If an oxygen nucleus consists of eight protons and eight neutrons, the charge on that nucleus is positive. Since even I learned in high school that like charges repel, such a nucleus would find all its positive protons repelling and quickly fall apart." How would you answer his argument?

The binding energy present in the atomic nucleus that holds the protons and the neutrons together and its magnitude is one million times stronger than the electron binding energy in small atoms

Explanation:

The minimum required force to dismember an atomic nucleus into its constituent components, of protons and nucleus (collectively called nucleons) in known as the nuclear binding energy.

Energy is required in separating the nucleons hence the binding energy of a nucleus is always positive

According to Einstein's Energy and light relation E = mc², when a nucleus is formed from the number of free protons and neutrons, the sum of their individual masses is more than the mass of the formed atomic nucleus. The mass deficit of the neutron, also known as the 'missing mass' or mass defect indicates the amount of energy released in forming of the nucleus which therefore has different characteristics from its constituents as mentioned above

The amount of mass that is equivalent to the binding energy of the nucleus as shown in the Einstein's equation (E=mc²) is represented by the missing mass or mass defect of the formed nucleus or the difference in mass between the nuclear mass and that of the sum of the individual masses of its constituent protons and neutrons