We now know that the neutron is one of the fundamental particles that, together with protons, form the nucleus of atoms. Around the latter there are electron clouds, which are responsible for conducting electrical current in conducting materials, for example.
The discovery of the existence of this particle was possible thanks to the great success of the application of the Principle of Conservation of Momentum. According to this, the conservation of the total amount of movement of a system occurs if the resultant of external forces acting on the system is nil. This principle gained enormous importance, so that it became known as one of the fundamental laws of nature, being applied by scientists in all fields of physical science.
The discovery of the neutron happened in the year of 1932 with the English physicist James Chadwick. Using momentum conservation, he performed an experiment that proved the existence of the neutron. However, twelve years before this event, the famous English scientist Rutherford had already predicted the existence of this particle. According to him, a possible connection of a proton with an electron would originate a particle without an electrical charge, but with a mass equal to that of the proton. This particle he called a neutron, but he wasn't sure of its existence.
The experience that J. Chadwick's work basically consisted of making beams of alpha particles collide with a sample of beryllium (a chemical element belonging to the 2A family of the periodic table). From that collision, a type of radiation appeared that led many scientists to believe that it was gamma rays. After performing several calculations, James concluded that these were not gamma rays, the invisible radiation was formed by neutrons. To prove that they really were neutrons, Chadwick measured the mass of these particles, because according to Rutherford they had a mass equal to that of the proton. With this achievement and for his important works, in 1935 James was awarded the Nobel Prize in Physics.
By Marco Aurélio da Silva
Source: Brazil School - https://brasilescola.uol.com.br/fisica/breve-historia-descoberta-neutron.htm