explain what is meant by nuclear fusion and nuclear fission

Mass defect is the difference between the mass of a nucleus and the sum of the masses of its individual protons and neutrons.

When nucleons bind together, a tiny amount of mass is lost and converted into energy according to Einstein’s famous equation:

$$E = \Delta m\,c^2$$

That lost mass is called the mass defect ($\Delta m$).

Binding energy per nucleon tells us how tightly each nucleon is held in the nucleus. It’s calculated as:

$$\frac{E_b}{A} = \frac{(\text{mass of nucleons} - \text{mass of nucleus})\,c^2}{A}$$

Higher values mean a more stable nucleus.

Fusion is like two Lego blocks snapping together to form a bigger block. When light nuclei (like hydrogen) combine, they form a heavier nucleus (like helium) and release energy.

Key equation:

$$\text{D} + \text{T} \rightarrow \, ^4\text{He} + n + 17.6\,\text{MeV}$$

💡 Why 17.6 MeV? It’s the difference in binding energy between the reactants and the product.

🔍 Exam Tip: Remember that fusion releases energy when binding energy per nucleon increases from reactants to product.

Fission is like splitting a big Lego block into two smaller blocks. Heavy nuclei (e.g., uranium-235) absorb a neutron, become unstable, and split into two lighter nuclei plus some neutrons.

Typical reaction:

$$^{235}\text{U} + n \rightarrow\, ^{140}\text{Xe} + ^{94}\text{Sr} + 3n + 200\,\text{MeV}$$

💡 The released energy comes from the higher binding energy per nucleon of the fission fragments.

🔍 Exam Tip: In fission questions, calculate the mass defect of the reactants and products to find energy released. Use $E = \Delta m\,c^2$.

✨ Final Exam Reminder: Always check whether the reaction moves towards or away from the peak of the binding energy curve. That determines if energy is released or absorbed.