With today&#x27;s technology, for some compounds... you can count atoms by physical measurements and known structure, and figure mass by atomic weight.<p><a href="http:&#x2F;&#x2F;www.nist.gov&#x2F;pml&#x2F;si-redef&#x2F;kg_new_silicon.cfm" rel="nofollow">http:&#x2F;&#x2F;www.nist.gov&#x2F;pml&#x2F;si-redef&#x2F;kg_new_silicon.cfm</a>