Neutrons  are subatomic particles,  with the symbol n or n, with no net electric charge and a mass slightly larger than that  protons. Protons and neutrons, each with mass approximately one atomic mass unit, constitute the nucleus of an atom, and they are collectively referred to as nucleons.  Their properties and interactions are described by nuclear physics.

The quark structure of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons. image: wikipedia

The nucleus consists of Z protons, where Z is called the atomic number, and N neutrons, where N is the neutron number. The atomic number defines the chemical properties of the atom, and the neutron number determines the isotope or nuclide. 

The discovery of the neutron: how do we know it is there?

The story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century, leading ultimately to the atomic bomb in 1945. 

In the 1911 Rutherford model, the atom consisted of a small positively charged massive nucleus surrounded by a much larger cloud of negatively charged electrons. In 1920, Rutherford suggested that the nucleus consisted of positive protons and neutrally-charged particles, suggested to be a proton and an electron bound in some way. 

Electrons were assumed to reside within the nucleus because it was known that beta radiation consisted of electrons emitted from the nucleus.  Rutherford called these uncharged particles neutrons, by the Latin root for neutralis (neuter) and the Greek suffix -on (a suffix used in the names of subatomic particles, i.e. electron and proton).  References to the word neutron in connection with the atom can be found in the literature as early as 1899.

Throughout the 1920s, physicists assumed that the atomic nucleus was composed of protons and "nuclear electrons”  but there were obvious problems. It was difficult to reconcile the proton–electron model for nuclei with the Heisenberg uncertainty relation of quantum mechanics. 

The Klein paradox discovered by Oskar Klein in 1928, presented further quantum mechanical objections to the notion of an electron confined within a nucleus. 

Observed properties of atoms and molecules were inconsistent with the nuclear spin expected from the proton–electron hypothesis. Since both protons and electrons carry an intrinsic spin of ½ ħ, there is no way to arrange an odd number of spins ±½ ħ to give a spin integer multiple of ħ. Nuclei with integer spin are common, e.g., 14N. 

Then, in 1931, Walther Bothe and Herbert Becker found that if alpha particle radiation from polonium fell on beryllium, boron, or lithium, an unusually penetrating radiation was produced. 

This radiation was not influenced by an electric field, so Bothe and Becker assumed it was gamma radiation. 

The following year Irène Joliot-Curie and Frédéric Joliot in Paris showed that if this "gamma" radiation fell on paraffin, or any other hydrogen-containing compound, it ejected protons of very high energy. 

Neither Rutherford nor James Chadwick at the Cavendish Laboratory in Cambridge were convinced by the gamma ray interpretation. 

 Chadwick quickly performed a series of experiments that showed that the new radiation consisted of uncharged particles with about the same mass as the proton.

 These particles were neutrons. Chadwick won the Nobel Prize in Physics for this discovery in 1935. 

source adapted from: Neutron. (2017, January 20). In Wikipedia, The Free Encyclopedia. Retrieved 08:48, January 23, 2017, from