1. The problem statement, all variables and given/known data Calculate the rest mass of Deuterium nucleus. 2. Relevant equations Is it correct to calculate its mass using hydrogen? i.e. 2*rest mass of hydrogen?.. 3. The attempt at a solution As stated in text book, the rest mass of deuterium = 3.34330*10^-27 kg If it is calculated from hydrogen (my "theory" as above) = 2*(1.67338*10^-27) = 3.34676*10^-27 kg My question is.. Is my theory correct or there is something wrong with my concept?.. Is there any correct ways to find out the mass of deuterium without looking from textbook's data? Thank you!