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C. Protons and neutrons (collectively called nucleons)
D. Electrons and protons
Answer: C. Protons and neutrons (collectively called nucleons)
Explanation: The nucleus of an atom consists of positively charged protons and electrically neutral neutrons, which together are referred to as nucleons.
2. The mass number (A) of a nucleus represents:
A. The number of protons only
B. The number of neutrons only
C. The total number of protons and neutrons
D. The atomic charge
Answer: C. The total number of protons and neutrons
Explanation: Mass number A is defined as the sum of the number of protons (Z) and the number of neutrons (N) present in the nucleus, i.e., A = Z + N.
3. Isotopes of an element have:
A. The same number of protons but different numbers of neutrons
B. The same number of neutrons but different numbers of protons
C. Different atomic numbers
D. Different chemical properties
Answer: A. The same number of protons but different numbers of neutrons
Explanation: Isotopes are atoms of the same element (same atomic number Z, hence same number of protons) that differ in their number of neutrons, and thus in mass number.
4. The radius of a nucleus with mass number A is given approximately by:
A. R = R0 A
B. R = R0 A^(1/3)
C. R = R0 A²
D. R = R0/A
Answer: B. R = R0 A^(1/3)
Explanation: Empirically, nuclear radius scales with the cube root of the mass number, R = R0 A^(1/3), where R0 is approximately 1.2 femtometres.
5. Nuclear density is found to be:
A. Different for every nucleus, increasing with A
B. Approximately the same (constant) for all nuclei, independent of mass number
C. Zero for heavy nuclei
D. Maximum for hydrogen only
Answer: B. Approximately the same (constant) for all nuclei, independent of mass number
Explanation: Since nuclear volume increases proportionally with mass number A (as R ∝ A^(1/3), volume ∝ A), nuclear density comes out to be nearly constant for all nuclei.
6. Mass defect of a nucleus is defined as:
A. The difference between the mass of the nucleus and the sum of the masses of its constituent nucleons
B. The total mass of the nucleus
C. The mass of the electrons in the atom
D. The mass converted into kinetic energy of neutrons
Answer: A. The difference between the mass of the nucleus and the sum of the masses of its constituent nucleons
Explanation: Mass defect (Δm) is the difference between the sum of the individual masses of free protons and neutrons and the actual measured mass of the bound nucleus, arising due to binding energy release.
7. Binding energy of a nucleus is related to mass defect (Δm) by:
A. BE = Δm/c²
B. BE = Δm × c²
C. BE = Δm × c
D. BE = Δm²c²
Answer: B. BE = Δm × c²
Explanation: Using Einstein's mass-energy relation, the binding energy that holds nucleons together corresponds to the mass defect converted to energy, given by BE = Δm c².
8. The binding energy per nucleon curve shows that nuclei with the highest stability (maximum binding energy per nucleon) lie around mass number:
A. A ≈ 1-5
B. A ≈ 56-60 (like iron)
C. A ≈ 238
D. A ≈ 500
Answer: B. A ≈ 56-60 (like iron)
Explanation: The binding energy per nucleon curve peaks around mass number 56-60 (elements like iron), indicating these mid-sized nuclei are the most tightly bound and stable.
9. Nuclear fission is the process in which:
A. A heavy nucleus splits into two lighter nuclei, releasing energy
B. Two light nuclei combine to form a heavier nucleus
C. A nucleus emits only gamma rays
D. Electrons are captured by the nucleus
Answer: A. A heavy nucleus splits into two lighter nuclei, releasing energy
Explanation: Nuclear fission involves a heavy unstable nucleus splitting into two or more lighter nuclei along with the release of a large amount of energy and often additional neutrons.
10. Nuclear fusion is the process in which:
A. A heavy nucleus splits apart
B. Two or more light nuclei combine to form a heavier nucleus, releasing energy
C. A nucleus captures an electron
D. Neutrons are converted into protons only
Answer: B. Two or more light nuclei combine to form a heavier nucleus, releasing energy
Explanation: Nuclear fusion is the combination of two or more light atomic nuclei into a single heavier nucleus, a process that releases enormous energy, as seen in stars.
