Describe the processes of nuclear fission and nuclear fusion as the splitting or joining of nuclei, to include the nuclide equation and qualitative description of mass and energy changes without values

5.1.2 The Nucleus

Learning Objective

Describe the processes of nuclear fission and nuclear fusion as the splitting or joining of nuclei. Write a correct nuclide equation for each process and give a qualitative description of the mass‑energy change (no numerical values).

1. Structure of the Nucleus

• Protons (p): charge +1 e, mass ≈ 1 u.
• Neutrons (n): charge 0 e, mass ≈ 1 u.
• Electrons (e⁻): charge –1 e, mass ≈ 0 u (outside the nucleus).

The nucleus therefore consists of protons + neutrons = nucleons. The total number of protons determines the element (its atomic number, Z); the total number of nucleons determines the mass number (A).

2. Nuclide Notation & Simple Calculations

Nuclide notation is written as \(\,^{A}_{Z}\text{X}\) where:

• \(Z\) = atomic number (number of protons).
• \(A\) = mass number (protons + neutrons).
• \(\text{X}\) = chemical symbol of the element.

Neutron count: \(N = A - Z\)

Example: \(^{14}_{6}\text{C}\) → \(N = 14 - 6 = 8\) neutrons.

3. Isotopes (Core Requirement)

Isotopes are nuclides of the same element (same Z) that have different mass numbers (different N).

Example: carbon‑12 \((^{12}{6}\text{C})\) and carbon‑14 \((^{14}{6}\text{C})\) are isotopes of carbon.

4. Writing Nuclide Equations – A Step‑by‑Step Template

1. Identify the reactants (including any incident particles such as neutrons).
2. Balance the total mass number (A) on both sides.
3. Balance the total atomic number (Z) on both sides.
4. Indicate any released particles (neutrons, photons, etc.) and write “+ energy”.

Worked example (U‑235 fission):

1. Reactants: \(^{235}{92}\text{U}\) + \(^{1}{0}\text{n}\).
2. Choose plausible fission fragments that together give A = 236 and Z = 92.
3. One common set: \(^{141}{56}\text{Ba}\) + \(^{92}{36}\text{Kr}\) + 3 \(^{1}_{0}\text{n}\).
4. Check: \(141+92+3 = 236\) and \(56+36+0 = 92\) – balanced.
5. Write the full equation with “+ energy”.

5. Nuclear Fission

Fission is the splitting of a heavy nucleus after it captures a neutron.

• The captured neutron makes the heavy nucleus unstable.
• The nucleus deforms and divides into two (occasionally three) fragments of roughly equal mass.
• 2–3 neutrons are emitted; these can trigger further fissions → a chain reaction.
• The total mass of the products is slightly less than the mass of the original nucleus + the incident neutron. The missing mass (mass defect) is released as energy (binding energy).
• In a nuclear reactor the chain reaction is controlled by control rods that absorb neutrons.

Typical nuclide equation (no numbers required):

\[

^{235}{92}\text{U} + ^{1}{0}\text{n} \;\rightarrow\; ^{141}{56}\text{Ba} + ^{92}{36}\text{Kr} + 3\,^{1}_{0}\text{n} + \text{energy}

\]

Qualitative mass‑energy description

Because the binding energy per nucleon is higher for the medium‑mass fission fragments than for the original heavy nucleus, the fragments together are more tightly bound. The increase in binding energy appears as a release of energy; the corresponding loss of mass is the mass defect.

6. Nuclear Fusion

Fusion is the joining of two light nuclei to form a heavier nucleus.

• Positively charged nuclei repel each other (Coulomb barrier). Overcoming this barrier requires extremely high temperature and pressure.
• When the nuclei are forced close enough, the short‑range strong nuclear force binds them together.
• The resulting nucleus has a slightly lower total mass than the sum of the original nuclei; the mass defect is released as energy.
• In the laboratory and proposed power‑plant concepts the most accessible reactants are the hydrogen isotopes deuterium (\(^{2}{1}\text{H}\)) and tritium (\(^{3}{1}\text{H}\)).

Typical nuclide equation (no numbers required):

\[

^{2}{1}\text{H} + ^{3}{1}\text{H} \;\rightarrow\; ^{4}{2}\text{He} + ^{1}{0}\text{n} + \text{energy}

\]

Qualitative mass‑energy description

The helium‑4 nucleus is more tightly bound per nucleon than the separate deuterium and tritium nuclei. The increase in binding energy is emitted as energy; the associated loss of mass is the mass defect.

7. Comparison of Fission and Fusion

8. Key Points to Remember

1. Both fission and fusion convert a tiny amount of mass into a large amount of energy (mass defect → binding energy).
2. Fission = splitting of a heavy nucleus after neutron capture; fusion = joining of two light nuclei.
3. The energy released per nucleon is greater for fusion than for fission.
4. Nuclide equations must balance both mass number (A) and atomic number (Z).
5. In fission the chain reaction is controlled with neutron‑absorbing control rods; in fusion the main challenge is achieving the required temperature and pressure.
6. Remember how to calculate neutrons: \(N = A - Z\) and the definition of isotopes (same Z, different A).