question_answer

The isotopes \[_{8}^{16}O\] has 8 protons, 8 neutrons and 8 electrons, while \[_{4}^{8}\] Be has 4 protons, 4 neutrons and 4 electrons. Yet the ratio of their atomic masses is not exactly 2. Why?

Answer:

                This is because of the fact that the mass of a nucleus is slightly less than the mass of the constituent nucleons. This decrease in mass is called mass defect .Since, the mass defect in case of \[_{8}^{16}O\] is not exactly twice of the mass defect in case of \[_{4}^{8}\] Be, the ratio of the atomic masses is not exactly 2.