Fission and fusion

Nuclear fission splits a large nucleus; nuclear fusion joins small ones. Both release huge amounts of energy from a tiny loss of mass, as Einstein’s E = mc² describes.

Key facts

• Fission: uranium or plutonium split when hit by a neutron, releasing more neutrons: a chain reaction.
• Nuclear power stations control the chain reaction with control rods.
• Fusion: hydrogen nuclei join to make helium; this powers stars.
• E = mc²: a small mass becomes a huge amount of energy.

Worked examples