Nuclear fission and fusion | ICSE Class 10 Physics Notes

This note covers nuclear structure and notation, nuclear fission and fusion, representative nuclear equations, conservation of mass number and charge, conditions for fusion, nuclear energy release, and differences between the two processes.

What do nuclear symbols tell us about a nucleus?

Which particles make up a nucleus?

The nucleus is the small central part of an atom, containing its positive charge and most of its mass. The plural of nucleus is nuclei. A proton is a positively charged nuclear particle; a neutron is an electrically neutral nuclear particle.

A proton or a neutron is called a nucleon. Electrons, the negatively charged particles in an atom, are outside the nucleus. Fission and fusion involve changes in nuclei, so their equations identify the nuclei and nuclear particles taking part.

Definition: The atomic number, represented by Z, is the number of protons in a nucleus. The mass number, represented by A, is the total number of protons and neutrons in that nucleus.

If N represents the neutron number, meaning the number of neutrons, then A = Z + N. Equivalently, N = A − Z. These symbols describe particle counts. Mass number is not the measured mass of a nucleus in kilograms.

How is a nuclear symbol read?

A nuclear symbol places A at the upper left and Z at the lower left of X, where X represents the chemical symbol of the element. For example, ²³⁵₉₂U represents uranium-235: U means uranium, 235 is its mass number, and 92 is its atomic number.

The hyphenated name uranium-235 gives the element and its mass number. The complete nuclear symbol also displays the atomic number. Reading both numbers helps distinguish the size of the nucleon count from the number that identifies the element.

Isotopes are forms of an element whose nuclei have the same number of protons but different numbers of neutrons. Deuterium and tritium are hydrogen isotopes. Deuterium has one proton and one neutron; tritium has one proton and two neutrons.

The nucleus of deuterium is called a deuteron, and the nucleus of tritium is a triton. These names refer to the nuclei, rather than to complete atoms with their surrounding electrons.

What happens during nuclear fission?

Definition: Nuclear fission is the splitting of a heavy nucleus into smaller nuclear fragments, with the release of energy. In neutron-induced fission, an incoming neutron initiates the nuclear reaction.

How does uranium-235 provide an example?

A uranium-235 nucleus can undergo fission when bombarded with a neutron. Here, bombardment means directing particles at nuclei so that a nuclear reaction can occur. The neutron joins the uranium nucleus, forming uranium-236, which then splits.

One possible reaction is ¹₀n + ²³⁵₉₂U → ²³⁶₉₂U → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + 3¹₀n. Here Ba means barium, Kr means krypton, the plus sign separates participating particles, and an arrow means “changes into” or “produces”.

The first arrow shows the formation of the uranium-236 nucleus. The second shows its division into barium-144 and krypton-89, with three neutrons released. Uranium-236 is an intermediate nucleus, meaning that it forms between the starting stage and the final products.

1. An incoming neutron interacts with a uranium-235 nucleus.
2. The intermediate nucleus has mass number 236 and atomic number 92.
3. This nucleus splits into the barium and krypton nuclei shown in the equation.
4. Three neutrons emerge, and energy is released in the fission process.

What does the example establish?

The two fragments have intermediate mass numbers compared with the original heavy nucleus and very light nuclei. They are separate nuclei. The process is therefore a change in nuclear structure, rather than the removal of electrons from an otherwise unchanged nucleus.

The energy released initially appears as kinetic energy, the energy associated with motion, of the fragments and neutrons. This energy is eventually transferred to surrounding matter and appears as heat. A fission equation represents both a rearrangement of nuclear matter and an energy-releasing process.

Draw a neutron approaching a nucleus labelled ²³⁵₉₂U. Use arrows to show ²³⁶₉₂U followed by nuclei labelled ¹⁴⁴₅₆Ba and ⁸⁹₃₆Kr and three separate neutrons. Add “energy released” beside the products.

The diagram should retain the labels and neutron count of this particular equation. It represents one possible fission outcome; it does not mean that uranium-235 produces this same pair of fragments in every fission event.

How can a fission equation be checked and balanced?

What must be counted on each side?

For the fission equations here, the total mass number and total atomic number agree on the two sides. Check them separately. Mass numbers count nucleons, while atomic numbers account for the positive nuclear charge carried by the protons.

A number placed before a nuclear symbol is a coefficient: it tells how many such particles are present. Thus 3¹₀n means three neutrons. Their total contribution is three to the mass-number sum and zero to the atomic-number sum.

