# MCQ Questions for Class 12 Physics Chapter 13 Nuclei with Answers

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## Nuclei Class 12 MCQs Questions with Answers

Solve these Nuclei Multiple Choice Questions of Class 12 Physics Chapter 13 MCQ, so as not to miss out on any concept from being clear about what they mean!

Question 1.
When a nucleus in an atom undergoes a radioactive decay, the electronic energy levels of the atom
(a) do not change for any type of radioactivity.
(c) change for α-radioactivity but not for others.
(d) change for ß-radioactivity but not for others.

Question 2.
A radioactive isotope has a half-life of T years. The time it takes its activity to reduce to 3.125% is
(a) 5 T
(b) 6.654 T
(c) 5.645 T
(d) 6.654 T

Question 3.
For a radioactive material, half-life is 10 minutes. If initially there are 600 number of nuclei, the time taken (in minutes) for the disintegration of 450 nuclei is:
(a) 20
(b) 10
(c) 30
(d) 15

Question 4.
A nuclear explosive is designed to deliver 1 MW power in the form of heat energy. If the explosion is designed with nuclear fuel consisting of U235 to run a reactor at this power level for one year, then the amount of fuel needed is (given energy per fission is 200 MeV)
(a) 1 kg
(b) 0.01 kg
(c) 3.84 kg
(d) 0.384 kg

Question 5.
When the radioactive isotope 88Ra226 decays in a series by emission of three alpha (α) and a beta (ß) particle, the isotope X which remains undecayed is
(a) 83X214
(b) 84X218
(c) 84X220
(d) 87X223

Question 6.
Fusion reaction takes place, at high temperature because:
(a) nuclei break up at high temperature
(b) atoms get ionised at high temperature
(c) kinetic energy is high enough to overcome the coulomb repulsion between nuclei
(d) molecules break up at high temperature

Answer: (c) kinetic energy is high enough to overcome the coulomb repulsion between nuclei

Question 7.
Half-lives of two radioactive elements A and B are 20 minutes and 40 minutes, respectively. Initially, the samples have equal number of nuclei. After 80 minutes, the ratio of decayed numbers of A and B nuclei will be:
(a) 1 : 16
(b) 4 : 1
(c) 1 : 4
(d) 5 : 4

Question 8.
Radioactive material ‘A’ has decay constant ‘8 λ’ and material ‘B’ has decay constant ‘λ’. Initially they have same number of nuclei. After what time, the ratio of number of nuclei of material ‘B’ to that ‘A’ will be 1/e?
(a) $$\frac { 1 }{7λ}$$
(b) $$\frac { 1 }{8λ}$$
(c) $$\frac { 1 }{9λ}$$
(d) $$\frac { 1 }{λ}$$

Answer: (a) $$\frac { 1 }{7λ}$$

Question 9.
A radioactive nucleus A with a half-life T decays into a nucleus B. At t = 0, there is no nucleus B. At some time, t the ratio of the number of B to that of A is 0.3. Then, t is given by:
(a) t = T log(1.3)
(b) t = $$\frac { T }{log(1.3)}$$
(c) t = $$\frac { T log(2) }{2log(1.3)}$$
(d) t = $$\frac { T log(1.3) }{log(2)}$$

Answer: (d) t = $$\frac { T log(1.3) }{log(2)}$$

II. Fill in the blanks

Question 1.
Isotopes of an element are the atoms of an element which have ……………….. but different atomic weights.

Question 2.
Isobars are the atoms of different elements which have same ……………….. but different atomic number.

Question 3.
Isotones are the nuclides which contain ………………..

Question 4.
One atomic mass unit is defined as ……………….. of mass of an atom of 6C12

Question 5.
Nuclear forces are the ……………….. force which hold together the nucleons in the tiny nucleus.

Question 6.
Nuclear forces are ……………….. Independent.

Question 7.
Nuclear forces do not obey ……………….. law.

Question 8.
The rest mass of a nucleus Is ……………….. than the sum of the rest masses of Its constituent nucleons.

Question 9.
The total energy required to liberate all the nucleons from the nucleus, i.e. to disintegrate the nucleus into its constituent particles, is called ………………..

Question 10.
The divergence of the nuclear mass from the nearest whole number (mass number) per nucleon is called ………………..

Question 11.
Natural ……………….. is defined as the spontaneous disintegration of a nucleus with the emission of certain particles and radiation.

Question 12.
During alpha decay atomic number of the daughter nucleus decreases by ……………….. and mass number decreases by ………………..

Question 13.
In negative beta decay there is no change in ………………..

Question 14.
The energy distribution of beta decay is ………………..

Question 15.
The number of nuclei disintegrating per second is ……………….. to the number of nuclei present at the time of disintegration.

Question 16.
A ……………….. is defined as the reciprocal of that time duration for which the number of nuclei of the radioactive sample decays to 1/e.

Question 17.
The ……………….. is the time required for the number of radioactive nuclei to decrease to one-half the original number.

Question 18.
A radioactive isotope of silver has half-life of 20 minutes. Tb fraction of the original activity that remains after one hour is ………………..

Question 19.
The rate of disintegration in a radioactive substance is known as its ………………..

Question 20.
The activity of a sample is said to be one ……………….. if it undergoes one decay per second.

Question 21.
Heavy water is a ……………….. which slows down fast moving neutrons to thermal velocities so that they can cause fission of 235U92 nuclei.

Question 22.
Two nuclei have mass numbers in the ratio 27 : 125. Then the ratio of their radii is ………………..

Question 23.
Complete the equation
is …………………

Answer: $$_{ n-2 }^{m-4}$$Y

Question 24.
The process responsible for energy production in the Sun is ………………..