An atom bomb (fission bomb) works mainly on which nuclear principle?

Introduction / Context:
Modern nuclear weapons and reactors rely on nuclear reactions to release very large amounts of energy. There is an important distinction between fission bombs, commonly called atom bombs, and fusion bombs, often called hydrogen bombs. This question asks about the basic principle behind an atom bomb, which is a key concept in nuclear physics and general knowledge.

Given Data / Assumptions:

• The device mentioned is an atom bomb, understood as a fission based weapon.
• Typical fissile materials are uranium 235 or plutonium 239.
• We consider the basic underlying nuclear principle, not the engineering details.

Concept / Approach:
Nuclear fission is the process in which a heavy nucleus splits into smaller nuclei, neutrons, and energy. In a chain reaction, neutrons from one fission event trigger further fissions. If this chain reaction is uncontrolled and grows very rapidly, it releases an enormous amount of energy in a very short time, which is the working principle of an atom bomb. In contrast, a nuclear reactor uses a controlled chain reaction where the rate is carefully limited to produce steady power. Fusion weapons are based on nuclear fusion, which is different again.

Step-by-Step Solution:
Step 1: Recognise that atom bombs historically used uranium 235 or plutonium 239 undergoing fission. Step 2: In fission, a heavy nucleus splits into lighter nuclei and more neutrons, releasing large amounts of energy according to E = m c^2. Step 3: If the emitted neutrons are not absorbed or controlled, they cause further fissions, leading to a rapidly multiplying chain reaction. Step 4: In an atom bomb, this chain reaction is deliberately made uncontrolled, so the energy is released almost instantaneously, causing an explosion.

Verification / Alternative check:
Descriptions of the first nuclear weapons, such as those used in the Second World War, clearly state that they relied on a supercritical mass of fissile material and a rapidly growing fission chain reaction. Nuclear power plants, on the other hand, use control rods and moderators to keep the reaction rate stable, showing the contrast between controlled and uncontrolled fission.

Why Other Options Are Wrong:
Nuclear fusion of light nuclei: This is the principle of hydrogen bombs and stellar energy, not typical atom bombs. Controlled chain reaction in a nuclear reactor: This describes nuclear power plants, not weapons. Purely chemical explosive reaction: Chemical explosives release much less energy per unit mass than nuclear reactions and are not the main mechanism of an atom bomb.

Common Pitfalls:
A frequent confusion is to mix up fission and fusion. An easy way to remember is that fission means splitting heavy nuclei, while fusion means joining light nuclei like hydrogen isotopes. Another mistake is to think that all nuclear devices use the same principle. For exams, always connect atom bomb with uncontrolled fission chain reaction and power reactor with controlled fission.

Final Answer:
An atom bomb works mainly on nuclear fission with an uncontrolled chain reaction.