Worked example 1. Check ¹₀n + ²³⁵₉₂U → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + 3¹₀n using the mass and atomic numbers displayed in the equation.

Answer: The initial mass-number sum is 1 + 235 = 236. The final sum is 144 + 89 + 3 × 1 = 236. The initial atomic-number sum is 0 + 92 = 92; the final sum is 56 + 36 + 3 × 0 = 92. Both checks agree.

Including the intermediate nucleus does not change these totals. Uranium-236 contains the incoming neutron as well as the nucleons originally present in uranium-235. Its mass number is therefore 236, while its atomic number remains 92 because the incoming neutron has no charge.

Are other fragment pairs possible?

Another possible equation is ¹₀n + ²³⁵₉₂U → ²³⁶₉₂U → ¹³³₅₁Sb + ⁹⁹₄₁Nb + 4¹₀n. Sb denotes antimony and Nb denotes niobium. Here the products include antimony-133, niobium-99 and four neutrons.

A further example is ¹₀n + ²³⁵₉₂U → ¹⁴⁰₅₄Xe + ⁹⁴₃₈Sr + 2¹₀n. Xe denotes xenon and Sr denotes strontium. This equation shows xenon-140, strontium-94 and two neutrons as products.

The different neutron counts belong to different fragment pairs. Remembering three neutrons from the barium-krypton example is not sufficient for completing every uranium fission equation. Read the displayed products and use their mass numbers to find the required number of neutrons.

Note: Balancing particle counts is a check on a proposed equation. It does not establish that any pair of nuclei chosen merely to satisfy the sums is an actual fission product pair.

What happens during nuclear fusion?

Definition: Nuclear fusion is the joining of light nuclei to form a larger nucleus. The light-nucleus fusion reactions described here release energy as the resulting nuclei become more tightly bound, meaning that their nucleons are held more strongly together.

How can deuterium nuclei combine?

Two deuterium nuclei can react to form helium-3 and a neutron. Helium-3 is a helium isotope whose nucleus contains two protons and one neutron. Its symbol is ³₂He, where He denotes helium, 3 is the mass number and 2 is the atomic number.

An equation for this reaction is ²₁H + ²₁H → ³₂He + ¹₀n + 3.27 MeV. The energy term shows the energy released. MeV means megaelectronvolt, an energy unit equal to one million electronvolts.

An electronvolt, abbreviated eV, is the energy gained by an electron accelerated through a potential difference of one volt. Potential difference means energy transferred per unit charge; the volt is its unit. The energy unit MeV is convenient for describing individual nuclear reactions.

The incoming deuterons have a total of four nucleons. Three form the helium-3 nucleus and one emerges as a neutron. Fusion therefore need not place every incoming nucleon into a single final nucleus without any other particle being emitted.

Draw two approaching nuclei, each labelled ²₁H. After an arrow, draw one nucleus labelled ³₂He and one neutron labelled ¹₀n. Write “3.27 MeV released” beside the products.

What other deuterium reaction is possible?

Two deuterons can also produce a triton and a proton: ²₁H + ²₁H → ³₁H + ¹₁H + 4.03 MeV. The symbol ¹₁H represents a hydrogen nucleus consisting of one proton, with mass number 1 and atomic number 1.

This second reaction gives different products and a different energy release. The 4.03 MeV value belongs to the tritium-and-proton reaction; 3.27 MeV belongs to the helium-3-and-neutron reaction. The energy values must stay with their respective equations.

Both examples begin with light hydrogen nuclei. The formation of a larger nucleus from these light nuclei identifies the process as fusion. The simultaneous emission of a neutron or proton does not change that classification.

How are fusion equations balanced and interpreted?

How do the two deuterium reactions balance?

Apply the same two separate checks used for fission: add the mass numbers, then add the atomic numbers. Use all particles shown in the reaction, including the neutron or proton appearing alongside the larger product nucleus.

Worked example 2. Check ²₁H + ²₁H → ³₂He + ¹₀n + 3.27 MeV. All particle mass numbers and atomic numbers are displayed.

Answer: Initially, the mass-number sum is 2 + 2 = 4 and the atomic-number sum is 1 + 1 = 2. Finally, the corresponding sums are 3 + 1 = 4 and 2 + 0 = 2. Both agree. The energy term is not an extra nucleon or a contribution to the atomic-number sum.

