Stability of nuclei with respect to proton-neutron ratio
It has been observed that the stability of nucleus depends upon the neutron to proton rati (n/P). In a plot of neutrons versus protons for nuclei of various elements, it has been observed that most th stable (non0radio active) elements lie in a belt, termed as stability belt or zone.
In the stability zone, for nuclei having atomic number up to 20, the n/p ratio is close to unity.
for nuclei having atomic number more than 20, the n/p ratio for stability exceeds unity and goes up to 1.5 for heavier nuclei.