NCERT Class 12 Physics Physics Part-II: Chapter 5 — 1 INTRODUCTION
This chapter, NCERT Class 12 Physics Part-II Chapter 13, delves into the structure and properties of atomic nuclei. It begins by explaining that the nucleus, located at the atom's center, contains almost all its mass and is significantly smaller than the atom itself. The chapter introduces the concept of atomic mass units (u) for measuring nuclear masses, defining 1 u as 1/12th the mass of a carbon-12 atom. It discusses isotopes, atoms of the same element with different masses, using chlorine as an example. The fundamental constituents of the nucleus, protons, are introduced, along with their mass and charge. The chapter also touches upon the instability of tritium and the exclusion of electrons from the nucleus based on quantum theory, setting the stage for further exploration of nuclear phenomena.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 12 |
| Subject | Physics |
| Book | Physics Part-II |
| Chapter | Chapter 5 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 705 |
Learning outcomes
- Understand the structure and relative size of the atomic nucleus.
- Define and use the atomic mass unit (u).
- Identify and explain the concept of isotopes.
- Recognize protons as fundamental constituents of the nucleus.
- Differentiate between atomic and nuclear dimensions.
Vocabulary
| Word | Meaning |
|---|---|
| Nucleus | The dense, positively charged center of an atom containing most of its mass. |
| Atomic mass unit (u) | A unit of mass defined as 1/12th the mass of a carbon-12 atom. |
| Isotopes | Atoms of the same element that have different numbers of neutrons and thus different masses. |
| Proton | A stable, positively charged particle found in the nucleus of an atom. |
| Mass spectrometer | An instrument used for accurately measuring the masses of atoms and molecules. |
| Deuterium | An isotope of hydrogen with one proton and one neutron. |
| Tritium | An isotope of hydrogen with one proton and two neutrons; unstable. |
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Practice questions
- What is the approximate size of a nucleus compared to an atom? Answer: The radius of a nucleus is about 10^4 times smaller than the radius of an atom, making the atom mostly empty space.
- Define the atomic mass unit (u). Answer: 1 atomic mass unit (u) is defined as 1/12th of the mass of a carbon-12 atom.
- What are isotopes? Answer: Isotopes are atomic species of the same element that have the same number of protons but differ in their number of neutrons, hence having different masses.
- What is the primary constituent of the positive charge in a nucleus? Answer: Protons are the constituents that carry the positive charge in the nucleus.
Practice MCQs
Q1. The radius of a nucleus is smaller than the radius of an atom by a factor of approximately:
Explanation: Experiments on scattering of a-particles showed that the radius of a nucleus is about 10^4 times smaller than that of an atom.
Q2. What is the definition of 1 atomic mass unit (u)?
Explanation: The atomic mass unit (u) is defined as 1/12th of the mass of a carbon-12 atom.
Q3. Atomic species of the same element differing in mass are called:
Explanation: Atoms of the same element that have different masses due to a different number of neutrons are known as isotopes.
Q4. Which of the following is a constituent of the nucleus carrying positive charge?
Explanation: Protons are positively charged particles found within the nucleus.
Q5. The mass of a proton is approximately:
Explanation: The mass of a proton is given as approximately 1.67262 × 10^-27 kg.
Frequently asked questions
What is the main difference in size between an atom and its nucleus?
The nucleus is significantly smaller than the atom, with its radius being about 10^4 times smaller than the atom's radius.
How is the atomic mass unit (u) defined?
One atomic mass unit (u) is defined as 1/12th of the mass of a carbon-12 atom.
What are isotopes and why are they important?
Isotopes are atoms of the same element with different masses. They are important because they explain why atomic masses are not always whole numbers, as seen in chlorine.
What particle carries the positive charge within the nucleus?
Protons are the particles within the nucleus that carry the positive charge.
Are electrons part of the nucleus?
No, electrons are located outside the nucleus and are not considered part of it, as established by quantum theory.
What is the difference between deuterium and tritium?
Deuterium and tritium are isotopes of hydrogen. Deuterium has one proton and one neutron, while tritium has one proton and two neutrons and is unstable.
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NCERT Class 12 Physics — Physics Part-II — Chapter 5 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.