Every atom contains a nucleus made up of protons and neutrons. Collectively, physicists refer to them as nucleons. The atomic mass of a nucleus is less than the combined mass of its nucleons. This discrepancy between the two masses is what physicists call mass defect. Physicists use amu, or atomic mass unit, to measure the mass of the nucleus and its nucleons.
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Determine the actual mass of the nucleus.
Calculate the total mass of the protons by multiplying the number of protons in the atom by the atomic mass for one proton. For example, if you wanted to calculate the total mass of the protons inside copper-63, you would note that there are 29 protons and the atomic mass for one proton is 1.00728 amu/proton. Therefore its total atomic mass would be 29.1112 amu/proton.
Calculate the total atomic mass of the neutrons inside the nucleus. Copper-63 contains 34 neutrons. The atomic mass for one neutron is 1.00867 amu/neutron. Therefore its total atomic mass would be 34.29478 amu/neutron.
Add the total masses of the nucleons and subtract the actual mass of the nucleus from this total. The result will be the mass defect. For example, in copper-63 the total mass of its nucleons would be 63.5059 amu. The mass defect would therefore be 63.5059 amu - 62.19637 amu (copper-63's actual nuclear mass) or .59223 amu.
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