Neutron

A neutron is one of the small particles that make up the atomic nucleus. It is counted alongside the proton; both are referred to as nucleons. The neutron carries no electric charge—it has neither positive nor negative charge—which is why its name derives from the word 'neutral'.

Its mass is slightly larger than that of the proton. The number of neutrons in an atom's nucleus determines the isotope of each element: two atoms with the same element but different numbers of neutrons are different isotopes.

Internal structure

A neutron is not indivisible. It is composed of quarks held together by the strong interaction. Therefore, the neutron belongs to the class of hadrons, specifically baryons, which are particles made of three quarks.

Stability and decay

When a neutron is inside a stable nucleus, it can persist for a very long time. However, a free neutron, separated from a nucleus, is not stable: it transforms into a proton, emitting an electron and something else extremely light. This process is called beta decay and is governed by the weak interaction. The lifetime of a free neutron is measured in minutes.

History of discovery

In the early 20th century, scientists recognized that the mass of the nucleus was greater than what could be accounted for by protons alone. In 1932, James Chadwick, building on work involving nuclear radiation experiments, proved the existence of a new uncharged particle—which was named the neutron. This discovery opened the way to understanding isotopes and nuclear reactions.

Applications

Because the neutron carries no electric charge, it can pass through matter without rapid interaction with electrons. This makes it a tool for investigating the structure of materials, in a technique called neutron diffraction. Neutrons are also part of the chain reaction in nuclear power reactors, and they play a role in some methods of treating certain diseases.