The atomic mass of oxygen is approximately 16 atomic mass units (amu) because it is a weighted average of the masses of its naturally occurring isotopes.
Oxygen has three naturally occurring isotopes: oxygen-16, oxygen-17, and oxygen-18. These isotopes have different numbers of neutrons in their nuclei while retaining the same number of protons.
Oxygen-16 is the most abundant isotope, constituting over 99% of naturally occurring oxygen. Oxygen-17 and oxygen-18 have much lower abundances, with oxygen-18 being the heaviest of the three isotopes.
The atomic mass of an element is calculated by taking into account the masses and abundances of its isotopes. In the case of oxygen, the atomic mass is primarily influenced by the isotopic masses and their relative abundances.
The atomic mass of oxygen is close to 16 amu because oxygen-16 has a mass of exactly 16 amu. The other isotopes, oxygen-17 and oxygen-18, contribute slightly to the overall atomic mass due to their lower abundances and slightly higher masses. However, their contributions are relatively small compared to oxygen-16, which dominates the calculation.
The exact atomic mass of oxygen may vary slightly due to the precise isotopic abundances, but 16 amu is a commonly used value for simplicity and convenience in most calculations.