It's because the atomic mass is not expressed in kilograms. You have to translate it... Think about it : Can an atom have a mass of 39 kilograms!!!!!?
You still have the same problem with the mass. An atom is much, much less heavy than .
Take a look at Atomic mass - Wikipedia, the free encyclopedia, especially at the
"Conversion factor between atomic mass units and grams"
EDIT :Yes, you have to use the Avogadro's number.6.022 x 10^23 ???