Atoms
Atoms are the fundamental building units of matter and the structure of atoms is one of the most vital components of physics. Thomson's and Rutherford's attempted to describe how positive and negative charges are distributed within an atom. Later on, Bohr's model described why atoms radiate light of definite wavelengths, introducing the concept of quantized energy levels. Atoms also define atomic spectra, which are directly related to the internal workings of atoms. Class 12 Physics chapter Atoms is quite significant as it connects classical physics with quantum concepts. It is regularly asked in board exams and also in JEE and NEET and also serves as the foundation for modern physics and applications such as lasers, X-rays and nuclear physics.
Atoms Class 12th Topics (NCERT Syllabus)
1. Introduction
Atoms are the basic building blocks of matter. Early models like Thomson’s plum pudding and Rutherford’s nuclear model tried to explain their structure, but could not account for atomic spectra. The study of hydrogen spectrum and scattering experiments led to modern atomic models, forming the basis of quantum theory.
2. Alpha-Particle Scattering and Rutherford’s Nuclear Model of Atom
Ernest Rutherford, through the famous gold foil experiment (1911), studied the scattering of alpha-particles directed at a thin gold foil. Most particles passed through undeflected, a few were deflected at small angles, and a very small number rebounded at large angles. This was impossible to explain with Thomson’s model.
Rutherford concluded that:
- Most of the atom is empty space.
- All the positive charge and most of the mass are concentrated in a small, dense region called the nucleus.
- Electrons revolve around the nucleus like planets around the Sun.
3. Atomic Spectra
When atoms are excited (by heating or electric discharge), they emit or absorb light at certain specific wavelengths. This set of discrete wavelengths forms the atomic spectrum of that element.
Emission Spectrum: Produced when excited atoms release energy as photons. Appears as bright lines on a dark background.
Absorption Spectrum: Produced when atoms absorb certain wavelengths from white light. Appears as dark lines on a continuous spectrum.
4. Bohr Model of the Hydrogen Atom
Niels Bohr (1913) proposed a new model to explain the stability of atoms and the line spectra of hydrogen.
Postulates of Bohr's Model:
1. Quantized Orbits: An electron revolves around the nucleus in certain stable circular orbits without radiating energy.
2. Angular Momentum Quantization: The angular momentum of the electron is restricted to discrete values:
$$
m v r=n \frac{h}{2 \pi}, \quad n=1,2,3, \ldots
$$
3. Energy Emission/Absorption: When an electron jumps between two orbits, it emits or absorbs a photon of energy:
$$
\Delta E=h \nu=E_i-E_f
$$
For hydrogen atom:
Radius of $n^{\text {th }}$ orbit:
$$
r_n=\frac{n^2 h^2 \varepsilon_0}{\pi m e^2}=n^2 a_0
$$
where $a_0 \approx 0.53 $ (Bohr radius).
Energy of $n^{t h}$ orbit:
$$
E_n=-\frac{13.6 \mathrm{eV}}{n^2}
$$
5. The Line Spectra of the Hydrogen Atom
When an excited hydrogen atom returns to a lower energy state, it emits light of specific wavelengths.
This results in discrete line spectra instead of a continuous spectrum.
According to Bohr's model:
$$
\frac{1}{\lambda}=R_H\left(\frac{1}{n_1^2}-\frac{1}{n_2^2}\right), \quad n_2>n_1
$$
6. De Broglie’s Explanation of Bohr’s Second Postulate
According to de Broglie, electrons have a wavelength $\lambda=\frac{h}{m v}$.
For a stable orbit, the circumference must fit an integer number of wavelengths:
$$
\begin{aligned}
& 2 \pi r=n \lambda \\
& m v r=\frac{n h}{2 \pi}
\end{aligned}
$$
Atoms Real Life Example
- Spectroscopy – Identifying elements in stars and gases using their atomic spectra.
- Lasers – Based on atomic transitions (used in medicine, industries, bar-code scanners).
- Atomic Clocks – Measure time very precisely using atomic vibrations.
- Nuclear Energy – Energy released from atoms in nuclear fission/fusion powers reactors and stars.
Exam-wise Weightage of Atoms
| Exam | Approximate Weightage | Remarks |
|---|---|---|
| NEET | 1 Question | Mostly theory-based; questions on Bohr’s model, hydrogen spectrum, and energy levels. |
| Board | 4 Marks | Conceptual + short derivations (Bohr’s model, line spectra) and simple numericals. |
| JEE | 1 Question | Conceptual + application-based questions; focus on Bohr’s postulates, spectral lines, and de Broglie explanation. |
Related Topics,
NCERT Notes Subject wise link:
- NCERT Notes for class 12 Physics.
