The system we use for counting Atoms is called "Gram Molecular Weight", or gram molecule.
A Gram Molecule is the "Quanity" of a substance (atomic or molecular) having a weight "in grams" numerically "equal to" its "Molecular Weight": also called mol, or mole.
A mole of a substance has about ...
individual members; either atoms or molecules, ( 6.02 X 1023). This is Avogadro's Number.
Hydro Ref Center Project Page
What is PPM, Parts Per Million ?