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Atoms and Nuclei question

2023 · 8 Apr · Shift 1 · Q60
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Atoms and Nuclei question

2023 · 8 Apr · Shift 1 · Q60

JEE MainPhysicsAtoms and NucleiMCQ+4 / −1
For a nucleus AAX{ }_{\mathrm{A}}^{\mathrm{A}} \mathrm{X}AA​X having mass number A\mathrm{A}A and atomic number Z\mathrm{Z}Z A. The surface energy per nucleon (bs)=−a1A2/3\left(b_{\mathrm{s}}\right)=-a_{1} A^{2 / 3}(bs​)=−a1​A2/3. B. The Coulomb contribution to the binding energy bc=−a2Z(Z−1)A4/3\mathrm{b}_{\mathrm{c}}=-a_{2} \frac{Z(Z-1)}{A^{4 / 3}}bc​=−a2​A4/3Z(Z−1)​ C. The volume energy bv=a3A\mathrm{b}_{\mathrm{v}}=a_{3} Abv​=a3​A D. Decrease in the binding energy is proportional to surface area. E. While estimating the surface energy, it is assumed that each nucleon interacts with 12 nucleons. (a1,a2a_{1}, a_{2}a1​,a2​ and a3a_{3}a3​ are constants) Choose the most appropriate answer from the options given below:
  1. A
    C, D only
  2. B
    B, C, E only
  3. C
    B, C only
  4. D
    A, B, C, D only
View written solutionFree

Correct answer: A

  1. Use the semi-empirical mass formula ideas

For a nucleus, the binding energy contains terms like:

B=avA−asA2/3−acZ(Z−1)A1/3+⋯B = a_v A - a_s A^{2/3} - a_c \frac{Z(Z-1)}{A^{1/3}} + \cdotsB=av​A−as​A2/3−ac​A1/3Z(Z−1)​+⋯

If we talk about binding energy per nucleon, divide by AAA:

BA=av−asA−1/3−acZ(Z−1)A4/3+⋯\frac{B}{A} = a_v - a_s A^{-1/3} - a_c \frac{Z(Z-1)}{A^{4/3}} + \cdotsAB​=av​−as​A−1/3−ac​A4/3Z(Z−1)​+⋯

Now check each statement.


  1. Statement A

It says the surface energy per nucleon is

bs=−a1A2/3b_s = -a_1 A^{2/3}bs​=−a1​A2/3

But surface energy itself is proportional to surface area:

Es∝−A2/3E_s \propto -A^{2/3}Es​∝−A2/3

So surface energy per nucleon is

bs=EsA∝−A−1/3b_s = \frac{E_s}{A} \propto -A^{-1/3}bs​=AEs​​∝−A−1/3

Hence statement A is false.


  1. Statement B

It says Coulomb contribution to the binding energy is

bc=−a2Z(Z−1)A4/3b_c = -a_2 \frac{Z(Z-1)}{A^{4/3}}bc​=−a2​A4/3Z(Z−1)​

But the Coulomb term in the total binding energy is

Ec=−acZ(Z−1)A1/3E_c = -a_c \frac{Z(Z-1)}{A^{1/3}}Ec​=−ac​A1/3Z(Z−1)​

The given power A−4/3A^{-4/3}A−4/3 corresponds to binding energy per nucleon, not total binding energy.

Therefore B is false as stated.


  1. Statement C

It says volume energy is

bv=a3Ab_v = a_3 Abv​=a3​A

The volume term in total binding energy is indeed proportional to AAA:

Ev=avAE_v = a_v AEv​=av​A

So C is true.


  1. Statement D

Surface effect reduces binding because nucleons at the surface have fewer neighbors. Since surface area of nucleus is proportional to

R2∝(A1/3)2=A2/3R^2 \propto (A^{1/3})^2 = A^{2/3}R2∝(A1/3)2=A2/3

Decrease in binding energy due to surface is proportional to surface area.

So D is true.


  1. Statement E

In estimating surface energy, the idea used is that each nucleon interacts only with near neighbors due to saturation of nuclear force. One often says each nucleon interacts with a limited number of neighboring nucleons, but the fixed number 12 is not a standard assumption of the liquid drop derivation here.

Hence E is not taken as correct in this context.


  1. Final evaluation
  • A: False
  • B: False
  • C: True
  • D: True
  • E: False

Therefore the correct choice is:

C, D only\boxed{\text{C, D only}}C, D only​

So the correct option is A.

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