Re: What practical purpose does the existence of isotopes serve?

In the absence of isotopes, the universe might not exist as we know it (a 
large volume of vacuum studded with large numbers of stars and other 
debris [such as planets and dust clouds]), depending on how the absence of 
neutron interactions would have affected the production of elements in the 
earliest phases of the Big Bang (neutron interactions help build heavier 
elements from lighter elements...the addition of a neutron to a light 
element results in, by definition, another isotope of the same element).  
If a universe similar to our own would exist in the absence of isotopes, 
it would still be a very different place than the one we know and love.  
The carbon cycle in the sun, for instance, (and all other stars, of 
course) requires both carbon 12 and carbon 13 to function.  In the absence 
of these two isotopes, the sun would be a very different object (which, of 
course, has implications for the earth). For another example, our bodies 
require water in the form of hydrogen(1) dioxide.  We cannot live long 
with large quantities of hydrogen(2) dioxide (deuterium dioxide, or heavy 
water), because the chemical rate constants which determine how water 
(etc.) is distributed in our bodies are based on "normal" water. 
Therefore, if only one isotope of hydrogen existed, and the "chosen" 
isotope was deuterium, life (if it existed in this very different 
universe) would be very different as well.

However, if we begin with the universe as it is, AND if we ignore the 
problem with the carbon cycle, AND we could pick-and-choose the isotopes 
which we are most interested in and discard the others, then the world 
might not be so different.  Right now I can't think of a REQUIREMENT for 
deuterium or tritium to exist, in order for us to live long and (fairly?) 
healthy lives.  However, science would be very different and much more 
ignorant (isotopic dating techniques, such as carbon, uranium, etc. would 
not exist), medicine wouldn't be as advanced (diagnostic [tracer] and 
theraputic radionuclides would be much rarer), and, worse of all, we 
health physicists would be out of a job fairly quickly :)

Great question! Keep that curiosity flowing!
ron