Get summaries, questions, answers, solutions, notes, extras, workbook solutions, PDF and guide of chapter 12 Radioactivity: ICSE Class 10 Physics which is part of the syllabus of students studying under the Council for the Indian School Certificate Examinations board. These solutions, however, should only be treated as references and can be modified/changed.
Summary
Atoms are tiny building blocks of everything. Inside an atom, there are even smaller particles. Electrons, which have a negative charge, move around a central part called the nucleus. The nucleus contains positively charged protons and neutral neutrons. Electrons travel in specific paths or shells around the nucleus, like planets around the sun. Each shell can hold a certain number of electrons. The number of protons in an atom’s nucleus is its atomic number, which defines what element it is. The total number of protons and neutrons is the mass number. Atoms are usually neutral because they have an equal number of protons and electrons.
Sometimes, atoms of the same element can have different numbers of neutrons. These are called isotopes. For example, hydrogen has isotopes called protium, deuterium, and tritium. Some isotopes are stable, but others are unstable, meaning their nuclei can change. This change is called radioactivity. Radioactivity is a natural process where an unstable nucleus spontaneously breaks down, or decays, releasing energy and particles. This process is not affected by chemical changes or physical conditions like temperature. The substances that undergo this decay are called radioactive substances.
There are three main types of radiation emitted during radioactive decay: alpha (α) particles, beta (β) particles, and gamma (γ) rays. Alpha particles are like tiny helium nuclei, with two protons and two neutrons, and have a positive charge. Beta particles are fast-moving electrons that come from the nucleus when a neutron changes into a proton, and they have a negative charge. Gamma rays are high-energy waves, similar to X-rays, and have no charge. These radiations have different properties. Alpha particles are highly ionizing but have low penetrating power, meaning they can be stopped easily. Beta particles are more penetrating than alpha particles. Gamma rays are the most penetrating and require thick materials like lead to stop them.
When a nucleus emits an alpha particle, its atomic number decreases by two and its mass number decreases by four, changing it into a new element. When it emits a beta particle, its atomic number increases by one, but its mass number stays the same, also forming a new element. Gamma emission often accompanies alpha or beta decay and happens when an excited nucleus releases excess energy to become more stable, without changing its atomic or mass number.
Radioactivity has many uses. Radioisotopes are used in medicine for diagnosing illnesses using “tracers” and treating diseases like cancer. In science, they help in dating ancient objects through “carbon dating” and in agricultural research. Industrially, they are used as fuel in nuclear power plants and for quality control. However, radiations can also be harmful, causing damage to living cells, leading to illnesses or genetic changes. So, safety precautions are very important when handling radioactive materials or working with nuclear energy. We are also constantly exposed to low levels of “background radiation” from natural sources in our environment and even within our bodies.
Besides natural decay, nuclei can undergo other changes. Nuclear fission is a process where a heavy nucleus, like uranium, splits into two smaller nuclei when hit by a neutron, releasing a large amount of energy and more neutrons. These new neutrons can cause further fissions, leading to a “chain reaction.” This can be uncontrolled, as in a nuclear bomb, or controlled, as in a nuclear reactor to generate electricity. Nuclear fusion is when two light nuclei, like hydrogen isotopes, combine to form a heavier nucleus, releasing even more energy per unit mass than fission. This process powers the sun and stars and requires extremely high temperatures.
Workbook Solutions
Exercise (A)
MCQ
1. The total number of protons in a neutral atom is equal to:
(a) atomic number
(b) neutrons
(c) electrons
(d) both (a) and (c)
Answer: (d) both (a) and (c)
2. By total number of nucleons, we mean :
(a) the number of protons + number of electrons
(b) the number of electrons + number of neutrons
(c) the number of electrons + number of protons + number of neutrons
(d) the number of neutrons + number of protons
Answer: (d) the number of neutrons + number of protons
3. If Z is the atomic number and A is the mass number of an atom, then which of the following relations is incorrect?
(a) Number of electrons = Z
(b) Number of protons = A – Z
(c) Number of neutrons = A – Z
(d) Number of neutrons + number of protons = A
Answer: (b) Number of protons = A – Z
4. A certain nucleus P has a mass number 15 and atomic number 7. The number of neutrons is :
(a) 7
(b) 15
(c) 8
(d) none of the above
Answer: (c) 8
5. The atoms of the same element, having the same atomic number Z but different mass number A are called:
(a) Isotopes
(b) Isobars
(c) Isotones
(d) None of the above
Answer: (a) Isotopes
6. Out of the following, which element has the largest number of isotopes ?
(a) Hydrogen
(b) Carbon
(c) Chlorine
(d) Tin
Answer: (d) Tin
7. ³⁹₁₉K and ⁴⁰₂₀Ca are :
(a) Isotopes
(b) Isobars
(c) Radioactive substances
(d) Isotones
Answer: (d) Isotones
8. Out of the following, radioactive substances are :
(a) Uranium
(b) Polonium
(c) Actinium
(d) All of the above
Answer: (d) All of the above
9. The heaviest nuclear radiation is:
(a) X radiation
(b) α radiation
(c) γ radiation
(d) β radiation
Answer: (b) α radiation
10. To study the age of excavated materials of archeological significance, we study the rate of decay of an isotope of:
(a) uranium
(b) cobalt
(c) carbon
(d) chlorine
Answer: (c) carbon
11. A radioactive nucleus emits a beta particle. The position of daughter nucleus in the periodic table as compared to the parent nucleus after emitting a beta particle is:
(a) at the same place
(b) one place higher
(c) one place lower
(d) two places lower
Answer: (b) one place higher
12. The speed of γ radiations is :
(a) of the order of 10⁶ ms⁻¹
(b) of the order of 10⁷ ms⁻¹
(c) equal to the speed of light, i.e. 3 × 10⁸ ms⁻¹
(d) none of the above
Answer: (c) equal to the speed of light, i.e. 3 × 10⁸ ms⁻¹
13. The correct representation of γ emission is :
(a) ᴬZP → ᴬZQ + γ
(b) ᴬP → ᴬZ-₂Q + ₂⁴γ
(c) ᴬZP* → ᴬZP + γ
(d) ᴬZP → ᴬZ+₁P + γ
Answer: (c) ᴬZP* → ᴬZP + γ
14. Assertion (A) : Out of α, β and γ radiation, α-particles have the maximum penetrating power. Reason (R) : The α-particles are the heaviest amongst the three.
(a) both A and R are true and R is the correct explanation of A
(b) both A and R are True and R is not the correct explanation of A
(c) assertion is false but reason is true
(d) assertion is true but reason is false
Answer: (c) assertion is false but reason is true
15. Assertion (A) : ¹⁴₆C and ¹⁴₇N are isobars. Reason (R) : ¹⁴C has 6 protons and 8 neutrons in its nucleus, whereas ¹⁴N has 7 protons and 7 neutrons in its nucleus.
