Even-odd effect:

This is known even-odd effect that arises essentially because of pairing of nucleons in the similar shell. Therefore, it might be remembered which pairing occurs among proton- proton and neutron-neutron but not between neutron-proton. It might be noted protons and neutrons occupy different shells and hence question of their pairing up does not arise. An odd Z-odd N nuclide contains at least one unpaired proton and neutron every in different shells and hence contributes little to the binding energy/stability of the nucleus.

Nuclei not falling on this curve or along with N/Z ratio deviating from the expected ratio within that region (as given in Table 12.2) are unstable and undergo decay through emitting features radiations (β- , β+ or electron capture) of definite energy and half-life. If the unstable nuclide is on the neutron rich side of the plot, it disintegrates by β- (beta particles are also called negatron because of the negative charge) emission and the daughter nuclide will contain one less neutron but one more proton than the parent nuclide

 (Parent radionuclide)                                                        (Daughter nuclide)

Alternatively, if a radionuclide is on the proton rich (or neutron deficient) side of the plot, it disintegrates through emitting a β+ (positron) or by an associated process called electron capture (EC). In both cases, resultant daughter nuclide will contain one proton less but one more neutron so that mass A remains unchanged.

For heavy nuclides with mass numbers in the region 200 or more (A ≥ 200), a more common mode of α-emission in observed. Then resultant daughter nuclide will contain two protons and neutrons less than the parent nuclide

More commonly, α and β (or β+) emission is accompanied through emission of electromagnetic (γ or IT) radiation but it is not always necessary as there are some pure β emitters also.