of: an atom
is given: by
where 
stands for one atomic mass unit. (Relative atomic masses of other particles may be defined in a similar way.) [P7.1, P8.1]
formerly was known: as 'atomic weight'. [P8.1]
is numerically equal: to the mass in grams of one mole of the substance. So it can be determined by comparing the masses of appropriately prepared macroscopic samples of matter. [P7.1, P7.2]
for a naturally occurring element, is obtained: by multiplying the relative atomic mass of each of its isotopes by the corresponding relative abundance and adding together the resulting products. [P8.1]
Copyright 1997, The Open University