(a) both A and R are true and R is the correct explanation of A
(b) both A and R are True and R is not the correct explanation of A
(c) assertion is false but reason is true
(d) assertion is true but reason is false
Answer: (a) both A and R are true and R is the correct explanation of A
Very Short Answer Type Questions
1. What is the name given to elements with the same mass number and different atomic numbers?
Answer: Isobars is the name given to elements with the same mass number and different atomic numbers.
2. Arrange the α, β and γ radiations in ascending order of their (i) ionising power, and (ii) penetrating power.
Answer:
(i) The ascending order of ionising power is γ < β < α.
(ii) The ascending order of penetrating power is α < β < γ.
3. State the speed of each of α, β and γ radiations.
Answer: The speed of α-particles is nearly 10⁷ m s⁻¹.
The speed of β-particles is about 90% of the speed of light or 2.7 x 10⁸ m s⁻¹.
The speed of γ-radiations is 3 x 10⁸ m s⁻¹ (in vacuum).
4. A certain radioactive nucleus emits a particle that leaves its mass number unchanged, but increases its atomic number by one. Identify the particle and write its symbol.
Answer: The particle is a beta particle. Its symbol is ⁰₋₁e or ⁰₋₁β.
5. What happens to the (i) atomic number, (ii) mass number of the nucleus of an element when (a) an α-particle, (b) a β-particle, and (c) γ radiation, is emitted?
Answer: (i) Atomic number:
(a) When an α-particle is emitted, the atomic number decreases by 2.
(b) When a β-particle is emitted, the atomic number increases by 1.
(c) When γ radiation is emitted, there is no change in the atomic number.
(ii) Mass number:
(a) When an α-particle is emitted, the mass number decreases by 4.
(b) When a β-particle is emitted, there is no change in the mass number.
(c) When γ radiation is emitted, there is no change in the mass number.
6. State whether the following nuclear disintegrations are allowed or not (star indicates an excited state). Give reason if it is not allowed.
(a) ᴬzX* → ᴬzX + γ
(b) ᴬzX → ᴬz₋₂X + ⁴₂He
Answer:
(a) ᴬzX* → ᴬzX + γ is allowed. This represents gamma emission where an excited nucleus comes to its ground state by emitting a gamma ray, with no change in mass number A or atomic number Z.
(b) ᴬzX → ᴬz₋₂X + ⁴₂He is not allowed. The daughter nucleus should be ᴬ⁻⁴z₋₂Y (a new element) if an alpha particle (⁴₂He) is emitted. The given equation shows the same element X with a changed atomic number, which is incorrect for alpha decay. The mass number of the daughter nucleus should also decrease by 4.
7. Complete the following sentences :
(a) The mass number and atomic number of an element are not changed when it emits ……….
(b) The atomic number of a radioactive element is not changed when it emits ……….
(c) During the emission of a beta particle, the ………. number remains same.
Answer:
(a) The mass number and atomic number of an element are not changed when it emits γ radiations.
(b) The atomic number of a radioactive element is not changed when it emits γ radiations.
(c) During the emission of a beta particle, the mass number remains same.
8. Complete the following nuclear changes :
(a) ᴬzP → …..Q + ⁰₋₁β
(b) ²³⁸₉₂U → ²³⁴₉₀Th + ….. + energy
(c) ²³⁸₉₂P → …..Q → …..R → …..S (α, β, β)
(d) ᴬzX → …..X₁ → …..X₂ → …..X₃ (α, γ, 2β)
(e) ᴬzX → …..X₁ → …..X₂ → …..X₃ (β, α, α)
Answer:
(a) ᴬzP → ᴬz₊₁Q + ⁰₋₁β
(b) ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He + energy
(c) ²³⁸₉₂P →(α)→ ²³⁴₉₀Q →(β)→ ²³⁴₉₁R →(β)→ ²³⁴₉₂S
(d) ᴬzX →(α)→ ᴬ⁻⁴z₋₂X₁ →(γ)→ ᴬ⁻⁴z₋₂X₂ →(2β)→ ᴬ⁻⁴zX₃
(e) ᴬzX →(β)→ ᴬz₊₁X₁ →(α)→ ᴬ⁻⁴z₋₁X₂ →(α)→ ᴬ⁻⁸z₋₃X₃
9. Why do we usually use isotopes emitting gamma radiations as radioactive tracers in medical science ? [Hint : gamma radiations are most penetrating]
Answer: We usually use isotopes emitting gamma radiations as radioactive tracers in medical science because gamma radiations are most penetrating and can be detected outside the body after being administered.
10. Which of the following is the radio isotope in each pair (a), (b) and (c) ?
(a) ¹²₆C, ¹⁴₆C (b) ³⁰₁₅P, ³²₁₅P (c) ³⁹₁₉K, ⁴⁰₁₉K
Give reason for your answer.
Answer:
(a) ¹⁴₆C is the radioisotope. Reason: In ¹⁴₆C, the number of neutrons (8) is more than the number of protons (6), making it unstable. ¹²₆C has an equal number of protons and neutrons (6 each) and is stable.
(b) ³²₁₅P is the radioisotope. Reason: In ³²₁₅P, the number of neutrons (17) is more than the number of protons (15), making it unstable. ³⁰₁₅P has an equal number of protons and neutrons (15 each) and is generally stable (though ³¹P is the most common stable isotope, the question implies ³⁰P is stable for comparison).
(c) ⁴⁰₁₉K is the radioisotope. Reason: In ⁴⁰₁₉K, the number of neutrons (21) is significantly more than the number of protons (19), making it unstable. ³⁹₁₉K has 20 neutrons and 19 protons and is a stable isotope.
Short Answer Type Questions
1. Name the three constituents of an atom and state mass and charge of each. How are they distributed in an atom?
Answer: The three main constituents of an atom are electrons, protons, and neutrons.
- Electron (e):
- Charge: -1.6 x 10⁻¹⁹ C (or -e)
- Mass: 9.1 x 10⁻³¹ kg
- Proton (p):
- Charge: +1.6 x 10⁻¹⁹ C (or +e)
- Mass: 1.67 x 10⁻²⁷ kg (approx.)
- Neutron (n):
- Charge: zero
- Mass: 1.67 x 10⁻²⁷ kg (approx.)
The protons and neutrons reside inside the nucleus of the atom which is at its centre, while the electrons revolve around the nucleus in some specific orbits or stationary shells.
2. Define the following terms :
(a) atomic number, and (b) mass number.
Answer:
(a) Atomic number (Z): The atomic number of an atom is equal to the number of protons in its nucleus (which is same as the number of electrons in a neutral atom).
(b) Mass number (A): The mass number of an atom is equal to the total number of nucleons (i.e., the sum of the number of protons and the number of neutrons) in its nucleus.
