In XIX century scientists found, from Avogadro's principle, 2g of H occuped 22.4L and they contained a certain number of molecules, but 16g of O2 occuped the same volume and they occuped the same number of molecules. So the it was born the concept of mole: the amount of substance which occupes in the gasses a volum of 22.4L.
But if the considered element isn't a gas to find how many atoms are in a mole it needs considere a number of grams equal at the number of molecular weight.
From the definition of mole we can deduce between the number of particeld and the number of moles:N°AVparticels ⇔1mol;; so N°AV/1=N°AVrepresents the number of particels which are equivalent at a mole;1/N°AVrepresents the number of moles which are equivalent at a particel.