Answer:
c
Explanation:
It is the weighted average of the isotopes. For example chlorine has two common masses for 2 isotopes.
The lighter one (and most common) is 34.96885
The heavier one is 36.96590
Now if you take the average of these two, you get 70.92/2 = 35.46 which is not the right answer. The right answer must take into account the abundance of the two isotopes.
36.590 * 0.2447 = 8.9536
34.96885*.7553=26.4120
The total is the periodic table mass which is 35.36557
The difference may not appear to be important, but an element has a lot of isotopes with quite a difference in them, the difference might be important.
