Mass Defect
Careful measurements have shown that the mass of a particular atom is always slightly less than the sum of the masses of the individual neutrons, protons, and electrons of which the atom consists. The difference between the mass of the atom and the sum of the masses of its parts is called the mass defect (Dm). The mass defect can be calculated using Equation (1-1). In calculating the mass defect it is important to use the full accuracy of mass measurements because the difference in mass is small compared to the mass of the atom. Rounding off the masses of atoms and particles to three or four significant digits prior to the calculation will result in a calculated mass defect of zero.
 

Δm = [ Z(m_p + m_e) + (A-Z)mn ] - m_atom          (1-1)
 

where:
Dm = mass defect (amu)
mp = mass of a proton (1.007277 amu)
mn = mass of a neutron (1.008665 amu)
me = mass of an electron (0.000548597 amu)
matom = mass of nuclide ^A_ZX (amu)
Z = atomic number (number of protons)
A = mass number (number of nucleons)

Example:
Calculate the mass defect for lithium-7. The mass of lithium-7 is 7.016003 amu.
Solution: