The difference between the two types of averages is that an arithmetic average are of equal importance, but a weighted average takes into account the importance of some values over others. But for average atomic mass we're calculating a weighted average. It we wanted the arithmetic average then it would be 5.5 because (5+6)/2 = 5.5. Also, scientists needed a pure isotope to base the system on. Scientists used carbon-12 because no other atom has exact whole-number masses in the amu scale. This is why the mass of carbon-12 is 12 amu, not 12.09. Some of the mass of the atom gets converted into binding energy, so the mass of the entire nucleus would actually be less than the mass of each individual proton and neutron added together. Because of energy-mass equivalence (E = mc^2), we know that energy and mass are interchangeable. Binding energy is the energy that holds the nucleus together. Since the nucleus is full of protons, which has positive charge, they would repel and the nucleus would fly apart. The missing mass is called mass defect, and it represents the binding energy. Following this logic, the carbon-12 atom's atomic mass should actually be 12.09 amu however, the mass is exactly 12 amu. So the mass of a proton is around 1.008 amu, not 1. The relative abundance of Deutrium (1 proton, 1 neutron) is so small that it is barely accounted for when calculating the average atomic mass. Protium is by far the must abundant isotope of hydrogen, and it only contains 1 proton, and no neutrons. The average atomic mass for hydrogen is actually around 1.008 amu. But after isotopes were discovered (oxygen-17, oxygen-18, etc), everything got really confusing, so scientists agreed to use carbon-12. Good question that requires a kind of long explanation!Ī long time ago, the standard for measuring average atomic mass was actually based on oxygen, and scientists thought all oxygen was oxygen-16 (8 protons, 8 neutrons).
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