3. State the atomic number and mass number of ²³₁₁Na and draw its atomic model.
Answer: For ²³₁₁Na:
Atomic number (Z) = 11
Mass number (A) = 23
The atomic model of sodium (²³₁₁Na) shows a nucleus at the center containing 11 protons and (23-11) = 12 neutrons. There are 11 electrons distributed in shells around the nucleus: 2 electrons in the K shell, 8 electrons in the L shell, and 1 electron in the M shell.
4. What are isotopes ? Give one example.
Answer: Isotopes are atoms of the same element, having the same atomic number Z, but different mass number A.
Example: Hydrogen has three isotopes: protium (¹₁H), deuterium (²₁H), and tritium (³₁H).
5. What are isobars ? Give one example.
Answer: Isobars are atoms of different elements which have the same mass number A, but different atomic number Z.
Example: ²³₁₁Na and ²³₁₂Mg are isobars.
6. Name the atoms of a substance having same atomic number, but different mass numbers. Give one example of such a substance. How do the structures of such atoms differ ?
Answer: The atoms of a substance having the same atomic number but different mass numbers are called isotopes.
Example: Carbon has isotopes ¹²₆C, ¹³₆C, and ¹⁴₆C.
The structures of such atoms differ in the number of neutrons in their nucleus. For example, ¹²₆C has 6 protons and 6 neutrons, ¹³₆C has 6 protons and 7 neutrons, and ¹⁴₆C has 6 protons and 8 neutrons. The number of protons and electrons remains the same for all isotopes of an element.
7. What is meant by radioactivity ? Name two radioactive substances.
Answer: Radioactivity is a nuclear phenomenon. It is the process of spontaneous emission of α or β and γ radiations from the nucleus of atoms during their decay.
Two radioactive substances are uranium and radium.
8. A radioactive substance is oxidised. What changes would you expect to take place in the nature of radioactivity ? Explain your answer.
Answer: No changes would be expected to take place in the nature of radioactivity. Radioactivity is a nuclear phenomenon and is not affected by chemical changes like oxidation, or physical changes such as temperature and pressure.
9. Explain why alpha and beta particles are deflected in an electric or a magnetic field, but gamma rays are not deflected in such a field. [Hint : alpha and beta particles are charged, but gamma rays are uncharged]
Answer: Alpha particles are positively charged (helium nuclei) and beta particles are negatively charged (fast-moving electrons). Due to their charge, they experience a force when moving through an electric or magnetic field, causing them to deflect. Gamma rays are electromagnetic radiations and are uncharged; therefore, they are not deflected by electric or magnetic fields.
10. Is it possible to deflect γ radiations in a way similar to α and β-particles, using the electric or magnetic field? Give reason.
Answer: No, it is not possible to deflect γ radiations using an electric or magnetic field in a way similar to α and β-particles. The reason is that γ radiations are uncharged electromagnetic waves, whereas α and β-particles are charged particles. Charged particles experience a force in electric and magnetic fields, leading to deflection, while uncharged radiations do not.
11. (a) What is the composition of α, β and γ radiations? (b) Can a hydrogen (¹₁H) nucleus emit an alpha particle? Give a reason for your answer. (c) Which one α, β or γ radiation has the least penetrating power?
Answer:
(a) Composition:
* α-radiation consists of alpha particles, which are helium nuclei (2 protons and 2 neutrons).
* β-radiation consists of beta particles, which are fast-moving electrons emitted from the nucleus.
* γ-radiation consists of electromagnetic waves (photons) of very short wavelength.
(b) No, a hydrogen (¹₁H) nucleus cannot emit an alpha particle. An alpha particle consists of 2 protons and 2 neutrons. A hydrogen nucleus (protium) consists of only 1 proton and no neutrons. Therefore, it does not have the constituents to form an alpha particle.
(c) α radiation has the least penetrating power.
12. An α-particle captures (i) one electron, (ii) two electrons. In each case, what does it change to ?
Answer:
(i) When an α-particle (He²⁺) captures one electron, it changes to a singly ionised helium atom (He⁺).
(ii) When an α-particle (He²⁺) captures two electrons, it changes to a neutral helium atom (He).
13. ‘Radioactivity is a nuclear phenomenon’. Comment on this statement.
Answer: Radioactivity is a nuclear phenomenon because it involves the spontaneous disintegration of the nucleus of an atom. The rate of radioactive decay and the nature of radiations emitted are not affected by physical changes (like temperature, pressure) or chemical changes (like oxidation, reaction with acids/alkalis). These external changes primarily affect the orbital electrons, not the nucleus. This clearly shows that radioactivity originates from the nucleus.
14. What happens to the position of an element in the periodic table when its nucleus emits (a) an α-particle, (b) a β-particle and (c) γ radiation ? Give reason for your answer.
Answer:
(a) When a nucleus emits an α-particle, its atomic number Z decreases by 2. Therefore, the element shifts two places to the left (earlier) in the periodic table.
(b) When a nucleus emits a β-particle, its atomic number Z increases by 1. Therefore, the element shifts one place to the right (later) in the periodic table.
(c) When a nucleus emits γ radiation, there is no change in its atomic number Z. Therefore, the position of the element in the periodic table remains unchanged.
15. What changes occur in the nucleus of a radioactive element when it emits (a) an alpha particle, (b) a beta particle, (c) gamma radiation ? Give one example, in each case (a) and (b) in support of your answer.
Answer:
(a) When a nucleus emits an alpha particle (⁴₂He), its mass number A decreases by 4 and its atomic number Z decreases by 2. A new element is formed.
Example: When uranium-238 (²³⁸₉₂U) emits an α-particle, it transforms into thorium-234 (²³⁴₉₀Th):
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
(b) When a nucleus emits a beta particle (⁰₋₁e), its mass number A remains unchanged, but its atomic number Z increases by 1. This occurs when a neutron in the nucleus converts into a proton and an electron (which is emitted as a β-particle). A new element is formed.
Example: When carbon-14 (¹⁴₆C) emits a β-particle, it transforms into nitrogen-14 (¹⁴₇N):
¹⁴₆C → ¹⁴₇N + ⁰₋₁e
(c) When a nucleus emits gamma radiation (γ), there is no change in its mass number A or atomic number Z. The nucleus transitions from an excited state to a lower energy state (or ground state) by releasing energy in the form of gamma rays. The element remains the same.
16. A nucleus ᴬzX emits 2 α particles and 1 β particle to form a nucleus ²²²₈₅R. Find the atomic number and mass number of X.
Answer:
Given:
Parent Nucleus = ᴬzX
Daughter Nucleus = ²²²₈₅R
Emitted particles = 2 α (alpha) particles and 1 β (beta) particle.
To find:
The atomic number (Z) of nucleus X.
The mass number (A) of nucleus X.
