**·**2011-06-19 06:06:00

The atomic weight you're talking about is the average atomic weight that is calculated according to the percentage of different isotopes of a particular element. But, suppose you'r given only one atom. The weight of that atom will be the sum of its protons and neutrons right, assuming that the weight of the electrons is negligible. Now, read the question again, it says "__The sum of the protons and neutrons in an atom is its__". I hope the answer is now justified. [1]