The mass of a helium atom (mass number 4) is amu, while the masses of a free neutron and proton are — Nuclear Chemistry and Radioactivity Chemistry Question
Question
The mass of a helium atom (mass number 4) is $4.0026$ amu, while the masses of a free neutron and proton are $1.0087$ amu and $1.0078$ amu respectively on the same scale. Assuming the mass of electrons is negligible here for mass defect calculation, the binding energy per nucleon in the helium atom is nearly how many MeV? (Use $1 \text{ amu} \approx 931.5 \text{ MeV}$ and round to the nearest integer).
💡 Solution & Explanation
Helium (${}_{2}^{4}He$) has 2 protons and 2 neutrons. Expected mass = $2 \times 1.0078 + 2 \times 1.0087 = 2.0156 + 2.0174 = 4.0330$ amu. Mass defect ($\Delta m$) = $4.0330 - 4.0026 = 0.0304$ amu. Total Binding Energy = $0.0304 \times 931.5 \approx 28.317$ MeV. Binding energy per nucleon = $28.317 / 4 \approx 7.07$ MeV. Nearest integer is 7.