Read the following passage and answer the questions.
The oxygen nucleus \({}^{16}_{8}\mathrm{O}\) contains 8 protons and 8 neutrons. A student adds up the masses of 8 free protons and 8 free neutrons. She then compares this total with the mass of the oxygen nucleus. The mass of the oxygen atom is measured very precisely with a mass spectrometer, and the mass of the nucleus is found from it. The nucleus turns out to be lighter than its separate nucleons put together. This difference in mass is called the mass defect. The nucleons stay together in the nucleus because of the nuclear force. The student also looks at a graph of the binding energy per nucleon against the mass number A for a large number of nuclei. The curve is smooth and changes slowly. At nuclides such as \({}^{4}\mathrm{He}\) and \({}^{16}\mathrm{O}\), it shows small peaks.
The small peaks of the curve at nuclides such as \({}^{4}\mathrm{He}\) and \({}^{16}\mathrm{O}\) are considered as evidence of
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → atom-like shell structure in nuclei
A peak on the curve means that the nuclide is bound a little more tightly than its neighbours. Such extra stability at certain nucleon numbers is like the extra stability of atoms with filled electron shells. So the nucleons in a nucleus are thought to fill shells too.
NCERT (2026-27):
Points to Ponder – “6. Although the binding energy (per nucleon) curve is smooth and slowly varying, it shows peaks at nuclides like 4He, 16O etc. This is considered as evidence of atom-like shell structure in nuclei.”