Solution:
The process involves a series of nuclear decays. We can represent the overall reaction as an equation. First, let’s define the particles involved.
An alpha (α) particle is a helium nucleus, represented as ⁴₂He. It has a mass number of 4 and an atomic number of 2.
A beta (β) particle is an electron, represented as ⁰₋₁e. It has a mass number of 0 and an atomic number of -1.
The nuclear reaction can be written as:
ᴬzX -> ²²²₈₅R + 2(⁴₂α) + 1(⁰₋₁β)
According to the law of conservation of mass and charge, the total mass number and total atomic number must be the same on both sides of the reaction.
1. Conservation of Mass Number (A):
The sum of the mass numbers on the left side must equal the sum of the mass numbers on the right side.
A = (Mass number of R) + 2 × (Mass number of α) + 1 × (Mass number of β)
=> A = 222 + 2(4) + 1(0)
=> A = 222 + 8 + 0
=> A = 230
2. Conservation of Atomic Number (Z):
The sum of the atomic numbers on the left side must equal the sum of the atomic numbers on the right side.
Z = (Atomic number of R) + 2 × (Atomic number of α) + 1 × (Atomic number of β)
=> Z = 85 + 2(2) + 1(-1)
=> Z = 85 + 4 – 1
=> Z = 89 – 1
=> Z = 88
Therefore, the atomic number of nucleus X is 88 and the mass number is 230.
17. What are radio isotopes ? Give one example of a radio isotope. State one use of radio isotopes.
Answer: Radio isotopes are isotopes of some elements (even those with Z < 82) that are radioactive. The nucleus of an atom becomes radioactive when the number of neutrons in the nucleus exceeds the number of protons inside it significantly.
Example: Cobalt-60 (⁶⁰₂₇Co) is a radioisotope.
One use of radioisotopes: Gamma radiations from cobalt-60 (⁶⁰Co) are used to treat cancer by killing the cells in the malignant tumour of the patient (radio-therapy).
18. Why are the alpha particles not used in radio therapy ? [Hint : alpha particles cannot penetrate the human skin]
Answer: Alpha particles are not used in radiotherapy primarily because they have very low penetrating power. They can be stopped by a thin sheet of paper or even the outer layer of human skin. Therefore, they cannot reach and treat tumours or cancerous cells located deep within the body.
19. When does the nucleus of an atom tend to become radioactive ?
Answer: The nucleus of an atom tends to become radioactive when the number of neutrons in the nucleus exceeds the number of protons inside it to a significant extent, leading to instability. For elements with atomic number higher than 82, the isotopes are generally radioactive because the number of neutrons is much more than the number of protons.
20. State the medical use of radioactivity.
Answer: One medical use of radioactivity is in radiotherapy, where gamma radiations from radioisotopes like cobalt-60 are used to treat cancer by killing malignant tumor cells. Another medical use is for diagnosis, where weak radioactive isotopes (tracers) like radio-sodium chloride are used to detect brain tumours and blood clots or to study blood circulation (radio cardiology).
21. Arrange the α, β and γ radiation in ascending order of their biological damage. Give reason.
Answer: The ascending order of biological damage for external exposure is generally α < β < γ.
Reason:
- α-particles have the least penetrating power and are stopped by the skin, causing less damage internally from external sources. However, if ingested or inhaled, they cause severe localized damage due to their high ionising power.
- β-particles are more penetrating than α-particles and can penetrate the skin, causing more biological damage than external α-particles.
- γ-radiations are highly penetrating and can pass through the human body, causing immense biological damage to tissues and cells throughout the body.
22. Name two main sources of nuclear radiations. How are the nuclear radiations harmful ?
Answer: Two main sources of harmful nuclear radiations are:
- Radioactive fall out from nuclear power plants (e.g., during accidents).
- Nuclear waste from used fuel rods in nuclear reactors.
Nuclear radiations are harmful because they interact with living tissues and can cause biological damage. This damage can manifest as short-term effects like diarrhea and hair loss, long-term irrecoverable effects like leukemia and cancer, and genetic effects that appear in later generations. They can damage DNA and RNA in living cells.
23. State two safety measures to be taken while establishing a nuclear power plant.
Answer: Two safety measures to be taken while establishing a nuclear power plant are:
(i) Ensure that the people working in it are not exposed to nuclear radiations and in case of any accident there is a minimum spread of radiations.
(ii) The nuclear reactor of the power plant must be shielded with lead and steel walls so as to stop radiations from escaping out to the environment during its normal operation.
24. What is meant by nuclear waste ? State one way for the safe disposal of nuclear waste.
Answer: Nuclear waste is the radioactive material after its use, such as fuel rods rejected from nuclear power reactors when their activity decreases below a certain level, or radioactive materials from laboratories and hospitals.
One way for the safe disposal of nuclear waste is to first keep it in thick casks and then bury these casks in specially constructed deep underground stores, made quite far from populated areas, or in useless mines which are then sealed.
25. State three safety precautions that you would take while handling the radioactive substances.
Answer: Three safety precautions to take while handling radioactive substances are:
(i) They should put on special lead-lined aprons and lead gloves.
(ii) They should handle the radioactive materials with long lead tongs.
(iii) The radioactive substance must be kept in a thick lead container with a very narrow opening to ensure radiations only come out through the opening.
26. Why should a radioactive substance not be touched by hand ?
Answer: A radioactive substance should not be touched by hand because the radiations emitted (alpha, beta, gamma) can cause biological damage to living cells and tissues. Direct contact can lead to harmful effects, including skin damage, and increase the risk of conditions like cancer.
27. What do you mean by background radiations ? Name its two sources. Is it possible for us to keep ourselves away from it ?
Answer: Background radiations are the radioactive radiations (such as α, β and γ) to which we all are exposed, even in the absence of a visible radioactive source.
Its two types of sources are:
- The internal source: radioactive substances such as potassium (K-40), carbon (C-14), and radium present inside our body.
- The external source: cosmic rays, naturally occurring radioactive elements such as radon-222, and solar radiations.It is not possible for us to keep ourselves completely away from background radiations.
Long Answer Type Questions
1. What is nucleus of an atom ? Compare its size with that of the atom. Name its constitutents. How is the number of these constituents determined by the atomic number and mass number of the atom ?
Answer: The nucleus is at the centre of an atom.
Its size is of the order of 10⁻¹⁵ m to 10⁻¹⁴ m, which is about 10⁻⁵ to 10⁻⁴ times the size of the atom (the atom’s size is of the order of 10⁻¹⁰ m).
The constituents of the nucleus are protons and neutrons (collectively called nucleons).
The number of these constituents is determined as follows:
- The number of protons in the nucleus is equal to the atomic number (Z) of the atom.
- The number of neutrons in the nucleus is equal to the difference between the mass number (A) and the atomic number (Z), i.e., Number of neutrons = A – Z.