For the reaction forming tritium and a proton, ²₁H + ²₁H → ³₁H + ¹₁H + 4.03 MeV, the mass-number check is again 2 + 2 = 3 + 1. The atomic-number check is now 1 + 1 = 1 + 1.

How can a missing particle be identified?

First find the missing mass number by subtraction. Then find the missing atomic number independently. A missing particle with mass number 1 and atomic number 0 is a neutron; one with mass number 1 and atomic number 1 is a proton.

For example, if the helium-3 product is given, the initial mass-number total is 4 and helium-3 accounts for 3. The missing particle therefore accounts for 1. The initial atomic-number total is 2 and helium-3 already accounts for 2, leaving 0 for the missing particle.

Show two nuclei labelled ²₁H on the left. After an arrow, show ³₁H and ¹₁H separately. Label the energy release 4.03 MeV and identify ¹₁H as a proton.

The nuclear label is essential: helium-3 and tritium have the same mass number but different atomic numbers. Exchanging their symbols without changing the accompanying particle would spoil the atomic-number balance even though the mass-number sums could still agree.

Why does fusion require very high temperatures?

What prevents light nuclei from joining readily?

Light nuclei carry positive electric charge. Two positively charged nuclei repel each other as they approach. This electric repulsion is called Coulomb repulsion. Fusion requires the nuclei to approach closely enough for the attractive nuclear force to act effectively.

The nuclear force is the strong, short-range force that binds nucleons in a nucleus. “Short-range” means that it acts effectively over very small nuclear distances. Bringing two nuclei together requires overcoming the barrier associated with their electric repulsion.

This repulsive energy barrier is called the Coulomb barrier. Its height depends on the charges and radii of the interacting nuclei. The nuclei need sufficient initial energy to overcome the barrier and get close enough for fusion.

What does heating achieve?

At very high temperatures, particles have high kinetic energies. When fusion is achieved by raising the temperature so that the particles have enough kinetic energy to overcome their electrical repulsion, the process is called thermonuclear fusion.

1. The approaching light nuclei repel one another because both have positive charge.
2. They must approach within the short range over which nuclear attraction is effective.
3. Very high temperature supplies the particles with the kinetic energy needed to overcome the repulsive barrier.
4. Fusion can then form more tightly bound nuclei and release energy.

The high temperature is an initial condition enabling the process. The energy released in the reaction has a different explanation: it results from the change in the nuclear system. Heating and nuclear energy release should not be treated as the same stage.

How is fusion connected with the Sun?

Thermonuclear fusion supplies energy in the interiors of stars. In the Sun, hydrogen in the core is converted into helium through a multistep process. The core means the central region. Hydrogen acts as the nuclear fuel for this process.

The conversion of hydrogen into helium in the Sun is therefore not a single collision represented by either deuterium equation above. Those equations illustrate particular light-nucleus reactions. The solar process involves a sequence of reactions whose overall effect is the conversion of hydrogen into helium.

Why can both fission and fusion release energy?

What is the connection between mass and energy?

Mass-energy equivalence means that mass has an associated energy. The relation is E = mc², where E is the energy equivalent, m is the mass, and c is the speed of light in a vacuum, meaning a region without matter.

The speed c is approximately 3 × 10⁸ metres per second. With mass in kilograms and speed in metres per second, the energy is in joules. The SI unit of energy is the joule, symbol J; SI means International System of Units.

For an energy-releasing nuclear reaction, the combined mass of the products is less than the combined mass of the starting particles. The difference corresponds to the released energy. Counting the same total number of nucleons therefore does not require identical total masses before and after the reaction.

Q, the energy released in a nuclear reaction, can be written as Q = Δmc². Here Δm, read “delta m”, means the sum of the initial masses minus the sum of the final masses. The symbol Δ indicates this difference.

Derivation: How does a mass difference give the released energy?

Let $m_i$ and $m_f$ be the sums of the initial and final masses, and $K_i$ and $K_f$ the corresponding total kinetic energies. Include the energy equivalent of mass when applying conservation of energy.

1. The initial and final total energies are equal: $m_i c^2 + K_i = m_f c^2 + K_f$.
2. Rearranging separates the gain in kinetic energy from the loss of mass energy: $K_f - K_i = (m_i - m_f)c^2$.
3. The reaction energy is defined by $Q = K_f - K_i$. Writing the mass difference as $\Delta m = m_i - m_f$ gives the mass form of the same relation.

Result: $Q = (m_i - m_f)c^2 = \Delta m c^2$. A positive mass difference gives a positive energy release. With masses in kilograms and $c$ in metres per second, $Q$ is in joules.

