Why is the atomic mass unit less than the mass of both a neutron and a proton? [duplicate]

The atomic mass unit is $$1.6605 \times 10^{-27}$$ kg.

This is less than the mean of the masses of 6 protons and 6 neutrons.

How do we account for the lower mass ?

My understanding is that some of the mass is in the form of energy somewhere, in the bonding maybe or in the kinetic energy.

Can someone clarify where this mass is please ?

• Binding a proton to a neutron releases energy. I've read that some gentleman named Albert Einstein figured out that this energy carries off the missing mass, $m=E/c^2$ is how he originally rendered it I think. Jan 2 at 7:53
• Does this answer your question? Units of mass on the atomic scale Jan 2 at 10:13
• Jan 2 at 10:15