The neutron was theorized about and named in 1920 by Ernest Rutherford, and discovered independently in 1932 based on data by Ettore Majorana and James Chadwick.
The following are related quotes:
“The answer to the question ‘what are we made of’ is: protons and neutrons, the heavy particles, or baryons, and electrons, the light particles, or leptons.”— Michael Robinson (1999), The Nine Number of the Cosmos 
1. Rowan-Robinson, Michael. (1999). The Nine Numbers of the Cosmos (pgs. 6, 8). Oxford University Press.
1. Neutron – Wikipedia.