NCERT Solutions Class 12 Physics Chapter 12 Atoms - PDF Download

If you want to score well in your final exams, you can refer to this solution as they provide study materials that prove beneficial for a better understanding of your subjects. NCERT Solutions provide a wide variety of study materials for all the chapters of your subject and allow you to gain maximum marks. Physics is indeed a tricky subject for many of you.

Chapter 12 of NCERT Solutions is all about Atoms. The structure of atoms, the origin of spectra, Bohr's Theory of Hydrogen Atoms, and X-Rays are the main topics of this chapter.

Chapter 12 highlights the different models of Atom - In this chapter you will learn about Thomson's, Rutherford’s, and Bohr's atomic models; their pros, cons, and limitations; production, properties, and application of X-Rays and the use of Mosley's law in it. So, when we start studying Atoms, it is going back to studying chemistry, but this time its application in machines. NCERT Solutions provide you with great content that is simplified as per the your convenience and help them gain good marks.

Topics covered in Class 12 Physics Chapter 12 Atoms are as follows

Alpha-Particle Scattering and Rutherford’s Nuclear Model of Atom

The observations made by Rutherford led him to conclude below points:

1. A major fraction of the α-particles bombarded on the gold sheet passed through the sheet without any deflection, and hence most of the space in an atom is empty.

2. Some of the α-particles were deflected by the gold sheet by very small angles, and therefore the positive charge in an atom is not uniformly distributed. The positive charge in an atom is concentrated in a very small volume or area.

3. Very few no. of α-particles were deflected back, that is only a few α-particles had nearly 180° angle of deflection. So the total volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.

Rutherford Atomic Model

Based on the Rutherford Atomic model observations and conclusions, He proposed the atomic structure of elements. 

1. The positive charge atom and most of the mass of an atom is concentrated in very small volume, he called this region of the atom as a nucleus.

2. Rutherford’s model says that the negatively charged electrons surround the nucleus of an atom. He also claimed that the electrons surrounding the nucleus revolve around it with very high speed in circular motion following a circular path. This circular path he named orbits

3. Electrons are negatively charged and the nucleus is a densely concentrated mass of positively charged particles held together by a strong electrostatic force of attraction.

Limitations of Rutherford Atomic Model

However the Rutherford atomic model was based on experimental observations, it failed to explain several things.

• Rutherford proposed that the electrons revolve around the nucleus in fixed paths which are called orbits. According to Maxwell, accelerated charged particles emit electromagnetic radiation and therefore an electron revolving around the nucleus should emit electromagnetic radiation. This radiation will carry energy from the motion of the electron which would come at the cost of shrinking of orbits. Finally the electrons would collapse in the nucleus. As per calculations have shown that Rutherford model, an electron would collapse into the nucleus in less than 10ⲻ8 sec. So the Rutherford model was not in agreement with Maxwell’s theory and could not explain the stability of an atom.

• One of the drawbacks of the Rutherford model was that he did not say anything about the arrangement of electrons in an atom which made his theory incomplete.

• In spite of the early atomic models were inaccurate and failed to explain certain experimental results, they formed the base for future developments in the world of quantum mechanics.

Alpha-particle Trajectory

When alpha particles are bombarded with some starting velocity, we observe the different trajectories followed by the alpha particle. The trajectory of an alpha particle always depends upon the impact parameter. Impact parameter is the perpendicular distance between the initial velocity vector and the centre of the nucleus.

Electron Orbits

Orbit is a well-defined circular path around the nucleus in which path electrons revolve around. The 3-D space around the nucleus where the probability of finding an electron is maximum is called an orbital. This represents the motion of an electron in one plane.

Atomic Spectra

Atomic spectra are defined as: the spectrum of the electromagnetic radiation absorbed by an electron during transitions between different energy levels within an atom. When an electron gets excited from one energy level to another energy levels, it either emits or absorbs light of a particular wavelength.

Bohr Model of the Hydrogen Atom

The Bohr model is called the Rutherford-Bohr Model. The Rutherford-Bohr Model of the hydrogen atom first proposed the planetary model, but later an assumption concerning the electrons was made. The theory was the quantization of the structure of atoms. Bohr proposed that electrons orbited the nucleus in specific orbits with a fixed radius. 

Energy Levels

Energy levels which are also known as electron shells are fixed distances from the nucleus of an atom where electrons may be found. As we go farther from the nucleus, electrons which are at higher energy levels have more energy

The Line Spectra of the Hydrogen Atom

The hydrogen spectrum is an essential piece of evidence to show the quantized electronic structure of an atom. The hydrogen atoms of the molecule dissociate into electric discharge as soon as it is passed through a gaseous hydrogen molecule. This results in the emission of electromagnetic radiation emanating from the energetically excited hydrogen atoms. The hydrogen emission spectrum includes radiation with discrete frequencies. These radiation series are named after the scientists who discovered them.