What does more tightly bound mean?

Binding energy is the energy required to separate a nucleus completely into its individual protons and neutrons. Binding energy per nucleon is the binding energy divided by the number of nucleons. It measures how strongly the nucleons are bound on average.

In the heavy-nucleus fission and light-nucleus fusion processes considered here, the resulting nuclear system is more tightly bound. Energy is released as the system changes. Splitting and joining are opposite descriptions of the rearrangement, but both can lead to an energy-releasing result.

Derivation: How is binding energy obtained from mass defect?

Let $M$ be the measured nuclear mass, $m_p$ the mass of one free proton and $m_n$ the mass of one free neutron. A nucleus with mass number $A$ and atomic number $Z$ contains $Z$ protons and $A-Z$ neutrons.

1. Add the masses of the separated nucleons: $m_{\mathrm{free}} = Zm_p + (A-Z)m_n$.
2. The bound nucleus has less mass than these separated constituents. Its mass defect is $\Delta M = m_{\mathrm{free}} - M$.
3. Separating the nucleus requires supplying the energy equivalent of this mass difference. Thus its binding energy is $E_b = \Delta M c^2$.

Result: $E_b = [Zm_p + (A-Z)m_n - M]c^2$. The same energy is released when the separated nucleons bind to form that nucleus. Use nuclear mass for $M$, excluding the surrounding electrons.

Fission of nuclei such as uranium releases energy of the order of 200 MeV per fissioning nucleus. “Of the order of” indicates the scale of the energy, not an exact energy assigned to every uranium fission event.

Note: Equal mass-number totals mean equal nucleon counts in these equations. They do not mean that the total mass, considered separately from energy, must remain unchanged.

How does the released energy become useful?

The fragments and neutrons produced in fission initially carry the released energy as kinetic energy. Transfer to surrounding matter produces heat. Nuclear reactors, systems that use nuclear reactions to release energy, use fission as the energy source for electricity production.

The enormous energy of an atom bomb comes from uncontrolled nuclear fission. This connection concerns the release of fission energy; it does not change the distinction between a heavy nucleus splitting and light nuclei joining in fusion.

How do nuclear fission and fusion compare?

Which differences identify the processes?

The central difference is the direction of the nuclear rearrangement. In fission, a heavy nucleus splits into smaller fragments. In fusion, light nuclei join to form a larger nucleus. Identify that change before considering the reaction’s energy release or application.

The table compares specified examples. It does not make neutron bombardment a definition of every possible fission process, or helium-3 the product of every fusion reaction. The definitions concern splitting heavy nuclei and joining light nuclei.

What do the two processes have in common?

Both are nuclear reactions, involving changes in nuclei. Both can release energy when the final nuclear system is more tightly bound. Their equations must account for the participating nuclei and emitted particles, and must be checked using the appropriate conserved quantities.

In the examples used here, mass-number totals and atomic-number totals match on both sides. Energy may be shown explicitly beside the products or discussed alongside the equation. Omitting a numerical energy value from a fission equation does not imply that no energy is released.

A complete explanation should identify the starting nuclei, describe the nuclear change and name the products in the chosen example. It should then explain the energy release or initial conditions if those are part of the comparison being made.

Note: Neither “energy is released” nor “a neutron appears among the products” alone distinguishes fission from fusion. Both statements apply to the uranium-barium-krypton reaction and to the deuterium-helium-3 reaction.

The most reliable comparison therefore begins with the nuclear transformation. The fact that one process releases neutrons, produces heat or has a practical application is additional information, not a replacement for stating what happens to the nuclei.

Glossary

• Nucleus — Small central part of an atom containing its positive charge and most of its mass.
• Nucleon — A proton or neutron, considered as a constituent particle of an atomic nucleus.
• Atomic number — The number of protons in a nucleus, represented by the symbol Z.
• Mass number — The total number of protons and neutrons in a nucleus, represented by A.
• Isotopes — Forms of an element with equal proton numbers but different neutron numbers in their nuclei.
• Deuteron — The nucleus of deuterium, containing one proton and one neutron.
• Triton — The nucleus of tritium, containing one proton and two neutrons.
• Nuclear fission — Splitting of a heavy nucleus into smaller nuclear fragments, releasing energy.
• Nuclear fusion — Joining of light nuclei to form a larger nucleus, releasing energy in the reactions discussed here.
• Kinetic energy — Energy associated with motion, initially carried by fragments and neutrons in fission.
• Coulomb repulsion — Electric repulsion between particles with charges of the same sign, including two positively charged nuclei.
• Thermonuclear fusion — Fusion achieved by heating particles sufficiently to overcome the electrical repulsion between their nuclei.
• Binding energy — Energy required to separate a nucleus completely into its individual protons and neutrons.
• Mass-energy equivalence — The relation between mass and its associated energy, expressed by E = mc².
• Megaelectronvolt — An energy unit equal to one million electronvolts, abbreviated MeV in nuclear reaction equations.

