Stoichiometry, Moles, and Molecular Weight

All nuclei of a given atom have the same "Z," or atomic number.  But, some atoms in an element have differing mass numbers (differing "A"s).  Nuclei with the same number of protons but different numbers of neutrons are called isotopes.  For example, silver (Ag) has two naturally occurring isotopes, silver-107 and silver-109.  The natural abundance (proportion occurring naturally) of silver-107 is 51.35% whereas silver-109 is 48.65% naturally abundant.

Central in Dalton's atomic theory was the idea that an atom of a particular element has a characteristic mass.  Elements are composed of certain mixtures of isotopes that are independent of where the element came from. Relative atomic masses, as calculated by Dalton are based on naturally occurring isotopic mixtures of the elements.