 Previous definitions of chemical quantity involved Avogadro 's number , a historical term closely related to the Avogadro constant , but defined differently : Avogadro 's number was initially defined by Jean Baptiste Perrin as the number of atoms in one gram @-@ molecule of atomic hydrogen , meaning one gram of hydrogen . This number is also known as Loschmidt constant in German literature . The constant was later redefined as the number of atoms in 12 grams of the isotope carbon @-@ 12 ( 12C ) , and still later generalized to relate amounts of a substance to their molecular weight . For instance , to a first approximation , 1 gram of hydrogen element ( H ) , having the atomic ( mass ) number 1 , has 6 @.@ 022 × 1023 hydrogen atoms . Similarly , 12 grams of 12C , with the mass number 12 ( atomic number 6 ) , has the same number of carbon atoms , 6 @.@ 022 × 1023 . Avogadro 's number is a dimensionless quantity , and has the same numerical value of the Avogadro constant given in base units . In contrast , the Avogadro constant has the dimension of reciprocal amount of substance . 
