[flashbulb]

I've come across an interesting question that I cannot find any information on, so I'm hoping someone here can help me out a little bit.

My question is in regard to isotopes and thier percent abundance. Can a single element with two or more isotopes have similar abundances under natural conditions? In other words, can the isotopes of the one element have similar natural abundances?

I hope I'm making myself clear enough, and I hope someone can help me.

Thanks,

Dave

[Mitch]

An isotopic ratio found in Canada should still be the same ratio found in Antarctica. But would be potentially different if you were on Pluto.

[flashbulb]

I'll clairify myself a little.

Does ONE isotope of an element generally predominate or can different isotopes of the same element have about the same natural abundance?

[gregpawin]

When you say dominate its hard to say exactly we're talking about.  The identity of isotopes is not static and as Mitch would tell you, is the outcome of several different processes happening at the same time.  There are the more stable ones but there are also the unstable isotopes still radioactive since the beginning of earth.  Even the stable ones can be subject to change by cosmic radiation, that for example produces carbon-14.  The producton of new elements as a byproduct of weapons and nuclear fuel production can affect the "natural abundance".  All these things have local variabilties and their "natural abundance" can deviate from the peroidic table's listed abundances.

If you're asking: neglecting all local variations and that your matter is shielded from further nuclear bombardment and contamination, can you get at least two different isotopes of the same element due to intrinsic nuclear stability, you'd have to ask Mitch one more time.

My answer would be yes but what does "equal" mean?  Exactly?

[Yggdrasil]

For most elements, one isotope is more predominant, but there are cases where two (or more) isotopes have approximately equal abundances.  For example, ⁷⁹Br and ⁸¹Br are found in approximately a 1:1 ratio in nature.

[Borek]

No idea what the question is precisely about, as it seems too ambiguous for me. However, there is an interesting side note (or at least that's what I think )

Take a look at atomic masses. If they are close to integer numbers (like in case of carbon or hydrogen) in most cases it means there is a predominant isotope. When they are far from integer numbers - you have some kind of mix of isotopes. For example Cl with 35.5 must be a mixture - and if I recall correctly, there are similar amounts of 35/36 isotopes in the chlorine.

The heavier the element the more isotopes it usually have, so above observation becomes more and more meaningless.

[Mitch]

Cl35 and Cl 37.

You can check: http://www.chemicalforums.com/index.php?page=isotopes

Still an other work in progress around here.

[Borek]

Cl25

But I think my observation still holds - the closer to an integer is atomic mass for light element, the less likely it is mixture of different isoptopes present in comparable amounts.