What is the mass of an oxygen atom (16O) in atomic mass units?

The mass of an oxygen atom, specifically the isotope (^{16}O), is approximately 15.995 atomic mass units (amu). This value represents the actual mass of the most abundant oxygen isotope found in nature, which contains 8 protons and 8 neutrons. The atomic mass scale is based on carbon-12, where one atomic mass unit is defined as one-twelfth the mass of a carbon-12 atom.

The slight difference between the whole number 16 and the measured mass (15.995 amu) is due to the binding energy of the nucleus, which causes the mass to be lower than the sum of the individual protons and neutrons. This phenomenon is also referred to as the mass defect and is a common occurrence in nuclear physics.

Thus, when looking for the mass of the oxygen atom, 15.995 amu accurately reflects the measurement we expect based on nuclear composition and mass-energy equivalence.