CBSE Class 12 Physics Revision Questions Atoms and Nuclei

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EASY AND SCORING AREAS:‐

1. Energy of orbit of Rutherford atomic model….E=‐13.6 Ev/n2

2. Bohr model of hydrogen atom………

3. Line spectra of hydrogen atom

4. De‐broglie explanation of Bohr’s second postulate

EASY AND SCORING AREAS:‐

1. Binding Energy

2. Mass energy relation

3. Law of radioactivity

4. α-decay, β-decay, γ-decay

5. Nuclear fission reaction, nuclear fusion reaction.

QUESTIONS

Q1. Define Nuclear forces and gives their important characteristics/properties.

Ans.The nucleus of an atom has a number of protons and neutrons (nucleons) which are held together by the forces known as Nuclear forces in the tiny nucleus, inspite of strong force of repulsion between protons.

Characteristics/Properties of nuclear forces:

1. Nuclear forces are strongest forces in nature.

2. Nuclear forces are short range forces.

3. Nuclear forces are basically strong attractive forces but contain a small component of repulsive forces.

4. Nuclear forces are saturated forces.

5. Nuclear forces are charge independent

6. Nuclear forces are spin- dependent

7. Nuclear forces are exchange forces

Q2.Define atomic mass unit (a.m.u.) and calculate its value in SI unit of mass. Also find energy equivalent in MeV corresponding to it. 

Q3. Define binding energy per nucleon and packing fraction? Draw the curve showing the variation of binding energy per nucleon with mass number (A). Discuss its conclusions and explain how nuclear fission and fusion processes are explained with its help.

PACKING FRACTION

The packing fraction of a nucleus is defined as the mass defect per nucleon of the nucleus. Packing fraction = MASS DEFECT/A

BINDING ENERGY CURVE

It is found that binding energy of ₃Li⁷ is greater than that of ₂He⁴, but the value of its binding energy per nucleon is lesser. However, ₂He⁴ is found to be more stable than ₃Li⁷. Therefore, it may be concluded that the stability of nucleus depends upon binding energy per nucleon rather than the total binding energy of nucleus. Fig. shows the graph between the binding energy per nucleon and mass number of different nuclei. From the binding energy curves the following conclusions can be drawn:

1. The binding energy per nucleon for light nuclei, such as ₁H², is very small.

2. The binding energy per nucleon increases rapidly for nuclei upto mass number 20 and the curve possesses peaks corresponding to nuclei ₂He⁴, ₆C¹² and ₈O¹⁶. The peaks indicate that these nuclei are more stable than those in their neighbourhood.

3. After mass number 20, binding energy per nucleon increases gradually and for mass number between 40 and 120 , the curve becomes more or less flat

4. After mass number 120, binding energy nucleon starts decreasing and drops to 7.6MeV for uranium such nuclei are unstable and are found to disintegrate. 

5. The binding energy per nucleon has a low value for both very high and very heavy nuclei in order to attain very higher value of binding energy per nucleon, the lighter nuclei may unite together to form a heavier nucleus (process ofnuclear fusion ) or a heavier nucleus may split into lighter nuclei (process of nuclear fusion). In both the nuclear processes, the resulting nucleus acquires greater value of binding energy per nucleon along with the liberation of energy.

Q4. What is radioactivity? State the law of radioactive decay. Show that the radioactive decay is exponential in nature.

Ans. Radioactive decay The spontaneous emission of radiation from a radioactive element is called radioactive decay.Decay Law The number of nuclei disintegrating per second of a radioactive sample at any instant is directly proportional to the number of undecayed nuclei present in the sample at that instant.

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