Answer:- Isotope for the first blank and sixteen for second blank.
Explanations:- The sum of protons and neutrons makes the mass number of the element. Since atomic mass is the average of isotopes of the element, it is not a whole number but the mass number is a whole number as it is the sum of number of protons and neutrons.
15.9994 is almost 16 means the mass number for the most common isotope of oxygen is 16.