In nuclear physics and weapons technology, the main process that releases energy in an atom bomb (such as those dropped on Hiroshima and Nagasaki) is which of the following?

Introduction / Context:
Atomic bombs and nuclear weapons derive their enormous destructive power from nuclear reactions rather than ordinary chemical reactions. Understanding whether these weapons primarily use nuclear fission or nuclear fusion is an important topic in basic physics and general knowledge. This question asks you to identify the principal nuclear process that operated in the early atom bombs used during the Second World War, such as the ones dropped on Hiroshima and Nagasaki.

Given Data / Assumptions:

• The term used is atom bomb, referring to early nuclear fission weapons.
• Options include nuclear fusion, nuclear fission, radioactive disintegration, and chemical decomposition.
• We assume standard historical information about the first atomic weapons.
• We focus on the dominant reaction releasing large amounts of energy.

Concept / Approach:
Nuclear fission is the process in which a heavy nucleus such as uranium 235 or plutonium 239 splits into two smaller nuclei, releasing a large amount of energy and additional neutrons. In an atom bomb, a rapid uncontrolled chain reaction of fission events takes place, producing a massive energy release in a fraction of a second. Nuclear fusion, by contrast, involves light nuclei combining and is used in hydrogen bombs, which are more advanced. Radioactive disintegration is a slower process and chemical decomposition cannot explain the huge energy release observed. Therefore, the main process in an atom bomb is nuclear fission.

Step-by-Step Solution:
Step 1: Recall that the early atomic bombs used fissile materials like uranium 235 and plutonium 239. Step 2: Understand that these isotopes undergo fission, splitting into lighter nuclei and releasing energy and neutrons. Step 3: Recognise that the design of an atom bomb allows a rapid uncontrolled chain reaction of many fission events. Step 4: Compare this with nuclear fusion, which requires extremely high temperatures and is characteristic of hydrogen bombs, not the first atom bombs. Step 5: Conclude that nuclear fission is the main process operating in an atom bomb and select it as the answer.

Verification / Alternative check:
Physics and history references describe the Hiroshima bomb as a uranium based device and the Nagasaki bomb as a plutonium based device, both relying on critical mass and fast neutron induced fission. They emphasise that hydrogen bombs came later and use an initial fission explosion to trigger fusion reactions. Radioactive disintegration processes occur in nature but at a much slower rate and do not produce the same sudden massive energy release. Chemical decomposition yields far less energy per mass. These facts confirm that nuclear fission is the principal mechanism in atom bombs.

Why Other Options Are Wrong:
Nuclear fusion is the dominant process in hydrogen bombs or thermonuclear weapons, not in the early atom bombs referenced in the question. Radioactive disintegration refers to spontaneous decay of unstable nuclei and is not fast or energetic enough by itself to produce a nuclear explosion. Chemical decomposition involves breaking chemical bonds and releases a tiny fraction of the energy available from nuclear reactions. Therefore, none of these processes can correctly describe the main energy releasing mechanism in an atom bomb except nuclear fission.

Common Pitfalls:
A common confusion is between fission and fusion because both involve changes in atomic nuclei and release large amounts of energy. Some students also misinterpret the term radioactive disintegration as referring to any nuclear process and may choose it incorrectly. To avoid these errors, remember that fission splits heavy nuclei and is used in nuclear power plants and early bombs, while fusion combines light nuclei and powers the sun and hydrogen bombs. Linking each term with typical examples helps clarify the correct choice in exam questions.

Final Answer:
The main process that releases energy in an atom bomb is nuclear fission of heavy nuclei such as uranium 235 or plutonium 239.