Neutron
A particle with no electric charge that, together with protons, makes up the atomic nucleus. It weighs almost the same as a proton.
In 1932 James Chadwick showed that a mysterious radiation produced when alpha particles strike beryllium is explained if it consists of uncharged particles of about the proton's mass[1]. Until then the nucleus was thought to be made of protons and electrons; the discovery settled the present picture of protons and neutrons.
Having no charge, a neutron is not repelled by the positive charge of a nucleus and can enter it. That property makes possible the chain reaction of fission and the reaction that breeds tritium from lithium in a fusion reactor. Because a magnetic field cannot hold neutrons, in a fusion reactor they fly into the walls and damage the materials. A neutron on its own outside a nucleus is unstable and turns into a proton in about 15 minutes.
Related terms
- ProtonA positively charged particle that, together with neutrons, makes up the atomic nucleus. The nucleus of hydrogen is a single proton. The number of protons decides which element an atom is.
- Atomic nucleusThe tiny, heavy core at the centre of an atom, made of protons and neutrons and carrying a positive charge.
- Alpha particleA heavy, positively charged particle made of two protons and two neutrons: the same thing as a helium nucleus. It is thrown out by radioactive substances.