The Balmer series is basically the part of the hydrogen emission spectrum responsible for the excitation of an electron from the second shell to any other level shell. Some of them are mentioned below,

• The transition from the first shell to any other shell – Lyman series

• The transition from the second shell to any other shell – Balmer series

• The Transition from the third shell to any other shell – Paschen series

• The transition from the fourth shell to any other shell – Brackett series

• The transition from the fifth shell to any other shell – Pfund series

De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation

Debroglie's explanation of Bohr's second postulate of quantization 

•  The second postulate of the Bohr atom model states that angular momentum of electrons orbiting around the nucleus is quantized.
• According to Debroglie, the electron in its circular orbit, as proposed by Bohr, must be seen as a wave particle.
• When a string fixed at two ends is plucked, a large number of wavelengths are excited and a standing wave is formed.
• It means that in a string, standing waves form when total distance travelled by a wave down the string and back is an integral number of wavelengths.
• According to Debroglie, a stationary orbit is that which contains an integral number of De Broglie waves associated with the revolving electron.

Benefits of NCERT Solutions For Class 12 Physics

• Studying from a NCERT book can be tiring and difficult for you due to the lack of explanations of basic concepts.

• Many of you may lose interest in Physics because you struggle to understand the concepts and theories presented in books.

• Learning Physics becomes enjoyable when students can easily grasp and connect the chapters.

• Answering exercise questions successfully boosts students' morale and motivates them to study harder.

• NCERT Solutions provide course material that simplifies hard topics and helps students understand solutions easily.

• NCERT Solutions for Class 12 Physics offer a wide range of solutions, eliminating the need to search through multiple books.

• The main concepts covered in this chapter include the alpha-particle scattering experiment, the Bohr model, hydrogen spectrum, Rutherford's model of an atom, and energy levels.

• NCERT Exemplar Book Solutions are beneficial for you to understand and solve the questions related to Chapter 12.

• You can utilize revision notes, important questions, sample papers, and other study materials to gain a comprehensive understanding of the concepts covered in the chapter.

Conclusion

NCERT Solutions for Class 12 Physics Chapter 12 Atoms provide support and resources for students studying this important topic. The chapter covers important concepts such as the alpha-particle scattering experiment, the Bohr model, Rutherford's model of an atom, hydrogen spectrum, and energy levels. By offering easy solutions and explanations, NCERT Solutions enable students to understand and solve the questions effectively. As well as, the availability of revision notes, important questions, sample papers, and related study materials further enhances the students' understanding of the chapter. With NCERT Solutions, you can overcome the challenges of studying atoms and gain confidence in their knowledge and problem-solving abilities. These solutions is a valuable tool in helping students succeed in their Class 12 Physics examinations.

Frequently Asked Question

Question 1 : What are the topics in chapter 12 Atoms?

Answer : Following are the topics and sub topics of chapter 12 : 

12.1 Introduction

12.2 Alpha-Particle Scattering and Rutherford’s Nuclear Model of Atom

12.2.1 Alpha-particle Trajectory

12.2.2 Electron Orbits

12.3 Atomic Spectra

12.3.1 Spectral Series

12.4 Bohr Model of the Hydrogen Atom

12.4.1 Energy Levels

12.5 The Line Spectra of the Hydrogen Atom

12.6 De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation

Question 2 : What is the Rutherford Atomic model?

Answer: According to the Rutherford atomic model:

1. The positive charge atom and most of the mass of an atom is concentrated in very small volume, he called this region of the atom as a nucleus.

2. Rutherford’s model says that the negatively charged electrons surround the nucleus of an atom. He also claimed that the electrons surrounding the nucleus revolve around it with very high speed in circular motion following a circular path. This circular path he named orbits

3. Electrons being negatively charged and nucleus being a densely concentrated mass of positively charged particles are held together by a strong electrostatic force of attraction.

Limitations of Rutherford Atomic Model

However the Rutherford atomic model was based on experimental observations, it failed to explain several things.

• Rutherford proposed that the electrons revolve around the nucleus in fixed paths which are called orbits. According to Maxwell, accelerated charged particles emit electromagnetic radiation and therefore an electron revolving around the nucleus should emit electromagnetic radiation. This radiation will carry energy from the motion of the electron which would come at the cost of shrinking of orbits. Finally the electrons would collapse in the nucleus. As per calculations have shown that Rutherford model, an electron would collapse into the nucleus in less than 10ⲻ8 sec. So the Rutherford model was not in agreement with Maxwell’s theory and could not explain the stability of an atom.

• One of the drawbacks of the Rutherford model was that he did not say anything about the arrangement of electrons in an atom which made his theory incomplete.

• In spite of the early atomic models were inaccurate and failed to explain certain experimental results, they formed the base for future developments in the world of quantum mechanics.