Re: what are two important physial properties of isotopes (radionuclides)?

First, I am sorry to be so late to answer this question; I just returned 
from travel to Cambodia and California and I was out of contact for a few 
weeks.  Anyhow...first a quick review.

The number of electrons surrounding an atom is what determines that 
atom's chemical properties because it is the number of available "slots" 
for electrons that make an atom more or less chemically reactive.  Since 
atoms are, by default, electrically neutral, the number of electrons is 
the same as the number of protons in the nucleus.  This is why every atom 
of a particular element has the same number of protons, and we can 
identify an element by its atomic number - the number of protons in the 
nucleus.

However, it is possible to add neutrons to a nucleus without changing the 
chemical  properties at all.  So, for any given element, one can have 
multiple isotopes (also called nuclides) of different atomic weights.  
Perhaps the best-known example of this is carbon.  Carbon 12 has 6 
protons and 6 neutrons and is non-radioactive.  Carbon 13 has 6 protons 
and 7 neutrons, and it is also non-radioactive.  Carbon 14 (used in 
carbon dating) has 8 neutrons and 6 protons and it is radioactive.

In general, radioactive isotopes can be used to determine the age of 
rocks or archeological artifacts.  They can also be used as tracers, to 
see where groundwater flows for example.  For radioactive isotopes, the 
most important properties are teh half-life (which determines how long 
the isotope will be detectable), the type of radiation emitted (alpha, 
beta, or gamma), and the energy of the radiation given off.

Stable isotopes are also used, primarily to look for signs of life or to 
help judge temperatures in the past.  For example, carbon 13 is slightly 
heavier than carbon 12 (the number refers to the mass of the isotope).  
We find both of these isotopes in nature, but the amount of C-13 in 
living organisms is different from that in rocks - this is discussed in 

Mad Scientist question 1054683752.Bc (Biochemistry).  

When used as a paleo-thermometer, we are also looking at the difference 
in masses.  Water molecules containing O-18, for example, are slightly 
heavier than are water molecules with more common O-16.  These molecules 
are less willing to evaporate at low temperatures, so water that 
evaporates from teh oceans at low temperatures tend to be "enriched" in O-
16 and  "depleted" in O-18 compared to sea water.  As temperatures rise, 
the amount of O-18 in the evaporated  water also increases.  Since rain 
comes  from evaporated water, this means that freshwater is enriched in O-
18 at higher temperatures, as is glacier ice.  Thus, by measuring O-18 
concentrations in glacial ice in the past, we can learn what the 
temperature was at the time the ice formed.

In this case, we are looking at stable isotopes, so what is most 
important is the weight of the two stable isotopes and, more importantly, 
the size of that difference with respect to the mass of the isotopes.  
For hydrogen, going from hydrogen (1 atomic mass unit) to deuterium (2 
amu) is a mass difference of 100%.   This is much more significant than 
the difference between C-12 and C-13 (about 9%).  By the time you get to 
atoms with a mass of about 40, the fractional difference is so small  
that you don't tend to see this sort of fractionation.

For more reading on this, there are some very good books.  At the college 
level is a great book by Gunter Faure (I studied from Isotope Geology).  
Another good book, at the high school level, is "The Age of the Earth" by 
Dalrymple, although this only really covers  isotope geology.

Happy reading!