The total number of nucleons (protons + neutrons) is equal to the mass number (A) of the atom.
2. A radioactive source emits three type of radiations.
(a) Name the three radiations.
(b) Name the radiations which are deflected by the electric field.
(c) Name the radiation which is most penetrating.
(d) Name the radiation which travels with the speed of light.
(e) Name the radiation which has the highest ionising power.
(f) Name the radiation consisting of the same kind of particles as the cathode rays.
Answer: (a) The three radiations are alpha (α) radiations, beta (β) radiations, and gamma (γ) radiations.
(b) Alpha (α) radiations and beta (β) radiations are deflected by the electric field.
(c) Gamma (γ) radiation is the most penetrating.
(d) Gamma (γ) radiation travels with the speed of light (3 x 10⁸ m s⁻¹ in vacuum). Beta (β) particles travel at speeds up to about 90% of the speed of light.
(e) Alpha (α) radiation has the highest ionising power.
(f) Beta (β) radiation consists of the same kind of particles (electrons) as cathode rays.
3. A radioactive source emits three type of radiations.
(a) Name the radiation of zero mass.
(b) Name the radiation which has the lowest ionising power.
(c) Name the radiation which has the lowest penetrating power.
(d) Give the charge and mass of particles composing the radiation in part (c).
(e) When the particle referred to in part (c) becomes neutral, it is found to be the atom of a rare gas. Name this rare gas and draw a model of its neutral atom.
Answer: (a) Gamma (γ) radiation has zero rest mass.
(b) Gamma (γ) radiation has the lowest ionising power.
(c) Alpha (α) radiation has the lowest penetrating power.
(d) The particles composing alpha (α) radiation (referred to in part c) are alpha particles.
* Charge of an alpha particle: +3.2 x 10⁻¹⁹ C (or +2e, twice the charge of a proton).
* Mass of an alpha particle: Approximately 6.68 x 10⁻²⁷ kg (four times the mass of a proton).
(e) The particle referred to in part (c) is an alpha particle (He²⁺). When it becomes neutral by gaining two electrons, it forms a helium atom (He). Helium is a rare gas.
A model of a neutral helium atom shows a nucleus at the center containing 2 protons and 2 neutrons, with 2 electrons revolving around the nucleus in the K shell.
4. The diagram shows a radioactive source S placed in a thick lead walled container. The radiations given out are allowed to pass through a magnetic field. The magnetic field (shown as x) acts perpendicular to the plane of paper inwards. Arrows show the paths of the radiations A, B and C.
(a) Name the radiations labelled A, B and C.
(b) Explain clearly how you used the diagram to arrive at the answer in part (a).
Answer: (a) Radiation A is beta (β) particles.
Radiation B is gamma (γ) radiations.
Radiation C is alpha (α) particles.
(b) The magnetic field is directed inwards, perpendicular to the plane of the paper.
- Radiation C deflects to the left. Using Fleming’s left-hand rule, if the force is to the left and the field is inwards, the current (or direction of positive charge) must be upwards. This indicates that radiation C consists of positively charged particles, which are alpha particles.
- Radiation A deflects to the right, which is more than the deflection of C. This indicates that radiation A consists of negatively charged particles. Since the deflection is opposite to that of positive charges and more pronounced (suggesting lighter particles), these are beta particles.
- Radiation B passes undeviated. This indicates that radiation B is uncharged, which are gamma radiations.
5. The figure shows a mixed source R of alpha and beta particles in a thick lead walled container. The particles pass through a magnetic field in a direction perpendicular to the plane of paper inwards as shown by x. (a) Show in the diagram how the particles get affected. (b) Name the law used in part (a).
[Hint : alpha particles will deflect to the left while beta particles to the right]
Answer: (a) In the diagram , when the mixed source R emits alpha and beta particles into the magnetic field directed inwards:
The alpha (α) particles, being positively charged, will deflect towards the left. The beta (β) particles, being negatively charged, will deflect towards the right. The deflection of beta particles will be more than that of alpha particles because beta particles are much lighter.
(b) The law used to determine the direction of deflection of charged particles in a magnetic field is Fleming’s Left-Hand Rule.
6. The figure shows a radioactive source S in a thick lead walled container having a narrow opening. The radiations pass through an electric field between the plates A and B.
(a) Complete the diagram to show the paths of α, β and γ radiations.
(b) Why is the source S kept in a thick lead walled container with a narrow opening ?
(c) Name the radiation which is unaffected by the electrostatic field.
(d) Which radiation is deflected the most. Give reason.
(e) Which among the three radiations causes the least biological damage?
[Hint : α radiations will deflect towards the negative plate, β radiations towards the positive plate and γ radiations remain undeflected (c) γ radiation (d) beta, because of its low mass (e) alpha radiation]
Answer: (a) In the completed diagram, with plate A being positive and plate B being negative (assuming standard convention, or as indicated if plates are marked):
* Alpha (α) radiations, being positively charged, will deflect towards the negative plate (plate B).
* Beta (β) radiations, being negatively charged, will deflect towards the positive plate (plate A). The deflection of β particles will be greater than that of α particles.
* Gamma (γ) radiations, being uncharged, will pass undeviated through the electric field.
(b) The source S is kept in a thick lead-walled container to absorb most of the harmful radiations emitted in all directions, allowing only a fine beam to emerge through the narrow opening for experimental purposes. This ensures safety and a collimated beam of radiation.
(c) Gamma (γ) radiation is unaffected by the electrostatic field.
(d) Beta (β) radiation is deflected the most. Reason: Beta particles have a much smaller mass compared to alpha particles, and for a given charge and field strength, lighter particles experience greater acceleration and hence greater deflection. (Specific charge e/m is much higher for beta particles).
(e) Alpha (α) radiation causes the least biological damage when considering external exposure and penetration, as it is easily stopped by the skin. (However, if ingested or inhaled, it is very damaging due to high ionisation). The hint provided suggests alpha radiation for least biological damage in this context.
7. State following four properties each of α, β and γ radiations: (a) nature, (b) charge, (c) mass, and (d) effect of electric field.
Answer: α-radiations:
(a) Nature: Stream of positively charged particles, i.e., helium nuclei (2 protons and 2 neutrons).
(b) Charge: Positive charge, +3.2 x 10⁻¹⁹ C (or +2e).
(c) Mass: Rest mass is four times the mass of a proton, i.e., 6.68 x 10⁻²⁷ kg.
(d) Effect of electric field: Deflected towards the negative plate.
β-radiations:
(a) Nature: Stream of negatively charged particles, i.e., energetic electrons emitted from the nucleus.
(b) Charge: Negative charge, -1.6 x 10⁻¹⁹ C (or -e).
(c) Mass: Rest mass is equal to the mass of an electron, i.e., 9.1 x 10⁻³¹ kg.