Common errors and misconceptions

• Misconception: Fission means joining nuclei and fusion means splitting them. Correct: Fission splits a heavy nucleus; fusion joins light nuclei to form a larger nucleus.
• Misconception: The upper number in a nuclear symbol counts protons. Correct: The upper number is the mass number, counting protons and neutrons together. The lower atomic number counts protons.
• Misconception: A neutron contributes nothing when balancing an equation. Correct: Its atomic number is zero, but its mass number is one. Each emitted neutron must therefore enter the mass-number sum.
• Misconception: Uranium-235 fission must release exactly three neutrons. Correct: Three neutrons belong to the barium-144 and krypton-89 example. Other displayed fragment pairs release four or two neutrons.
• Misconception: Fusion must leave just one product nucleus and no other particles. Correct: The deuterium reactions shown produce a larger nucleus together with a neutron or proton.
• Misconception: Fusion releases energy, so no initial energy is needed. Correct: Positively charged nuclei repel one another and need sufficient initial energy to approach closely enough for fusion.
• Misconception: Equal mass-number totals mean identical total masses before and after reaction. Correct: Mass number counts nucleons. A decrease in the combined mass corresponds to the energy released.
• Misconception: Every uranium fission releases exactly 200 MeV. Correct: The energy is of the order of 200 MeV per fissioning nucleus; this is an approximate scale.

Exam-style questions with model answers

Q1. Define nuclear fission and nuclear fusion. [2 marks]

1. Nuclear fission is the splitting of a heavy nucleus into smaller nuclear fragments with the release of energy.
2. Nuclear fusion is the joining of light nuclei to form a larger nucleus, with energy released in the light-nucleus reactions considered here.

Q2. In ¹₀n + ²³⁵₉₂U → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + x¹₀n, x is the number of emitted neutrons. Find x, check the atomic-number balance and identify the process. All mass and atomic numbers are shown. [3 marks]

1. The initial mass-number total is 1 + 235 = 236. The two product nuclei account for 144 + 89 = 233, leaving three neutrons. Therefore x = 3.
2. The initial atomic-number total is 0 + 92 = 92. The final total is 56 + 36 + 3 × 0 = 92, so it balances.
3. The process is nuclear fission because a heavy uranium nucleus splits into smaller barium and krypton nuclei.

Q3. Complete ²₁H + ²₁H → ³₂He + a missing particle + 3.27 MeV. Use ¹₀n for a neutron and ¹₁H for a proton. Show the mass-number and atomic-number checks, then write the complete equation. [3 marks]

1. The two deuterium nuclei have a combined mass number of 2 + 2 = 4. Helium-3 accounts for 3, so the missing particle must have mass number 1.
2. The initial atomic-number sum is 1 + 1 = 2. Helium-3 already accounts for 2, so the missing particle has atomic number 0 and is a neutron.
3. The complete balanced equation is ²₁H + ²₁H → ³₂He + ¹₀n + 3.27 MeV.

Q4. Explain why thermonuclear fusion requires a very high temperature. Give four linked points. [4 marks]

1. The light nuclei that must combine carry positive electric charge, so they experience electric repulsion as they approach one another.
2. The attractive nuclear force acts over a very short range, so the nuclei must approach very closely before it can bind them.
3. They therefore need sufficient initial kinetic energy to overcome the energy barrier associated with their electrical repulsion.
4. A very high temperature provides the particles with high kinetic energies. Fusion achieved by heating in this way is called thermonuclear fusion.

