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Mass Defect (Δm)

It is found that the mass of a nucleus is always less than the sum of masses of it’s constituent nucleons in free state. This difference in masses is called mass defect. Hence mass defect
 
Δm = Sum of masses of nucleons – Mass of nucleus
 
106399.png
 
where mp = mass of proton, mn = mass of each neutron, me = mass of each electron, M = mass of nucleus, Z = Atomic number, A = mass number, and M’ = mass of atom as a whole.

Packing fraction

Mass defect per nucleon is called packing fraction
 
Packing fraction (f ) 106405.png
 
where M = mass of nucleus, A = mass number
 
Packing fraction measures the stability of a nucleus. Smaller the value of packing fraction, larger is the stability of the nucleus.
  • Packing fraction may be of positive, negative, or zero value.
  • At A = 16, f Zero
106865.png
Fig. 3

Binding energy (BE)

The neutrons and protons in a stable nucleus are held together by nuclear forces and energy is needed to pull them infinitely apart (or the same energy is released during the formation of the nucleus). This energy is called the binding energy of the nucleus.
 
or
 
The binding energy of a nucleus may be defined as the energy equivalent to the mass defect of the nucleus.
 
If Δm is mass defect, then according to Einstein’s mass energy relation:
 
Binding energy = Δm  c2 = [{mpZ + mn(A – Z)} – M]⋅ c2
 
(This binding energy is expressed in joule, because Δm is measured in kg)
 
If Δm is measured in amu, then binding energy = Δm amu = [{mpZ + mn(AZ)} – M] amu = Δm × 931 MeV

Binding energy per nucleon

The average energy required to release a nucleon from the nucleus is called binding energy per nucleon.
 
Binding energy per nucleon
106411.png
106417.png
 
Binding energy per nucleon ∝ Stability of nucleus




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