[dkv (OP)]

And Why?

[DrDick]

Many elements have no naturally occurring isotopes.  All of the elements after plutonium in the periodic table are man-made, so do not naturally occur in the earth's crust. A few are found in trace amounts.  These are radioactive, so are constantly disappearing, but are also being formed from the decay of other radioactive elements.

The why has to do with stability.  The largest elements are only made in stellar explosions (e.g., novae).  These elements are very unstable, some with half-lives measured in seconds or minutes.  Over time, even with very long half-lives, they disappear (e.g., technetium-98, with a half-life of 1.5 million years is almost completely gone).  Some radioactive elements can be continuously formed (as mentioned above) by decay of (generally) larger radioactive elements.

Dick

[dkv (OP)]

Thnx. The heavier elements may or may not have isotopes. Simply because those elements are rare it becomes to difficult to identify isotopes. I think rest of the elements like H ,He ,C,N, O, etc ,which are plenty ,definitely known to have isotopes.
With reagrds to technetium are you suggesting that its parent element also went extinct?
How can an element go extinct if there is continuous
process of formation and decay?

[DrDick]

With regard to technetium, I was referring to the naturally produced technetium from red giant stars (where it has been detected).  Any technetium present in the initial formation of the earth, even with a half-life of 1.5 million years, would have long since disappeared.  Even in the time since the dinosaurs (~60 million years ago), we have had roughly 40 of these half-lives, so if a million metric tons of technetium were to have appeared with that giant asteroid, only about a milligram would remain.

So, any technetium remaining in the earth must be a result of production from radioactive decay of other elements.

I'm not sure you're using the word "isotope" correctly.  Fluorine has only one naturally occurring isotope with any sort of real lifetime.  That means only one version of fluorine is to be found:  fluorine-19.  Every element has at least one isotope, since the word refers to a specific number of protons and a specific number of neutrons.  Perhaps your original question was actually whether or not any elements had only one naturally occurring isotope?

Dick

[dkv (OP)]

Yes I was referring to the fact that every element has at least one isotope. The number of neutrons differ. And these isotopes are naturally ocurring.
The question is : Why every element has at least one  natural isotope? (for heavier elements it difficult to ascertain this fact due to small quantitative presence)
Fluorine-19  also has an isotope which is naturally occuring. The life time is immaterial.
Another question which came up:
Why Technetrium went extinct ?
Answer I suppose refers to some initial amount during the formation of earth without referring to its parent. 
Which elements can be considered parents of this extinct species?
If that parent went extinct then which was parent's parent and so on...
Given the amount of Uranium I guess a path was available for the bulk formation of Tech-98 even 1.5 millions of years ago.

[dkv (OP)]

How do we explain formation of isotopes?
Atoms are driven by quantum rules.
Either the pair forms or it doesnt form ... either the nucleus divides qunatum mechanically or does not.