The mass of the parts is always the mass of the whole when looking at nuclear masses.

When looking at nuclear masses we speak of the processes nuclear fision and nuclear fusion. In fission a nucleus breaks up, into two nuclei. In fusion on the other hand two light nuclei combine to form one heavier nucleus. The relation 
E=m*c^2. explains the difference in masses. 
So, in case of nuclear fusion the mass of the parts is always more than the mass of the whole when looking at nuclear masses. In case of nuclear fusion. And in case of nuclear fision, the mass of the parts is always less than the mass of the whole when looking at nuclear masses. In case of nuclear fusion

As newton's first law states a object in motion stays in motion