Topic 1 : Composition and Size of Nucleus

1. ⇒ (JEE Main 2023 (Online) 30th January Evening Shift)

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R

Assertion A: The nuclear density of nuclides 5 10   B , 3 6 Li , 26 56 Fe , 10 20 Ne and 83 209 Bi can be arranged as ρ Bi N > ρ Fe N > ρ Ne N > ρ B N > ρ Li N

Reason R: The radius R of nucleus is related to its mass number A as R = R 0 A 1 / 3 , where R 0 is a constant.

In the light of the above statements, choose the correct answer from the options given below

A. B o t h   A and R are true and R is the correct explanation of A

B. Both A and R are true but R is NOT the correct explanation of A

C. A is false but R is true

D. A is true but R is false

The Correct Answer is Option (C)

R = R 0 A 1 3 , using this

ρ = M 4 3 π R 3 = A m P 4 3 π R 0 3 A = m P 4 3 π R 0 3

ρ is independent of mass number.

A is false

2. ⇒ (JEE Main 2023 (Online) 25th January Morning Shift)

The ratio of the density of oxygen nucleus ( 8 16 O ) and helium nucleus ( 2 4 He ) is

A. 4:1

B. 1:1

C. 2:1

D. 8:1

The Correct Answer is Option (B)

Nuclear density is independent of mass number

As nuclear density = Au 4 3 π R 3

Also, R = R 0   A 1 3

And R 3 = R 0 3 A

Nuclear density = Au 4 3 π R 0 3   A

Nuclear density = 3 u 4 π R 0 3

Nuclear density is independent of A

3. ⇒ (JEE Main 2023 (Online) 11th April Evening Shift)

A nucleus disintegrates into two nuclear parts, in such a way that ratio of their nuclear sizes is 1 : 2 1 / 3 . Their respective speed have a ratio of n : 1 . The value of n is __________.

The Crrect Answer is

Let the masses of the two nuclear parts be m 1 and m 2 , and their respective speeds be v 1 and v 2 . According to the problem, the ratio of their nuclear sizes is 1 : 2 1 / 3 . Since the nuclear size is proportional to the cube root of the mass, we can write:

m 1 m 2 = ( 1 2 1 / 3 ) 3 = 1 2

Now, according to the conservation of linear momentum, the momentum before disintegration is equal to the momentum after disintegration:

m 1 v 1 = m 2 v 2

From the problem statement, the ratio of their respective speeds is n : 1 , so we can write:

v 1 = n v 2

Substitute the expression for v 1 into the momentum conservation equation:

m 1 ( n v 2 ) = m 2 v 2

We know the mass ratio, so substitute that into the equation:

1 2 m 2 ( n v 2 ) = m 2 v 2

Divide both sides by m 2 v 2 :

1 2 n = 1

Now, solve for n :

n = 2

Thus, the value of n is 2.

4. ⇒ ( JEE Main 2023 (Online) 25th January Evening Shift)

A nucleus disintegrates into two smaller parts, which have their velocities in the ratio 3 : 2. The ratio of their nuclear sizes will be ( x 3 ) 1 3 . The value of ' x ' is :-

The Crrect Answer is

JEE Main 2023 (Online) 25th January Evening Shift Physics - Atoms and Nuclei Question 32 English Explanation

v 1 v 2 = 3 2   m 1 v 1 = m 2 v 2 m 1   m 2 = 2 3

Since, Nuclear mass density is constant

m 1 4 3 π r 1 3 = m 2 4 3 π r 2 3 ( r 1 r 2 ) 3 = m 1   m 2 r 1 r 2 = ( 2 3 ) 1 3  So,  x = 2

5. ⇒ (JEE Main 2023 (Online) 24th January Morning Shift)

Assume that protons and neutrons have equal masses. Mass of a nucleon is 1.6 × 10 27 kg and radius of nucleus is 1.5 × 10 15   A 1 / 3 m. The approximate ratio of the nuclear density and water density is n × 10 13 . The value of n is __________.

The Crrect Answer is

Radius = 1.5 × 10 15 A 1 / 3

Volume  Volume  = 4 π 3 r 3

Mass of nucleus = ( 1.6 × 10 27 ) A kg

Density of nucleus   Density of nucleus  = 1.6 × 10 27 × A 4 3 × π × ( 1.5 × 10 15 A 1 3 ) 3

= 1.6 × 3 × 8 × 10 18 4 π × 27 = 32 9 π × 10 17

Density of water = 1000   kg / m 3

Density of nucleus Density of water   Density of nucleus   Density of water  = 32 9 π × 10 17 1000

= 320 9 π × 10 13

= 11.32 × 10 13

value of n = 11

6. ⇒(JEE Main 2022 (Online) 26th July Evening Shift)

A nucleus of mass M at rest splits into two parts having masses M 3 and 2 M 3 ( M < M ) . The ratio of de Broglie wavelength of two parts will be :

A. 1 : 2

B. 2 : 1

C. 1 : 1

D. 2 : 3

The Correct Answer is Option (C)

Linear momentum is conserved

so, p M / 3 = p 2 M / 3

so, λ M / 3 λ 2 M / 3 = 1 1

7. ⇒(JEE Main 2022 (Online) 26th July Evening Shift)

Mass numbers of two nuclei are in the ratio of 4 : 3 . Their nuclear densities will be in the ratio of

A. 4 : 3

B. ( 3 4 ) 1 3

C. 1 : 1

D. ( 4 3 ) 1 3

The Correct Answer is Option (C)

R = R 0 A 1 3

R 1 R 2 = ( A 1 A 2 ) 1 3 = ( 4 3 ) 1 3

Density ratio, ρ 1 ρ 2 = A 1 / V 1 A 2 / V 2

= ( A 1 A 2 ) × ( R 2 R 1 ) 3

= 1 : 1

8. ⇒(JEE Main 2022 (Online) 25th June Evening Shift)

Which of the following figure represents the variation of l n ( R R 0 ) with l n A (if R = radius of a nucleus and A = its mass number)

A.

