One atomic mass unit is _______ of the mass of one atom of _________ isotope.
, Carbon-12
One atomic mass unit is 112th of the mass of one atom of carbon-12 isotpoe. This implies that to get 1 unit of this mass we will have to take one carbon-12 atom, cut it up into 12 equal parts and weigh one of these parts.
For atomic mass we consider only the C-12 isotope and not C-13 or C-14 isotopes for some scientific reasons.