(d) Effect of electric field: Deflected towards the positive plate; deflection is more than that of alpha particles.
γ-radiations:
(a) Nature: Highly energetic electromagnetic radiations (photons).
(b) Charge: No charge (neutral).
(c) Mass: No rest mass (rest mass is zero).
(d) Effect of electric field: Unaffected (pass undeviated).
8. How are γ radiations produced ? Mention two common properties of the gamma radiations and visible light.
Answer: γ radiations are produced when a nucleus is in an excited state (i.e., it has an excess of energy), often after an alpha or beta emission. This extra energy is released in the form of electromagnetic radiation known as γ-radiation (or γ-ray photon) as the nucleus transitions to a lower energy state or its ground state.
Two common properties of gamma radiations and visible light are:
- Both are electromagnetic waves.
- Both travel with the same speed in vacuum (or air), which is 3 x 10⁸ m s⁻¹.
9. What kind of change takes place in a nucleus when a β-particle is emitted ? Express it by an equation. State whether (a) atomic number, and (b) mass number are conserved in a radioactive β-decay ?
Answer: When a β-particle is emitted from a nucleus, a neutron within the nucleus converts into a proton, and an electron (the β-particle) is created and emitted. The change can be represented by the equation:
¹₀n → ¹₁p + ⁰₋₁e (+ anti-neutrino)
If a parent nucleus ᴬzP emits a β-particle, it changes to a daughter nucleus ᴬz₊₁Q:
ᴬzP → ᴬz₊₁Q + ⁰₋₁e
(a) Atomic number (Z) is not conserved for the parent nucleus alone; it increases by 1 for the daughter nucleus. However, the total charge is conserved in the reaction (Z = (Z+1) + (-1)).
(b) Mass number (A) is conserved. The mass number of the parent nucleus is the same as the mass number of the daughter nucleus.
10. (a) An atomic nucleus A is composed of 84 protons and 128 neutrons. The nucleus A emits an α-particle and is transformed into a nucleus B. What is the composition of B ?
(b) The nucleus B emits a β-particle and is transformed into a nucleus C. What is the composition of C ?
(c) What is the mass number of the nucleus A ?
(d) Does the composition of nucleus C change if it emits the γ radiation?
Answer: (a) Nucleus A has 84 protons and 128 neutrons.
Atomic number of A (Zᴬ) = 84.
Mass number of A (Aᴬ) = 84 (protons) + 128 (neutrons) = 212.
When A emits an α-particle (⁴₂He), its atomic number decreases by 2 and mass number decreases by 4.
So, nucleus B will have:
Protons in B = 84 – 2 = 82 protons.
Neutrons in B = Mass number of B – Protons in B = (212 – 4) – 82 = 208 – 82 = 126 neutrons.
Composition of B: 82 protons and 126 neutrons.
(b) Nucleus B has 82 protons and 126 neutrons.
When B emits a β-particle (⁰₋₁e), its atomic number increases by 1, and its mass number remains unchanged. A neutron converts to a proton.
So, nucleus C will have:
Protons in C = 82 + 1 = 83 protons.
Neutrons in C = (Number of neutrons in B – 1) = 126 – 1 = 125 neutrons. (Alternatively, Mass number of C = Mass number of B = 208. Neutrons in C = 208 – 83 = 125).
Composition of C: 83 protons and 125 neutrons.
(c) The mass number of nucleus A is 84 (protons) + 128 (neutrons) = 212.
(d) No, the composition (number of protons and neutrons) of nucleus C does not change if it emits γ radiation. Gamma emission only involves the release of excess energy from the nucleus; it does not alter the number of protons or neutrons.
11. A certain nucleus A (mass number 238 and atomic number 92) is radioactive and becomes a nucleus B (mass number 234 and atomic number 90) by the emission of a particle.
(a) Name the particle emitted.
(b) Explain how you arrived at your answer.
(c) State the change in the form of a reaction.
Answer: (a) The particle emitted is an alpha (α) particle.
(b) The mass number of nucleus A is 238 and it changes to 234 for nucleus B. The decrease in mass number is 238 – 234 = 4.
The atomic number of nucleus A is 92 and it changes to 90 for nucleus B. The decrease in atomic number is 92 – 90 = 2.
An alpha particle (⁴₂He) has a mass number of 4 and an atomic number of 2. The observed changes (decrease of 4 in mass number and decrease of 2 in atomic number) correspond exactly to the emission of an alpha particle.
(c) The change in the form of a reaction is:
²³⁸₉₂A → ²³⁴₉₀B + ⁴₂He (or ⁴₂α)
12. A nucleus ²⁴₁₁Na is β-radioactive.
(a) What are the numbers 24 and 11 called ?
(b) Write the equation representing β-decay.
(c) What general name is given to the product nucleus with respect to ²⁴₁₁Na?
Answer: (a) In ²⁴₁₁Na:
The number 24 is called the mass number (A).
The number 11 is called the atomic number (Z).
(b) The equation representing β-decay of ²⁴₁₁Na is:
²⁴₁₁Na → ²⁴₁₂Mg + ⁰₋₁e (+ anti-neutrino)
(c) The product nucleus (²⁴₁₂Mg) has the same mass number (24) as the parent nucleus (²⁴₁₁Na) but a different atomic number (12 for Mg, 11 for Na). Therefore, the product nucleus is an isobar of the parent nucleus.
13. A nucleus of stable phosphorus has 15 protons and 16 neutrons.
(a) What is its atomic number and mass number ?
(b) The nucleus of radio phosphorus has one neutron more than the stable nucleus. What will be its atomic number and mass number ?
(c) What will be the atomic number and mass number of new nucleus formed by the decay of a β-particle by the radio phosphorus in part (b) ?
Answer: (a) For stable phosphorus:
Number of protons = 15. So, atomic number (Z) = 15.
Number of neutrons = 16.
Mass number (A) = Number of protons + Number of neutrons = 15 + 16 = 31.
So, its atomic number is 15 and mass number is 31 (³¹₁₅P).
(b) For radio phosphorus:
It has one neutron more than the stable nucleus.
Number of protons remains the same for an isotope, so atomic number (Z) = 15.
Number of neutrons = 16 (stable) + 1 = 17.
Mass number (A) = Number of protons + Number of neutrons = 15 + 17 = 32.
So, its atomic number will be 15 and mass number will be 32 (³²₁₅P).
(c) The radio phosphorus (³²₁₅P) decays by emitting a β-particle.
In β-decay, the mass number remains the same, and the atomic number increases by 1.
So, for the new nucleus formed:
Mass number = 32 (same as radio phosphorus).
Atomic number = 15 (of radio phosphorus) + 1 = 16.
The new nucleus will have atomic number 16 and mass number 32 (³²₁₆S, which is Sulphur).
