Chapter 13 Nuclei
NCERT Class 12 Physics - Chapter 13: Nuclei
Topics Covered
- Introduction to Nuclei
- Composition of Nucleus
- Size of the Nucleus
- Nuclear Force
- Mass-Energy Relation
- Binding Energy Per Nucleon
- Radioactivity
- Law of Radioactive Decay
- Alpha, Beta, and Gamma Decay
- Half-life and Mean Life
- Nuclear Energy
Introduction to Nuclei
The nucleus is the small, dense center of an atom, composed of protons and neutrons. It contains almost all the mass of the atom.
Example
A helium nucleus consists of 2 protons and 2 neutrons.
Composition of Nucleus
The nucleus is made up of nucleons, which are protons and neutrons. Protons are positively charged, while neutrons are neutral.
Example
A carbon nucleus has 6 protons and 6 neutrons.
Size of the Nucleus
The size of the nucleus is very small compared to the size of the atom. It is measured in femtometers (1 fm = 10-15 meters).
Example
The radius of a typical nucleus is about 1-10 femtometers.
Nuclear Force
Nuclear force is the strong force that holds protons and neutrons together in the nucleus, overcoming the electrostatic repulsion between protons.
Example
Nuclear force is much stronger than the electromagnetic force but acts over a very short range.
Mass-Energy Relation
According to Einstein's mass-energy equivalence principle, mass can be converted into energy and vice versa, represented by the equation E = mc2.
Example
In nuclear reactions, a small amount of mass is converted into a large amount of energy.
Binding Energy Per Nucleon
Binding energy per nucleon is the energy required to remove a nucleon from the nucleus. It indicates the stability of the nucleus.
Example
Iron-56 has one of the highest binding energies per nucleon, making it very stable.
Radioactivity
Radioactivity is the spontaneous emission of particles or radiation from an unstable nucleus. It was discovered by Henri Becquerel.
Example
Uranium-238 undergoes radioactive decay to form thorium-234.
Law of Radioactive Decay
The law of radioactive decay states that the rate of decay of a radioactive substance is proportional to the number of undecayed nuclei present.
Example
The decay rate of carbon-14 is used in radiocarbon dating.
Alpha, Beta, and Gamma Decay
Alpha decay involves the emission of an alpha particle (2 protons and 2 neutrons). Beta decay involves the emission of a beta particle (electron or positron). Gamma decay involves the emission of gamma radiation (high-energy photons).
Example
Radon-222 undergoes alpha decay to form polonium-218. Carbon-14 undergoes beta decay to form nitrogen-14.
Half-life and Mean Life
Half-life is the time taken for half the number of radioactive nuclei in a sample to decay. Mean life is the average lifetime of a radioactive particle before it decays.
Example
The half-life of uranium-238 is about 4.5 billion years.
Nuclear Energy
Nuclear energy is released during nuclear reactions, such as fission (splitting of a nucleus) and fusion (combining of nuclei). It is a powerful source of energy.
Example
Nuclear reactors use fission to produce energy, while the sun uses fusion.
Word Meanings
Nucleus
The small, dense center of an atom, composed of protons and neutrons.
Proton
A positively charged subatomic particle found in the nucleus of an atom.
Neutron
A neutral subatomic particle found in the nucleus of an atom.
Nucleon
A particle found in the nucleus of an atom, either a proton or a neutron.
Femtometer
A unit of length equal to 10-15 meters, used to measure nuclear sizes.
Nuclear Force
The strong force that holds protons and neutrons together in the nucleus.
Mass-Energy Equivalence
The principle that mass can be converted into energy and vice versa, represented by the equation E = mc2.
Binding Energy
The energy required to separate a nucleon from the nucleus.
Radioactivity
The spontaneous emission of particles or radiation from an unstable nucleus.
Alpha Decay
The emission of an alpha particle (2 protons and 2 neutrons) from a nucleus.
Beta Decay
The emission of a beta particle (electron or positron) from a nucleus.
Gamma Decay
The emission of gamma radiation (high-energy photons) from a nucleus.
Half-life
The time taken for half the number of radioactive nuclei in a sample to decay.
Mean Life
The average lifetime of a radioactive particle before it decays.
Nuclear Fission
The splitting of a large nucleus into smaller nuclei, releasing energy.
Nuclear Fusion
The combining of small nuclei to form a larger nucleus, releasing energy.
Radiocarbon Dating
A method of dating ancient objects by measuring the amount of carbon-14 they contain.
Photon
A quantum of light or electromagnetic radiation.
Electromagnetic Force
The force between charged particles, including the repulsion between protons in the nucleus.
Relativistic Effects
Effects that become significant when particles move at speeds close to the speed of light.
FAQs
1. What is a nucleus?
The nucleus is the small, dense center of an atom, composed of protons and neutrons.
2. What are nucleons?
Nucleons are particles found in the nucleus of an atom, specifically protons and neutrons.
3. How is the size of a nucleus measured?
The size of a nucleus is measured in femtometers (1 fm = 10-15 meters).
4. What is nuclear force?
Nuclear force is the strong force that holds protons and neutrons together in the nucleus.
5. What is mass-energy equivalence?
Mass-energy equivalence is the principle that mass can be converted into energy and vice versa, represented by E = mc2.
6. What is binding energy per nucleon?
Binding energy per nucleon is the energy required to remove a nucleon from the nucleus, indicating the stability of the nucleus.
7. What is radioactivity?
Radioactivity is the spontaneous emission of particles or radiation from an unstable nucleus.
8. What is alpha decay?
Alpha decay is the emission of an alpha particle (2 protons and 2 neutrons) from a nucleus.
9. What is beta decay?
Beta decay is the emission of a beta particle (electron or positron) from a nucleus.
10. What is gamma decay?
Gamma decay is the emission of gamma radiation (high-energy photons) from a nucleus.
11. What is half-life?
Half-life is the time taken for half the number of radioactive nuclei in a sample to decay.
12. What is mean life?
Mean life is the average lifetime of a radioactive particle before it decays.
13. What is nuclear fission?
Nuclear fission is the splitting of a large nucleus into smaller nuclei, releasing energy.
14. What is nuclear fusion?
Nuclear fusion is the combining of small nuclei to form a larger nucleus, releasing energy.
15. What is radiocarbon dating?
Radiocarbon dating is a method of dating ancient objects by measuring the amount of carbon-14 they contain.
16. What is a photon?
A photon is a quantum of light or electromagnetic radiation.
17. What is the electromagnetic force?
The electromagnetic force is the force between charged particles, including the repulsion between protons in the nucleus.
18. What are relativistic effects?
Relativistic effects are effects that become significant when particles move at speeds close to the speed of light.
19. How does nuclear energy benefit us?
Nuclear energy provides a powerful source of energy used in power plants and medical applications.
20. Who discovered radioactivity?
Henri Becquerel discovered radioactivity in 1896.