e =mc^2. m is the defected mass, or mass that isn\'t there and is instead used as binding energy. For Mg, you have 25 nucleons: 12 protons and 13 neutrons. So your expected mass would be 1.007825x12 + 1.008665x13 = 25.206545. The defected mass is the supposed mass minus the actual mass. so its 25.206545-24.985839= .220706 amu. Then you have to convert this into kilograms. so, divide 220706 amu by 1 kg = 6.022 x 10^26 and you get: 3.664996*10^-28 Solution e =mc^2. m is the defected mass, or mass that isn\'t there and is instead used as binding energy. For Mg, you have 25 nucleons: 12 protons and 13 neutrons. So your expected mass would be 1.007825x12 + 1.008665x13 = 25.206545. The defected mass is the supposed mass minus the actual mass. so its 25.206545-24.985839= .220706 amu. Then you have to convert this into kilograms. so, divide 220706 amu by 1 kg = 6.022 x 10^26 and you get: 3.664996*10^-28.