Q5. Compare the reactions ¹₀n + ²³⁵₉₂U → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + 3¹₀n and ²₁H + ²₁H → ³₂He + ¹₀n + 3.27 MeV. U, Ba, Kr, H and He denote uranium, barium, krypton, hydrogen and helium; n denotes a neutron. Compare the nuclear change, starting nuclei, products, neutron involvement and mass-number balance. [5 marks]

1. The uranium reaction is fission: a heavy nucleus splits into smaller nuclei. The deuterium reaction is fusion: two light nuclei combine to form a larger nucleus.
2. The first reaction begins with a uranium-235 nucleus and an incoming neutron. The second begins with two deuterium nuclei, each having mass number 2.
3. The fission products shown are barium-144, krypton-89 and three neutrons. The fusion products shown are helium-3 and one neutron, with 3.27 MeV released.
4. The fission equation has a neutron among its starting particles and three among its products. The fusion equation has no incoming free neutron but releases one neutron.
5. The fission mass-number totals are 1 + 235 = 144 + 89 + 3 = 236. The fusion totals are 2 + 2 = 3 + 1 = 4.

Q6. A pupil claims that uranium-235 must always form barium-144 and krypton-89. Evaluate this claim using the supplied alternative equation ¹₀n + ²³⁵₉₂U → ¹³³₅₁Sb + ⁹⁹₄₁Nb + 4¹₀n. Sb means antimony and Nb means niobium. State your conclusion and check both number balances. [3 marks]

1. The claim is incorrect. The supplied equation gives another possible product pair, antimony-133 and niobium-99, so barium-144 and krypton-89 are not the compulsory pair.
2. The initial mass-number total is 1 + 235 = 236. The final total is 133 + 99 + 4 × 1 = 236.
3. The atomic-number total is 0 + 92 = 92 initially and 51 + 41 + 4 × 0 = 92 finally, so this balance also agrees.

Q7. Explain why equal mass-number totals in an energy-releasing nuclear reaction do not require equal combined masses before and after it. [3 marks]

1. Mass number counts the protons and neutrons in a nucleus. Equal total mass numbers therefore establish the equality of the nucleon counts in the reactions considered.
2. The measured mass of a bound nuclear system is related to its binding energy; it is not simply specified by its nucleon count.
3. The combined product mass is less than the combined starting mass in an energy-releasing reaction. The mass difference corresponds to released energy through mass-energy equivalence.

Q8. State the fuel and the overall nuclear conversion responsible for the Sun's fusion energy. [2 marks]

1. Hydrogen in the Sun's core acts as the fuel for its energy-producing nuclear fusion process.
2. A multistep sequence of nuclear reactions converts hydrogen into helium, releasing energy in the process.

Key takeaways

• Atomic number counts protons; mass number counts protons and neutrons. Read both numbers when interpreting a nuclear equation.
• Nuclear fission splits a heavy nucleus into smaller fragments, while fusion combines light nuclei into a larger nucleus.
• Uranium-235 can yield different fragment pairs, so the number of emitted neutrons depends on the reaction shown.
• Check mass-number totals and atomic-number totals independently, including every neutron or proton displayed in the equation.
• Two deuterons can form helium-3 and a neutron, or tritium and a proton, with different energy releases.
• Very high temperatures enable thermonuclear fusion by giving particles sufficient kinetic energy to overcome their electrical repulsion.
• Both processes can release energy when the final nuclear system is more tightly bound than the initial system.
• Nuclear fission supplies energy in electricity-producing reactors; thermonuclear fusion supplies energy in the interiors of stars.

Test yourself

What does the lower number in ²³⁵₉₂U represent?

It is the atomic number, 92, which states how many protons the uranium nucleus contains.

Why does 3¹₀n contribute three to the mass-number sum?

It represents three neutrons. Each neutron has mass number one, giving a combined mass number of three.

What intermediate nucleus appears after uranium-235 absorbs a neutron?

Uranium-236, written ²³⁶₉₂U, forms as the intermediate nucleus before splitting in the displayed fission reaction.

Which particle accompanies helium-3 in the fusion reaction between two deuterons?

A neutron accompanies helium-3, giving ²₁H + ²₁H → ³₂He + ¹₀n + 3.27 MeV.

Which particle accompanies tritium in the alternative deuterium reaction?

A proton accompanies tritium, and the energy released in this reaction is 4.03 MeV.

Why must fusing nuclei approach very closely?

The attractive nuclear force is short-ranged, so the nuclei must approach closely enough for it to act effectively.

In what form does fission energy initially appear?

It initially appears as kinetic energy of the nuclear fragments and neutrons, later becoming heat in surrounding matter.

Does an energy release of the order of 200 MeV mean exactly 200 MeV?

No. The phrase describes the approximate scale of the energy per fissioning nucleus, rather than an exact universal value.