JEE Main 2022 (Online) 25th June Evening Shift Physics - Atoms and Nuclei Question 71 English Option 1

B.

JEE Main 2022 (Online) 25th June Evening Shift Physics - Atoms and Nuclei Question 71 English Option 2

C.

JEE Main 2022 (Online) 25th June Evening Shift Physics - Atoms and Nuclei Question 71 English Option 3

D.

JEE Main 2022 (Online) 25th June Evening Shift Physics - Atoms and Nuclei Question 71 English Option 4

The Correct Answer is Option (B)

We know that

R = R 0 A 1 / 3

ln ( R R 0 ) y = 1 3 m ln ( A ) x

Straight line

9. ⇒(JEE Main 2021 (Online) 25th July Morning Shift)

A particle of mass 4M at rest disintegrates into two particles of mass M and 3M respectively having non zero velocities. The ratio of de-Broglie wavelength of particle of mass M to that of mass 3M will be :

A. 1 : 3

B. 3 : 1

C. 1 : 3

D. 1 : 1

The Correct Answer is Option ()

λ = h p

both the particles will move with momentum same in magnitude & opposite in direction.

So De-Broglie wavelength of both will be same i.e. ratio 1 : 1

10. ⇒(JEE Main 2020 (Online) 3rd September Evening Slot)

The radius R of a nucleus of mass number A can be estimated by the formula
R = (1.3 × 10–15)A1/3 m.
It follows that the mass density of a nucleus is of the order of :

(Mprot. Mneut 1.67 × 10–27 kg)

A. 1024 kg m–3

B. 1010 kg m–3

C. 1017 kg m–3

D. 103 kg m–3

The Correct Answer is Option (C)

R = ( 1.3 × 10 15 ) A 1 3

We know, m = p V

p = m V

p = m p A 4 3 π R 3

p = m p A 4 3 π × ( 1.3 × 10 15 ) 3 A

p 10 17 k g / m

11. The ratio of mass densities of nuclei of 40Ca and 16O is close to :-⇒ (JEE Main 2019 (Online) 8th April Evening Slot)

A. 1

B. 5

C. 0.1

D. 2

The correct answer is option (A)

Densities of nucleus happens to be constant, irrespective of mass number.

   

12. An unstable heavy nucleus at rest breaks into two nuclei which move away with velocities in the ratio of 8 : 27. The ratio of the radii of the nuclei (assumed to be spherical) is :⇒ ( JEE Main 2018 (Online) 15th April Evening Slot)

A. 8 : 27

B. 4 : 9

C. 3 : 2

D. 2 : 3

The correct answer is option (C)

The two nuclei have velocity in ratio 8 : 27. By conservation of momentum, we have

m 1 v 1 = m 2 v 2 v 1 v 2 = m 2 m 1 m 2 m 1 = 8 27

Now, since m = ρ 4 3 π r 3

Therefore, m 2 m 1 = ρ 4 3 π r 2 3 ρ 4 3 π r 1 3 m 2 m 1 = ( r 2 r 1 ) 3 ( r 2 r 1 ) 3 = 8 27

r 2 r 1 = 2 3

Thus, ratio of radii of nuclei r 1 : r 2 = 3 : 2 .

   

13. If radius of the 27 13 A l nucleus is estimated to be 3.6 fermi then the radius of 125 52 T e nucleus is estimated to be nearly⇒ ( AIEEE 2005)

A. 8

B. 6

C. 5 fermi

D. 4

The correct answer is option (B)

KEY CONCEPT : R = R 0 ( A ) 1 / 3

Here A = Mass number

R 1 R 2 = ( A 1 A 2 ) 1 / 3

= ( 27 125 ) 1 / 3 = 3 5

R 2 = 5 3 × 3.6 = 6 fermi

   

14. A nucleus disintegrated into two nuclear parts which have their velocities in the ratio of 2 : 1. The ratio of their nuclear sizes will be⇒ (AIEEE 2004)

A. 3 1 / 2 : 1

B. 1 : 2 1 / 3

C. 2 1 / 3 : 1

D. 1 : 3 1 / 2

The correct answer is option (B)

From conservation of momentum m 1 v 1 = m 2 v 2

( m 1 m 2 ) = ( v 2 v 1 ) given v 1 v 2 = 2

m 1 m 2 = 1 2

r 1 3 r 2 3 = 1 2

( r 1 r 2 ) = ( 1 2 ) 1 / 3

15. The wavelengths involved in the spectrum of deuterium ( 1 2 D ) are slightly different from that of hydrogen spectrum, because ⇒(AIEEE 2003)

A. the size of the two nuclei are different

B. the nuclear forces are different in the two cases

C. the masses of the two nuclei are different

D. the attraction between the electron and the nucleus is different in the two cases

The correct answer is option (C)

The wavelength of spectrum is given by

1 λ = R z 2 ( 1 n 1 2 1 n 2 2 )

where R = 1.097 × 10 7 1 + m M m = mass of electron

M = mass of nucleus.

For different M , R is different and therefore λ is different