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Why are uranium-235 and plutonium-239 used in atomic bombs?

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U.S. hydrogen bomb test in the Marshall Islands — nuclear weapon A test of a U.S. thermonuclear weapon (hydrogen bomb) at Enewetak atoll in the Marshall Islands, November 1, 1952.

Uranium-235 and plutonium-239 are used in atomic bombs because they are fissile materials, meaning they can sustain a nuclear chain reaction when bombarded by neutrons.12

When these nuclei are struck by neutrons, they undergo nuclear fission, splitting into lighter atoms while releasing significant energy and additional free neutrons. In a weapon, this process is designed to grow rapidly: the neutrons released by one fission event induce further fissions in surrounding nuclei, liberating a massive amount of energy in a very short time.12

These specific isotopes are chosen for several reasons:
* Fissility: Unlike more common isotopes such as uranium-238, which are fissionable but not fissile, uranium-235 and plutonium-239 can be split by both fast and slow neutrons, making them ideal for sustaining the necessary chain reaction.12
* Energy Density: The fission of these materials produces an immense amount of energy—approximately 2,000,000 times more than the burning of an equivalent mass of coal.2
* Critical Mass: These materials can reach a critical mass—the minimum amount required to sustain a chain reaction—in manageable quantities, especially when compressed or surrounded by neutron reflectors.3