Neutron

The vertical axis of this graph is the number of protons, and the horizontal axis is the number of neutrons. The black squares are stable elements. All the coloured squares show unstable radioactive elements. Note that Lead, atomic number 82, is the last chemical element with a stable isotope.

A neutron is a subatomic particle. The electrically neutral neutron and the positively charged proton, where the first two nucleons to be described. Prior to the discovery of subatomic particles the various atom on the periodic table were described as indivisible.

Neutrons and protons formed the core of an atom's nucleus, and they were surrounded by a cloud of electrons, another sub-atomic particle, much lighter than the other two particles. Normally, each atom is surrounded by the same number of electrons, as the number of protons found in its nucleus.

The chemical bonds between the atoms that make up chemical compounds, is mediated by the electromagnetic forces from the outer electrons in each atoms cloud of electrons.

The ratio between the number of protons and the number of neutrons in an atom's nucleus complicates chemical reactions, and atomic reactions.

For example, most atoms of Hydrogen, the simplest chemical element, consist of just a single proton circled by a single electron. But two other isotopes of Hydrogen are found in nature. Deuterium atoms have a nucleus with both a proton and a neutron. Ordinary water can be purified, to separate out the heavy water, that has a Deuterium atom in place of an garden variety Hydrogen. Heavy water not only has slightly different melting and boiling temperatures, it also has the ability to slow down neutrons emitted from other atoms. This property is exploited in nuclear engineering, including in Candu reactors.

Tritium is the third isotope of Hydrogen found in nature. It has two neutrons in its nucleus, and it radioactive, meaning it can spontaneously emit its second neutron. This property is exploited in Hydrogen bombs.