The chemists used a scale such that the natural mixture of oxygen isotopes had an atomic mass 16 , while the physicists assigned the same number 16 to the atomic mass of the most common oxygen isotope ( containing eight protons and eight neutrons ) .
A carbon-12 atom has a mass of 12 u. Because electrons are so light , we can say that the mass of a carbon-12 atom is made of 6 protons and 6 neutrons .