows
- The notes are prepared by subject matter experts by carrying out extensive research. Previous year question papers and sample papers are used for reference.
- Chapter 12 NCERT solutions class 12 Physics includes key points, formulas, equations that allows quick reference.
- The solutions are made lucidly in an easy language to enhance the students’ exam preparation level.
- In case of any doubts or difficulties, students are advised to use NCERT Solutions Class 12 Physics Chapter 12 to understand the topic and clear their doubts.
- Students can access these notes on any device like mobiles, tabs, laptops, etc.
- It covers all topics with plenty of examples and informative diagrams and graphs.
Q.1 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.)
(a) The size of the atom in Thomson’s model is no different from the atomic size in Rutherford’s model.
(b) In the ground state of Thomson’s model electrons are in stable equilibrium, while in Rutherford’s model electrons always experience a net force.
(c) A classical atom based on Rutherford’s model is doomed to collapse.
(d) An atom has a nearly continuous mass distribution in a Thomson’s model but has a highly non-uniform mass distribution in Rutherford’s model.
(e) The positively charged part of the atom possesses most of the mass in 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. (Hydrogen is a solid at temperatures below 14 K) What results do you expect?
In the alpha- particle scattering experiment if we use a thin sheet of solid hydrogen in place of gold foil, then the scattering angle would be very small. The reason behind it is that the mass of the target nucleus (Hydrogen of mass 1.67 × 10-27 kg) is very small as compared to the mass of scattering particles (Alpha–particle of mass 6.64 × 10-27 kg) and due to this repulsive force between them is very small and could not deflect the alpha particle backwards.
Q.3 What is the shortest wavelength present in the Paschen series of spectral lines?
Q.4 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.5 The ground state energy of hydrogen atom is -13.6 eV. What are the kinetic and potential energies of the electron in this state?
As the potential energy of an electron of charge, e revolving in an orbit of radius, r around the nucleus of an atom is given by
Q.6 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.7 (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.8 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.9 A 12.5 eV electron beam is used to bombard gaseous hydrogen at room temperature. What series of wavelength will be emitted?
Q.10 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.11 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?
(a) The average angle of deflection of α-particles by a thin gold foil predicted by Thomson’s model is about the same as predicted by Rutherford’s model because we are considering average deflection angle.
(b) The probability of backward scattering (i.e. scattering of α-particles at angles greater than 90°) predicted by Thomson’s model is much less than the predicted value because the massive core is absent in Thomson’s model.
(c) It suggest that the scattering of α-particle is primarily because of the single collision. The chance of single collision increases with the number of target atoms hence, collision increases linearly with the thickness of the foil.
(d) In Thomson’s model of atom, as single collision causes very little deflection thus the observed average scattering can be explained only by considering multiple scattering. Multiple collisions can‘t be ignored in this model because positive charge is spread throughout in this model.
Q.12 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.13 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.14 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, me 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, me and e will yield the right atomic size. Construct a quantity with the dimension of length from h, me and e and confirm that its numerical value has indeed the
correct order of magnitude.
Q.15 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?
(a) Using E = – K.E, the kinetic energy of the electron in this state is +3.4 eV.
(b) As P.E = – 2K.E, thus P.E of the electron is – 6.8 eV.
(c) Kinetic energy does not depend upon the choice of zero of potential energy. Therefore, its value remains unchanged; however, the potential energy gets changed with the change in the zero level of potential energy.
Q.16 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.17 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).
FAQs (Frequently Asked Questions)
1. Which reference books should I use to study CBSE Class 12 Physics?
Using the reference books to prepare for examinations will help to get an in-depth understanding of the questions. The best reference books for Physics class 12 include
- Concepts of Physics: Volume 1 and 2 by H C Verma
- CBSE Chapter-wise Solutions – Physics Class 12 by S. K. Singh
- CBSE Chapter-wise Physics Solutions by Singh S
- New Simplified Physics: Volume 1 and 2 by Dhanpat Rai
2. What concepts are included in the NCERT Solutions Class 12 Physics Chapter 12?
The NCERT Solutions include concepts like Alpha-Particle Scattering, Alpha-Particle Trajectory, and Atomic Spectra. A detailed explanation of the atomic models given by scientists like Thomson, Rutherford, and Bohr is mentioned in the chapter. Furthermore, students will get a thorough understanding of energy levels, line spectra of the hydrogen atom, and De Broglie’s Explanation of Bohr’s second postulate based on quantisation using the NCERT Solutions Class 12 Physics Chapter 12.