- NCERT Notes for class 12 Chemistry.
- NCERT Notes for class 12 Mathematics.
- NCERT Notes for class 12 Biology.
- NCERT solutions for class 12 physics chapter atoms
- NCERT exemplar solutions for class 12 physics chapter atoms
- NCERT notes for class 12 physics chapter atoms
NCERT Solutions Subject wise link:
- NCERT solutions for class 12 Physics.
- NCERT solutions for class 12 Chemistry.
- NCERT solutions for class 12 Mathematics.
- NCERT solutions for class 12 Biology.
NCERT Exemplar Solutions Subject wise link:
Frequently Asked Questions (FAQs)
Thomson's Model is also known as Plum Pudding Model. From the beginning of Chapter 12 of Class 12 Physics, you will study about the many atomic model theories. The Thompson Model is one of them. The atom is said to be like a plum or a watermelon in this concept. There is a reddish orangish fleshy portion with seeds embedded in it, similar to watermelon or plum. Thompson believed that the positive charge of the atom is represented by the fleshy section, while the negatively charged electrons are represented by the seeds.
It was unable to explain the origin of hydrogen and other atoms' spectrum series, as well as large angle scattering of – particles.
During its orbital motion, an electron should generate energy in the form of an electromagnetic wave, according to Maxwell's electromagnetic wave theory. As a result, the radius of the electron's orbit would steadily decrease, eventually falling inside the nucleus, and the Rutherford atomic model will be unable to describe the atomic line spectrum.
The radius of the initial orbit of the hydrogen atom is defined as Bohr's radius. It has a value of 5.29 x 10-11m ~ 0.53 Å.
"Atoms" is the title of Chapter 12 of Physics in CBSE Class 12. The many atomic model theories developed by various physicists are explored in this chapter. Thomson's Model of Atom, Rutherford Atomic Model, and Bohr's Model of Atom, commonly known as Bohr's Theory, developed by Neil Bohr, are some of the model hypotheses explored.
Questions related to
On Question asked by student community
Hello Aditya,
Here’s a short plan to help you catch up:
-
Conceptual Clarity : Start with one-shot videos (Unacademy, Atoms) to quickly understand missed concepts from 11th.
-
Problem-Solving : After understanding concepts, practice NCERT problems and previous year papers to build problem-solving skills.
-
Structured Lectures : Consider enrolling in live or recorded lectures for a more detailed approach. Recommended teachers:
- Physics : Kushagra Pathak (Unacademy)
- Chemistry : Vikas Singh (Unacademy)
-
Maths
: Rajeev Malhotra (Unacademy)
-
Gradual Progress : Start with easy subjects and gradually move to harder topics.
-
Mock Tests : Take regular mock tests for practice.
-
Consistency : Study for 4-6 hours daily, balancing breaks and exercise.
Stay focused, and gradually you’ll improve both your concept understanding and problem-solving skills!
I hope this answer helps you. If you have more queries then feel free to share your questions with us we will be happy to assist you.
Thank you and wishing you all the best for your bright future.
Correct Answer: Involvement with the complexities of the day to day life.
Solution : The correct option is 3.
Baconian liberal education is the involvement with the complexities of the day to day life.
Explanation:
It is stated that "The magnificent ideals that have ever haunted of though the human mind and given us our highest proofs of future immortality by reason of the impossibility of their fulfillment here are splintered into atoms by contact with life's realities."
- Baconian liberal education is clearly linked to everyday life.
Correct Answer: Avogadro made a distinction between atoms and molecules.
Solution : The correct option is Avogadro made a distinction between atoms and molecules.
Avogadro distinguished molecules from atoms for the first time in 1811. He also proposed Avogadro's Law: equal amounts of gas at the same pressure and temperature have the same number of molecules.
Correct Answer: To be in pursuit of the ideal in the company of immortals
Solution : The correct option is 1
Explanation:
As stated in the passage, "Why such thinkers fly to the solitude of their own thoughts, or he the silent companionship of the immortals and if they care to present their verse to the world that these views take a sombre and melancholy setting from "the pale cast of thought" in which they were engendered."
- Therefore, great thinkers enjoy spending time alone with their thoughts in order to pursue the ideal in the company of immortals.
Correct Answer: Isotopes
Solution : The correct answer is Isotopes.
Isotopes are those atoms which have the same number of protons but different numbers of neutrons.
For example- There are three isotopes of Hydrogen. They are Protium, Deuterium, and Tritium. Where Protium only has one electron and one proton, Deuterium has one electron, one proton, and one neutron, while Tritium has one electron, one proton, and two neutrons.