11. The primary source of energy in the Sun and other stars is:
A. Nuclear fission of uranium
B. Nuclear fusion of hydrogen nuclei into helium
C. Chemical combustion
D. Radioactive decay of heavy elements only
Answer: B. Nuclear fusion of hydrogen nuclei into helium
Explanation: Stars, including the Sun, generate their enormous energy output through nuclear fusion reactions, primarily fusing hydrogen nuclei into helium in their cores.
12. Natural radioactivity was discovered by:
A. Marie Curie
B. Henri Becquerel
C. Ernest Rutherford
D. Niels Bohr
Answer: B. Henri Becquerel
Explanation: Henri Becquerel discovered natural radioactivity in 1896 when he found that uranium salts spontaneously emitted radiation capable of exposing photographic plates.
13. During alpha (α) decay, a nucleus emits an alpha particle, which is equivalent to:
A. A single electron
B. A helium nucleus (2 protons and 2 neutrons)
C. A high-energy photon
D. A single neutron
Answer: B. A helium nucleus (2 protons and 2 neutrons)
Explanation: An alpha particle consists of two protons and two neutrons bound together, which is identical to the nucleus of a helium-4 atom, and its emission reduces the parent's mass number by 4.
14. During beta-minus (β⁻) decay, a nucleus emits an electron, and within the nucleus this corresponds to:
A. A proton converting into a neutron
B. A neutron converting into a proton (along with an electron and antineutrino)
C. Two neutrons combining
D. A photon being absorbed
Answer: B. A neutron converting into a proton (along with an electron and antineutrino)
Explanation: In beta-minus decay, a neutron inside the nucleus transforms into a proton, emitting an electron (beta particle) and an antineutrino, thereby increasing the atomic number by one.
15. Gamma (γ) decay involves the emission of:
A. Alpha particles
B. Beta particles
C. High-energy electromagnetic photons, with no change in atomic or mass number
D. Neutrons only
Answer: C. High-energy electromagnetic photons, with no change in atomic or mass number
Explanation: Gamma decay occurs when an excited nucleus releases excess energy as high-energy photons (gamma rays) while transitioning to a lower energy state, without changing its atomic or mass number.
16. The half-life of a radioactive substance is the time in which:
A. All the radioactive nuclei decay completely
B. Half of the initial number of radioactive nuclei decay
C. The substance doubles its mass
D. The decay constant becomes zero
Answer: B. Half of the initial number of radioactive nuclei decay
Explanation: Half-life (T½) is defined as the time required for exactly half of the radioactive nuclei present in a sample to undergo decay, and it is a characteristic constant for each radioactive isotope.
17. The relationship between decay constant (λ) and half-life (T½) is:
A. T½ = 0.693/λ
B. T½ = λ/0.693
C. T½ = λ × 0.693
D. T½ = 1/λ²
Answer: A. T½ = 0.693/λ
Explanation: Half-life and decay constant are related by T½ = ln(2)/λ = 0.693/λ, showing that a larger decay constant corresponds to a shorter half-life.
18. The law of radioactive decay expresses the number of undecayed nuclei (N) at time t as:
A. N = N0 e^(−λt)
B. N = N0 e^(λt)
C. N = N0 (1 − λt)
D. N = N0 λt
Answer: A. N = N0 e^(−λt)
Explanation: Radioactive decay follows an exponential law, N = N0 e^(−λt), where N0 is the initial number of nuclei, λ is the decay constant, and t is elapsed time.
19. The mean life (average life) of a radioactive sample is related to the decay constant (λ) by:
A. Mean life = λ
B. Mean life = 1/λ
C. Mean life = λ²
D. Mean life = 0.693λ
Answer: B. Mean life = 1/λ
Explanation: The mean life of a radioactive substance, representing the average time a nucleus survives before decaying, is given by the reciprocal of the decay constant, τ = 1/λ.
20. In a nuclear reactor, a moderator (such as heavy water or graphite) is used to:
A. Absorb all neutrons and stop the reaction
B. Slow down fast neutrons to increase the probability of further fission
C. Speed up the neutrons
D. Convert neutrons into protons
Answer: B. Slow down fast neutrons to increase the probability of further fission
Explanation: A moderator slows down the fast neutrons produced during fission to thermal speeds, since slow neutrons are far more effective at inducing further fission reactions and sustaining a chain reaction.
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