Answer:
16
Explanation:
The atomic mass of an element is defined as the average of the isotopic masses, weighted according to the naturally occurring abundances of the isotopes of the element. Although oxygen has nine isotopes with mass numbers ranging from 13 to 21, oxygen-13, oxygen-14 and oxygen-15 are not naturally occurring isotopes. So, we do not need to include them when we calculate the relative atomic mass of oxygen.
