There is no subscript after oxygen (O), which means only one atom is present. The atomic mass of an element is a weighted average of all the element's isotopes based on their natural abundance. And this, right over here, is gonna have one more So one plus one is equal to two. Although the masses of the electron, the proton, and the neutron are known to a high degree of precision (Table 1.5.1), the mass of any given atom is not simply the sum of the masses of its electrons, protons, and neutrons. Direct link to Vica Kelly's post This is probably a very s, Posted 8 years ago. /*