I was looking for a way to calculate the atomic mass of an Isotope. Taking into account that the mass of proton, neutron and electron are: 1.007276466812, 1.0086649156 and 0,0005485799094 atomic mass units (u) respectively, I was trying to calculate the atomic mass of 40Ca, it was: 40,3292507 u (using 19 electrons) or 40,3297992 u (using 20 electrons). However, the atomic mass reported for this element is: 39.962591 u. This situation is similar for other elements, and in general, the atomic mass calculated using the procedure I have showed is higher than the reported atomic mass measured by mass spectrometry. Does anyone know what happen about this situation?