Final answer:
The average atomic mass of oxygen on Earth is closer to 16 amu because the most abundant isotope is oxygen-16. Other isotopes like oxygen-17 and oxygen-18 exist but are less common, which establishes the average mass near 16 amu.
Step-by-step explanation:
The average atomic mass of oxygen on Earth is closer to 16 amu. This is because the most common isotope of oxygen is oxygen-16, which has an atomic mass very close to 16 amu. Natural oxygen also includes other isotopes like oxygen-17 and oxygen-18, but these are much less abundant.
For example, in a mass spectrometer, when separating isotopes from glacial ice, common oxygen-16 from rarer oxygen-18 can be differentiated based on their masses, with a ratio of 16 to 18. Since the atomic mass unit (amu) is defined using carbon-12 as the reference standard, one atomic mass unit is equal to one twelfth the mass of an atom of carbon-12. Therefore, since naturally occurring oxygen is mostly oxygen-16, the average atomic mass is indeed closer to 16 amu.