This hypothesis was put forward by Heinsberg after the discovery by Chadwick.
According to this hypothesis, a nucleus of mass number A and atomic number Z contains Z protons, and (A – Z) neutrons. As an atom is electrically neutral, therefore, number of peripheral electrons must be equal to Z, the number of protons inside the nucleus.
This hypothesis explained all the discrepancies of proton electron hypothesis.
As is known, a neutron is a neutral particle carrying no charge. A proton carries a unit positive charge. Proton is slightly lighter than a neutron.
A proton and a neutron may be regarded as two different charge states of the same particle, called nucleon.
As the atom is electrically neutral, number of electrons in an atom is equal to the number of protons inside the nucleus.
Atomic number of an element is the number of protons present inside the nucleus of an atom of the element. It is also equal to the number of electrons revolving in various orbits around the nucleus of the atom. It is represented by Z.
Mass number of an element is the total number of protons and neutrons present inside the atomic nucleus of the element. It is represented by A.
Thus, in an atom, number of protons = Z
Number of electrons = Z
Number of nucleons = A
Number of neutrons = (A – Z)