The atomic mass of oxygen is 15.999 amu on the periodic table. What does this value mean?

The Atomic Mass of oxygen, 15.999 amu (atomic mass units), represents the average mass of an oxygen atom, taking into account the relative abundance of its isotopes. This value is not a whole number because it is a weighted average, reflecting the presence of different isotopes of oxygen, primarily oxygen-16, oxygen-17, and oxygen-18. The atomic mass is crucial for calculating the mass of compounds and understanding stoichiometry in chemical reactions.