How magic numbers,N/Z ratio and Even odd no. Of neuclons can stable a Isotopes?

Magic numbers refer to filled shells, which have increased stability. Even numbers represent filled energy levels, which also have increased stability.

Isotope is stable when there is not allowed decay mode to other isotope with smaller mass-energy.

That's it, if decay would violate one or more conservation, decay is not allowed.

 

The most important is conservation of energy. Deuterium is not decaying to free proton and free neutron, because there is missing 2.22 MeV energy. When this energy is delivered from external source (such as alpha particle with kinetic energy higher than 2.22 MeV), Deuterium can disintegrate. And it's used as source of free neutrons in other experiments for instance.

Edited August 27, 201510 yr by Sensei

An isotope is an element that has the same atomic number but different atomic mass compared to the periodic table. Every element has a proton, neutron, and electron. The number of protons is equal to the atomic number, and the number of electrons is equal the protons, unless it is an ion. To determine the number of neutrons in an element you subtract the atomic number from the atomic mass of the element. Atomic mass is represented as (A) and atomic number is represented as (Z) and neutrons are represented as (N).

A=N+Z(1)

atomic mass = number of neutrons + atomic number

To determine the stability of an isotope you can use the ratio of neutrons to protons (N:Z)