A mystery of the nature of the nucleus was not completely solved. The nucleus contains most of the atom's mass as well as the positive charge so the protons were thought to account for this mass. However, a nucleus with twice the charge of another should have twice the number of protons and twice the mass. But this is not so. Rutherford speculated in 1920 that there are electrically neutral particles with the protons that make up the missing mass but no one accepted his idea at the time.
It wasn't until 1932 that the English physicist James Chadwick finally did discover the neutron and found it to be slightly heavier than the proton with a mass of 1840 electrons and is neutral. The proton-neutron together, are called the nucleon.
