Problem 62 - Chemistry: An Atoms-Focused Approach

Question

Why is energy released in a nuclear fusion process when the product is an element preceding iron in the periodic table?

Step-by-Step Solution

Answer

Answer: Elements lighter than iron release energy during nuclear fusion because their binding energy per nucleon increases as their nucleus becomes heavier. This results in the products of the fusion reaction having a higher binding energy per nucleon than the reactants, and the energy difference is released during the process. For elements heavier than iron, the binding energy per nucleon starts to decrease, which means that fusion of these elements requires energy input instead of releasing energy.

Step 1: Understand nuclear fusion

Nuclear fusion is the process of combining two lighter atomic nuclei to form a heavier nucleus. In this process, a tremendous amount of energy is released, which powers the nuclear reactions occurring in stars like the Sun.

Step 2: Learn about nuclear binding energy

Nuclear binding energy is the energy required to disassemble an atomic nucleus into its constituent protons and neutrons. It is also the energy released when a nucleus is formed from its components. The binding energy per nucleon varies with the size of the nucleus; it generally increases with the number of nucleons up to iron (Fe) and then decreases for heavier elements.

Step 3: Energy release in nuclear fusion: elements lighter than iron

The energy released during nuclear fusion comes from the difference in binding energy between the reactants (lighter nuclei) and the products (heavier nuclei). For elements lighter than iron in the periodic table, the binding energy per nucleon increases as the nucleus becomes heavier. This means the products of the fusion reaction have a higher binding energy per nucleon than the reactants, and this difference in energy is given off during the process.

Step 4: Compare binding energy per nucleon for elements lighter and heavier than iron

As nuclei get heavier than iron, the trend changes. The binding energy per nucleon starts to decrease, which means fusing two heavy nuclei together would require energy input rather than release energy. That is why heavier elements like uranium undergo nuclear fission, where a heavy nucleus splits into two lighter nuclei, releasing energy.

Step 5: Summary and conclusion

In summary, energy is released in a nuclear fusion process when the product is an element preceding iron in the periodic table because their binding energy per nucleon increases with their size. This means that the products have more nuclear binding energy than the reactants, and this energy difference is given off as fusion energy. Once we reach elements heavier than iron, the binding energy per nucleon starts to decrease, meaning that fusion of these elements requires energy input instead of releasing energy.