Atomic Physics - rest mass of Deuterium

[joeykeys]

Homework Statement

Calculate the rest mass of Deuterium nucleus.

Homework Equations

Is it correct to calculate its mass using hydrogen?
i.e. 2*rest mass of hydrogen?..

The Attempt at a Solution

As stated in textbook,
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!

 

[Berdi]

I think the problem arises in the fact that a Deuterium nucleus is a Proton and a Neutron, not 2 Protons, and the masses are slightly different.