Discovery of neutrons
The discovery of neutrons: Consider the element helium (2He4). It was found that helium has two protons and two electrons however its mass was found to be four times that of Hydrogen. Similarly, the masses of some other elements were also found to be double or more than double the number of protons.
This problem was solved on the discovery of another particle ‘neutron’ by James Chadwick In 1932 by bombarding beryllium with alpha particles.
4Be9 + 2He4→ 6C12 + 0n1
‘Neutron’ is a neutral particle with a mass 1.6749 X10-27Kg ; which is nearly equal to mass of proton 1.6726 X10-27Kg.
Mass of Neutron and Proton for simple calculations ; considered as 1 Unit or 1amu (atomic mass unit).