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The process in which the nucleus of a heavy atom splits into two smaller nuclei, releasing energy, is called:
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Answer: Option A — Nuclear fission
Detailed Solution
Nuclear fission is the process in which the nucleus of a heavy atom, such as uranium-235 or plutonium-239, splits into two smaller nuclei of roughly similar size, when it is struck by a slow (thermal) neutron. During fission, two or three new neutrons and a large amount of energy are released, mainly as the kinetic energy of the fragments and as gamma radiation.
A typical example is: uranium-235 + neutron → barium-141 + krypton-92 + 3 neutrons + about 200 MeV of energy.
The energy released in fission is millions of times greater than the energy released in a chemical reaction, such as burning coal. It comes from the conversion of a small amount of mass into energy (E = mc²).
Nuclear fission was discovered in December 1938 by the German chemists Otto Hahn and Fritz Strassmann. Lise Meitner and her nephew Otto Frisch explained it as the splitting of the nucleus and named it "fission" in early 1939. Hahn received the Nobel Prize in Chemistry in 1944.
The neutrons released by one fission can cause fission in other nuclei, which in turn release more neutrons. This is called a chain reaction:
In a nuclear reactor, the chain reaction is controlled. Control rods made of boron or cadmium absorb extra neutrons. A moderator, such as heavy water or graphite, slows down the neutrons so they can cause more fission. The heat produced is used to make steam to generate electricity. The first controlled chain reaction was achieved by Enrico Fermi in Chicago in 1942.
In an atom bomb, the chain reaction is uncontrolled, releasing a huge amount of energy in an instant. Atom bombs were dropped on Hiroshima (uranium) and Nagasaki (plutonium) in August 1945. India conducted nuclear tests at Pokhran in 1974 and 1998.
Why not the other options?
- ✗Nuclear fusion is the joining of light nuclei, as in the Sun.
- ✗Radioactive decay is the spontaneous emission of radiation from unstable nuclei.
- ✗Ionisation is the removal or addition of electrons to form ions.
Quick tip
Remember: Fission = splitting heavy nuclei (U-235, discovered by Hahn and Strassmann, 1938), chain reaction, nuclear reactors and atom bombs; control rods = boron or cadmium; moderator = heavy water or graphite.
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Modern Physics & Nuclear Energy — Preparation
This question tests your understanding of Modern Physics & Nuclear Energy, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers atomic and nuclear physics: the discovery of the electron, proton and neutron, radioactivity and its types, nuclear fission and fusion, nuclear reactors, X-rays, semiconductors, and important laws and theories, such as Einstein's mass-energy relation. Questions usually ask who discovered a particle (J. J. Thomson discovered the electron, and James Chadwick the neutron), what powers the Sun (nuclear fusion), what happens in a nuclear reactor (controlled fission), and what moderators and control rods do. Learn the three types of radioactive radiation (alpha, beta and gamma) and how they differ. Practise the modern physics questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.