what is nuclear stability??..i mean in terms of physics??...?

Answer 1

It means that the nucleus will not spontaneously "decay" into some other nucleus plus various ejecta.

Nuclear physicists and astrophysicists talk of "the valley of nuclear stability," where the binding energy per nucleon for a given number of nucleons is at or close to its local maximum value. Some distance away from this on either side in the (proton number, atomic weight) space, nuclei are unstable.

For low atomic weight/number, the most stable form of a given element (and therefore the most common form of it found in nature) has EQUAL numbers of neutrons and protons --- thus helium (2p, 2n), carbon (6p, 6n), oxygen (8p, 8n) etc. In this range an equal number of neutrons helps provide some more strong force to overcome the mutual repulsion of the protons. (As far as I know, it's an amazing coincidence that precisely an EQUAL number will just do this nicely!) Further on in the table, the repulsive forces between the protons become so large that yet more neutrons are needed to do the job. That's why you have such elements as uranium 235 (92p, 143n).

Many very "heavy" nuclei are subject to spontaneous fission into two fairly substantial "daughter nuclei." Others emit alpha particles (bare helium nuclei). Still other nuclei, produced evanescently, may emit electrons or positrons, depending on which side of the valley of nuclear stability they happen to lie. This is called "spontaneous beta decay," after the name first given to "beta emissions" before it was realized that these were in fact electrons. (In the earliest days when radioactivity was discovered, the first three letters of the Greek alphabet --- for alpha partices, beta "rays" and gamma rays --- were assigned to these initially mysterious "emanations.")

Live long and prosper.

Answer 2

I think there are a few very good answers already!

I wanted to add one more observation (not mine, from Lise Meitner and others in the 30's): you can use a drop of water as a good analogy for the atomic nucleus.

Neglecting gravity, a drop of water has two competing forces: the thermal motion of the water molecules inside it (which want to explode the drop), and the binding force of surface tension on its surface.

Similarly, an atomic nucleus has the outward pressure of electrostatics (positively-charged protons pushing against each other), and the binding energy of the nuclear strong force (which acts like surface tension, pulling nucleons together).

A small water drop is very stable, since the surface tension is well able to contain the thermal motion. And a small nucleus is likewise stable, since the strong force counters the electrostatic repulsion of protons.

A large water drop is unstable, since the surface tension can barely hold all of the moving water molecules inside. After all, as the droplet gets bigger, the area (surface tension) scales as the square of the radius, but the volume (moving molecules inside) scales as the cube. So eventually the moving molecules dominate.

A large water droplet is very wobbly, and just a slight bump can cause it to rupture into multiple smaller droplets.

In the same way, a larger atomic nucleus is "wobbly", and a small bump (for instance, a wandering neutron) can cause the nucleus to rupture into multiple smaller nuclei. This is called fission.

This is a big simplification, and doesn't address the role of neutrons at all, but it is a pretty good mental model. Another answer described the "valley of stability": that is the range of nuclei for which the surface tension (strong force) is able to contain the outward electrostatic repulsion of the protons. Nuclei in that valley last for a very long time, and so are called "stable". Wobbly nuclei are unstable.

Answer 3

The atom changes volume as a function of temperature. As mass is added to the atom it changes its radius vector in order to maintain equilibrium. In this manner the atoms maintain a stable structure.Other wise an hygrogen atom in one location would not be the same hydrogen atom in another location.
Basically all elements in nature are isotopes. That means that not all atoms have the same stability in their species.
That is why atoms like Uranium are radioactive because they have excessive mass within their atomic structure and must lose that excessive mass to become a more stable atom,to reach a state of equilibrium.

Its very vely simple