Why is the total mass of a helium nucleus not equal to the mass of its individual parts?

The "missing" mass is due to inaccurate measurement on a very small scale.

The "missing" mass has been converted to energy.

The "missing" matter has been converted to mass.

The "missing" mass has been converted to matter.

Question

Why is the total mass of a helium nucleus not equal to the mass of its individual parts?
 
		
The "missing" mass is due to inaccurate measurement on a very small scale.
 		
The "missing" mass has been converted to energy.
 		
The "missing" matter has been converted to mass.
 		
The "missing" mass has been converted to matter.

anonymous · Answer

Energy is needed to bind the individual parts of a helium nucleus together. Where does that energy come from? As Einstein showed us, energy and mass are equivalent. The energy to form the bonds between the constituents of the nucleus can therefore come from some of the mass of the constituents themselves.