Electrons do have mass, so their values must be considered when observing the measurements at the end of the mass spectrometer. However, electrons have a mass of ~1/1836 AMU and are borderline negligible when measuring the overall atomic mass of an atom....
Copper has only two stable isotopes: ^63Cu and ^65Cu, with masses of 62.929597 amu and 64.927789 amu, respectively. The atomic mass of copper is known to be 63.546 amu. What is the isotopic abundance of 63^Cu based on thes...