The statement "the relative atomic mass of oxygen is 16" means that the average mass of oxygen atoms, taking into account the different isotopes and their abundances, is approximately 16 atomic mass units (amu).
Oxygen has three naturally occurring isotopes: oxygen-16 (O-16), oxygen-17 (O-17), and oxygen-18 (O-18). These isotopes have different numbers of neutrons in their nuclei, resulting in slightly different atomic masses.
The relative atomic mass of an element is a weighted average of the masses of all the naturally occurring isotopes of that element, taking into account their abundance in nature. The atomic mass is expressed relative to the mass of a carbon-12 atom, which is assigned a mass of exactly 12 atomic mass units.
In the case of oxygen, the most abundant isotope is oxygen-16, which is more common than the other isotopes. Oxygen-16 has a mass of approximately 16 atomic mass units. The other isotopes, oxygen-17 and oxygen-18, are less abundant but contribute to the overall atomic mass of oxygen.
Therefore, when it is stated that the relative atomic mass of oxygen is 16, it means that the average mass of all the naturally occurring isotopes of oxygen, weighted by their abundances, is approximately 16 atomic mass units.