How does an atomic bomb differ from a hydrogen bomb?

The primary difference between an atomic bomb and a hydrogen bomb lies in the physical process used to generate their explosive energy.1

• Atomic Bombs: These weapons rely exclusively on nuclear fission. They function by rapidly assembling a critical mass of fissile material—typically uranium-235 or plutonium-239—to initiate a runaway chain reaction, where heavy atomic nuclei split into lighter elements, releasing significant energy.23

• Hydrogen Bombs: Also known as thermonuclear weapons, these devices utilize nuclear fusion, a process where light atomic nuclei (isotopes of hydrogen) combine to form heavier helium nuclei. Because fusion requires extreme temperatures to overcome electrical repulsion between nuclei, these bombs use a fission-based "primary" stage to trigger the fusion reaction in a "secondary" stage.124

Hydrogen bombs are significantly more powerful than atomic bombs, often releasing thousands of times more energy. While atomic bomb yields are measured in kilotons, hydrogen bomb yields are frequently measured in megatons.21