14. An element P disintegrates by α emission and the new element suffers two further disintegrations, both by β emission, to form an element Q. Explain the fact that P and Q are isotopes.
Answer: Let element P have atomic number Z and mass number A, represented as ᴬzP.
- P disintegrates by α emission:
ᴬzP → ᴬ⁻⁴z₋₂X + ⁴₂He (α-particle)
The new element formed is X, with atomic number Z-2 and mass number A-4. - Element X suffers two further disintegrations, both by β emission:
First β emission from X:
ᴬ⁻⁴z₋₂X → ᴬ⁻⁴z₋₂₊₁Y + ⁰₋₁e (β-particle)
So, element Y has atomic number Z-1 and mass number A-4.
Second β emission from Y to form Q:
ᴬ⁻⁴z₋₁Y → ᴬ⁻⁴z₋₁₊₁Q + ⁰₋₁e (β-particle)
So, element Q has atomic number Z and mass number A-4.
Comparing element P (ᴬzP) and element Q (ᴬ⁻⁴zQ):
- Atomic number of P = Z
- Atomic number of Q = Z
- Mass number of P = A
- Mass number of Q = A-4
Since P and Q have the same atomic number (Z) but different mass numbers (A and A-4 respectively), they are isotopes of the same element.
Exercise (B)
MCQs
1. 1 amu is equivalent to :
(a) 931 eV
(b) 9.31 MeV
(c) 931 MeV
(d) 931 keV
Answer: (c) 931 MeV
2. The mass of an electron is :
(a) 1.6725 × 10⁻²⁷
(b) 1.6725 × 10⁻³¹
(c) 1.6748 × 10⁻³¹
(d) 9.1 x 10⁻³¹
Answer: (d) 9.1 x 10⁻³¹
3. The particle used in nuclear fission for bombardment is :
(a) α particle
(b) proton
(c) β particle
(d) neutron
Answer: (d) neutron
4. According to the mass energy equivalence :
(a) E = Δmc²
(b) E = mc
(c) E = Δm/c²
(d) E = m/c²
Answer: (a) E = Δmc²
5. In each fission reaction, the __________ remains conserved.
(a) mass number
(b) atomic number
(c) mass
(d) both (a) and (b)
Answer: (d) both (a) and (b)
6. Which of the following is used as moderator(s) in a nuclear reactor ?
(a) Cadmium rods
(b) Graphite
(c) Heavy water
(d) Both (b) and (c)
Answer: (d) Both (b) and (c)
7. The source of energy of the sun and stars is obtained from the nuclear fusion of :
(a) uranium
(b) thorium
(c) hydrogen
(d) ²³⁵₉₂U
Answer: (c) hydrogen
8. The temperature required for the process of nuclear fusion is nearly :
(a) 1000 K
(b) 10⁴ K
(c) 10⁵ K
(d) 10⁷ K
Answer: (d) 10⁷ K
Very Short Answer Questions
1. Complete the following nuclear fission reactions:
(a) ²³⁵₉₂U + ¹₀n →
(b) ²³⁵₉₂U + ¹₀n →
Answer:
(a) ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + energy
(b) ²³⁵₉₂U + ¹₀n → ¹⁴⁸₅₇La + ⁸⁵₃₅Br + 3¹₀n + energy
2. Complete the following fusion reactions:
(a) ³₂He + ²₁H →
(b) ²₁H + ²₁H →
Answer:
(a) ³₂He + ²₁H → ⁴₂He + ¹₁H + energy
(b) ²₁H + ²₁H → ³₂He + ¹₀n + energy
3. (a) Name the process, nuclear fission or nuclear fusion, in which the energy released per unit mass is more? (b) Name the process, fission or fusion which is possible at ordinary temperature.
Answer:
(a) The process in which the energy released per unit mass is more is nuclear fusion.
(b) The process which is possible at ordinary temperature is nuclear fission.
4. What is the source of energy of sun or stars?
Answer: The source of energy of the sun or stars is nuclear fusion.
5. Name the following nuclear reactions:
(a) ²³⁵₉₂U + ¹₀n → ⁹⁰₃₈Sr + ¹⁴³₅₄Xe + 3 ¹₀n + γ
(b) ³₁H + ²₁H → ⁴₂He + ¹₀n + γ
Answer:
(a) The reaction ²³⁵₉₂U + ¹₀n → ⁹⁰₃₈Sr + ¹⁴³₅₄Xe + 3 ¹₀n + γ is fission.
(b) The reaction ³₁H + ²₁H → ⁴₂He + ¹₀n + γ is fusion.
Short Answer Type Questions
1. What do you mean by nuclear energy? What is responsible for its release?
Answer: In a nuclear change due to a radioactive phenomenon such as decay, fission or fusion, the total sum of masses of product nuclei is always less than the total sum of the masses of reactant nuclei. Thus, there is a loss in nuclear mass. In 1905, on the basis of theory of relativity Einstein suggested that mass and energy are interchangeable. Due to loss in mass Δm, the energy released E is given by E = (Δm)c². Here Δm is the loss in mass in kg, c is the speed of light (= 3 × 10⁸ m s⁻¹) and E is the energy in joule (J). The energy so obtained is called the nuclear energy.
The loss in mass (Δm) during the nuclear reaction is responsible for its release, as this mass is converted into energy.
2. Write down the Einstein’s mass-energy equivalence relation, explaining the meaning of each symbol used in it.
Answer: The Einstein’s mass-energy equivalence relation is E = (Δm)c².
In this relation:
E: is the energy released, measured in joules (J).
Δm: is the loss in mass, measured in kilograms (kg).
c: is the speed of light in vacuum, which is equal to 3 × 10⁸ m s⁻¹.
3. (a) What is a.m.u? Express 1 a.m.u. in MeV. (b) Write the approximate mass of a proton, neutron and electron in a.m.u.
Answer: (a) The mass of atomic particles is expressed in atomic mass unit (a.m.u) or unified unit (u). 1 a.m.u. is defined as 1/12th the mass of one atom of carbon-12. Numerically, 1 a.m.u. = 1.66 × 10⁻²⁷ kg. 1 a.m.u. is equivalent to 931 MeV of energy.
(b) The approximate masses are:
Electron: 0.00055 a.m.u.
Proton: 1.00727 a.m.u.
Neutron: 1.00865 a.m.u.
4. What is nuclear fission? Name the substance used for it. Write one fission reaction.
Answer: Nuclear fission is the process in which a heavy nucleus splits into two lighter nuclei of nearly the same size, when bombarded with slow neutrons.
A common substance used for nuclear fission is uranium-235 (²³⁵₉₂U).
One fission reaction is:
²³⁵₉₂U + ¹₀n → (²³⁶₉₂U) → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + 3 ¹₀n + energy
5. Write the approximate value of the energy released in the fission of one nucleus of ²³⁵₉₂U. What is the reason for it?
