In general chemistry, isotopes are a group of substances that belong to the same element. An element is characterized in the periodic table by their atomic number, which is the number of protons in an atom. Therefore, these substances have the same atomic numbers, but differ in mass numbers. Mass number is the sum of the number of protons and neutrons in the nucleus of an atom.
To determine the atomic weight of an element, you take the average weight of all the existent isotopes of that said element. The calculation would require to multiply the exact mass of the isotope to its abundance. Then, sum them all up.
the answer is c. their atomic masses are different clearly because an atom of gold has 79 protons and the atom can be divided multiple times. An atom of silver has an atomic number of 47. 47 electrons. Clearly different. Hope it helps :)
