Read the passage below and answer the question.

Atom contains dense mass at its center, called nucleus. Nucleus is positively charged. Nucleus is made up of neutrons and protons. Therefore, it may be expected that the mass of the nucleus is equal to its individual protons and neutrons. However, the nuclear mass M is found to be always less than this. 

What is this difference in masses called?

mass defect

This difference in the masses is called mass defect.