CBSE Class 12-science Answered

The actual mass of a nucleus is always found to be less than the sum of the masses of the nucleons present in it . The mass difference is known as the mass defect and is denoted by  .

Consider nucleus of an element  _ZX^A with the mass number 'A' and atomic number 'z' . This element contains 'z' protons and ( A - z ) neutrons . Hence , mass of the constituent nucleons ( M' )  .

( M' )  =  z m_p  +  ( A - z ) m_n       ------------> (1)

Where m_p  and  m_n are the masses of the proton and neutron respectively .

If 'M' is the actual mass of the nucleus of the element then the mass defect is  

m   =   [ zm_p +  (A - z)m_n ]  -  m        

Binding energy B.E  =      931.5  MeV