Question

Explain the proton-neutron hypothesis in the structure of nucleus.

Hint:

After Chadwick found the neutron (1932), Heisenberg said the nucleus holds only protons and neutrons: \(Z\) protons and \((A-Z)\) neutrons, no electrons.

Answer 1

Step 1: The problem before 1932.
Earlier the proton-electron hypothesis was proposed, which assumed that a nucleus of mass number \(A\) and atomic number \(Z\) contained \(A\) protons and \((A-Z)\) electrons inside it. This model failed on several counts: (i) the wave nature of an electron confined to a nuclear size (\( \approx 10^{-15}\,\text{m}\)) would require it to have an impossibly large energy; (ii) it could not correctly explain nuclear spin and nuclear magnetic moment.

Step 2: Discovery of the neutron.
In 1932, James Chadwick discovered the neutron, a chargeless particle of mass almost equal to that of a proton (\(m_n \approx 1.67\times10^{-27}\,\text{kg}\)). This discovery removed the need to place electrons inside the nucleus.

Step 3: Heisenberg's proton-neutron hypothesis.
Werner Heisenberg proposed that the nucleus is made up of only two kinds of particles, protons and neutrons, together called nucleons. According to this hypothesis, a nucleus \(^{A}_{Z}X\) contains:
Number of protons \(= Z\) (equal to the atomic number)
Number of neutrons \(= A - Z\)
Total number of nucleons \(= Z + (A-Z) = A\) (the mass number).

Step 4: How it explains the observed facts.
The total positive charge of the nucleus is \(+Ze\), because only the \(Z\) protons carry charge \(+e\) each and the neutrons are neutral. The total mass is nearly \(A\) times the nucleon mass, since proton and neutron masses are almost equal. This correctly gives the atomic number, the charge, and the mass number of the atom.

Step 5: Superiority over the old model.
The proton-neutron model correctly explains nuclear spin, nuclear magnetic moment, and isotopes (same \(Z\), different \(N\)). Hence it is the accepted model of nuclear structure.
\[\boxed{\text{Nucleus} = Z\ \text{protons} + (A-Z)\ \text{neutrons},\ \ \text{total } A\ \text{nucleons}}\]