Answer: The approximate value of the energy released in the fission of one nucleus of ²³⁵₉₂U is nearly 190 MeV.
The reason for this release of energy is that the sum of masses of the product nuclei is less than the sum of the mass of the parent nucleus and the mass of the neutron. This loss in mass is converted into energy according to Einstein’s mass-energy relation, E = (Δm)c².
6. What do you mean by the chain reaction in nuclear fission? How is it controlled?
Answer: When slow neutrons are bombarded on uranium-235 (²³⁵₉₂U), each uranium nucleus splits into two nearly equal fragments, with a release of three new neutrons and tremendous amount of energy (nearly 190 MeV). These new neutrons can cause fission in other uranium nuclei under suitable conditions. Thus, a chain of fission of nuclei is formed which once started, continues till the entire uranium is consumed. This is known as a chain reaction.
The chain reaction is controlled by absorbing some of the neutrons emitted in the fission process by means of cadmium rods and then making them slow by the moderators (such as graphite, heavy water, etc.). By controlling the number of neutrons available to cause further fissions, the rate of energy generated in fission can be controlled and utilised for constructive purposes.
7. State two uses of nuclear fission.
Answer: Two uses of nuclear fission are:
(i) For destructive use: The fission process is used in a nuclear bomb, where the energy released is fast and uncontrolled.
(ii) For constructive use: The fission process is used in a nuclear reactor where the rate of release of energy is slow and controlled. This energy is used to generate electric power.
8. Give two differences between the radioactive decay and nuclear fission.
Answer: Two differences between radioactive decay and nuclear fission are:
(i) Radioactive decay is a spontaneous process, whereas nuclear fission does not occur by itself; it is initiated when neutrons are bombarded on a heavy nucleus.
(ii) In radioactive decay, the nucleus emits either α or β particles with the emission of energy in the form of γ rays which is not very large. In nuclear fission, on bombardment of a heavy nucleus with neutrons, a tremendous amount of energy is released and the nucleus splits into two nearly equal fragments.
9. (a) What is nuclear fusion? Give one example and write its nuclear reaction. (b) What other name is given to nuclear fusion? Give reason.
Answer: (a) Nuclear fusion is the process in which two light nuclei combine to form a heavy nucleus. In this process also, a huge amount of energy is released. An example is when two deuterium nuclei (²₁H) fuse. The nuclear reaction is:
²₁H + ²₁H → ³₂He + ¹₀n + 3.3 MeV
(b) Another name given to nuclear fusion is the thermo-nuclear reaction. The reason is that when two nuclei approach each other, due to their positive charge, the electrostatic force of repulsion between them becomes so strong that they do not fuse. Hence to make the fusion possible, a high temperature (≈ 10⁷ K) and high pressure is required. At such a high temperature, both nuclei due to their thermal agitations acquire sufficient kinetic energy to overcome the force of repulsion, allowing them to get fused.
10. Why is a very high temperature required for the process of nuclear fusion? State the approximate temperature required.
Answer: A very high temperature is required for nuclear fusion because when two nuclei approach each other, their positive charges cause a strong electrostatic force of repulsion. To overcome this repulsion and allow the nuclei to fuse, they need to have very high kinetic energy. This high kinetic energy is achieved at very high temperatures, where thermal agitations are significant.
The approximate temperature required is ≈ 10⁷ K.
11. (a) Write one nuclear fusion reaction. (b) State the approximate value of energy released in the reaction mentioned in part (a). (c) Give reason for the release of energy stated in part (b).
Answer: (a) One nuclear fusion reaction is:
³₂He + ²₁H → ⁴₂He + ¹₁H + energy
(b) For the reaction ³₂He + ²₁H → ⁴₂He + ¹₁H, the energy released is 18.3 MeV.
(c) The reason for the release of energy in nuclear fusion is that the mass of the product nucleus is less than the sum of the masses of the two combining nuclei. This loss in mass is released in the form of energy according to the mass-energy equivalence relation E = (Δm)c².
12. (a) State one similarity in the process of nuclear fission and fusion. (b) State two differences between the process of nuclear fission and fusion.
Answer: (a) One similarity in the process of nuclear fission and nuclear fusion is the release of energy due to loss in mass.
(b) Two differences between nuclear fission and nuclear fusion are:
(i) In fission, when neutrons are bombarded on a heavy nucleus, it splits into two nearly equal light fragments. In fusion, at a very high temperature and high pressure, two light nuclei combine to form a heavy nucleus.
(ii) Fission is possible at ordinary temperature and ordinary pressure, while fusion is possible only at a very high temperature (≈ 10⁷ K) and a very high pressure.
13. Give two examples of nuclear fusion.
Answer: Two examples of nuclear fusion reactions are:
(i) ¹₁H + ¹₁H → ²₁H + ⁰₁e + 0.42 MeV (where ¹₁H is a proton, ²₁H is deuterium, and ⁰₁e is a positron)
(ii) ²₁H + ²₁H → ³₁H + ¹₁H + 4.0 MeV (where ³₁H is tritium)
Long Answer Type Questions
1. (a) Name two isotopes of uranium which are fissionable. (b) Which of the isotope mentioned in part (a) above is easily fissionable? Give reason. (c) State whether the neutron needed for fission reaction of the isotope mentioned in part (b) above, is slow or fast?
Answer: (a) Two isotopes of uranium which are fissionable are uranium-238 (²³⁸₉₂U) and uranium-235 (²³⁵₉₂U).
(b) The isotope uranium-235 (²³⁵₉₂U) is more easily fissionable than uranium-238 (²³⁸₉₂U). The reason is that the fission of ²³⁸₉₂U nucleus is possible only by fast neutrons, while the fission of ²³⁵₉₂U nucleus can occur even by slow neutrons.
(c) The neutron needed for the fission reaction of uranium-235 (²³⁵₉₂U) can be slow, as its fission can occur even with slow neutrons.
Numericals
1. In fission of one uranium-235 nucleus, the loss in mass is 0.2 a.m.u. Calculate the energy released.
Answer: Given that the loss in mass (Δm) = 0.2 a.m.u.
We know that 1 a.m.u. is equivalent to 931 MeV of energy.
Therefore, the energy released (E) = Δm × 931 MeV
E = 0.2 × 931 MeV
E = 186.2 MeV.
The energy released is 186.2 MeV.
2. When four hydrogen nuclei combine to form a helium nucleus in the interior of sun, the loss in mass is 0.0265 a.m.u. How much energy is released?
Answer: Given that the loss in mass (Δm) = 0.0265 a.m.u.
We know that 1 a.m.u. is equivalent to 931 MeV of energy.
Therefore, the energy released (E) = Δm × 931 MeV
E = 0.0265 × 931 MeV
E = 24.6715 MeV.
Approximately, the energy released is 24.7 MeV.
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