Zirconium half lives and relative abundance.

[Sorcerer]

This may belong in physics.

 

I was just reading the wiki on zirconium and it says this:

 

"Naturally occurring zirconium is composed of five isotopes. 90Zr, 91Zr, 92Zr and 94Zr are stable. 94Zr can undergo double beta decay (not observed experimentally) with a half-life of more than 1.10×1017 years. 96Zr has a half-life of 2.4×1019 years, making it the longest-lived radioisotope of zirconium. Of these natural isotopes, 90Zr is the most common, making up 51.45% of all zirconium. 96Zr is the least common, comprising only 2.80% of zirconium.[12]"

 

At first glance it seems counter intuitive that the most stable isotope is the least abundant. If earth formed with typical, for the rest of the universe, ratio of the isotopes, wouldn't the most stable be the most abundant remaining after 4 billion years of earth's existence, plus time from formation in a supernova or from decay from heavier elements. IE given time shouldn't the unstable isotopes be less abundant.

 

Why is this?

 

Is zirconium formed only from decay pathways and 96Zr the least probable product?

 

Does only small amounts form in supernovae and the rest is transitional from decay of heavier elements?

 

Has there simply not been enough time for the isotope ratios to level out to where 96Zr is the most abundant, will this happen eventually?

[Strange]

I think you have got it the wrong way round it says "90Zr is the most common" and "90Zr [and others] are stable". So one of the stable isotopes is the most common.

 

The only (known to be) unstable one is "96Zr has a half-life of 2.4×1019 years". And "96Zr is the least common".

 

So it seems to match your intuition.

 

I assume (but I don't know) that the relative abundances also depends on the proportions that were formed in stellar nucleosynthesis and not just decay since then. And I guess that the unstable isotopes are also less likely to be formed (less energetically favourable). Similarly, although the other four are all stable, presumably some are still lower energy configurations than others, hence their relative abundance

[Sorcerer]

96Zr has a half-life of 2.4×1019 years, making it the longest-lived radioisotope of zirconium.

96Zr is the least common, comprising only 2.80% of zirconium.

 

Read it again please, like I said counterintuitive.

Oh I see you're looking at 90Zr, I'm looking at 96Zr.

Perhaps is it just a typo?

[Strange]

Oh I see you're looking at 90Zr, I'm looking at 96Zr.

 

I see. I was assuming that 96Zr is the only unstable isotope, because it says "90Zr, 91Zr, 92Zr and 94Zr are stable."

 

I think that there is a typo. Where it says, "94Zr can undergo double beta decay" I think that should be 96Zr. If you follow the "double beta decay" link, it lists 96Zr as the isotope.

https://en.wikipedia.org/wiki/Double_beta_decay#Known_double_beta_decay_isotopes

 

But this page:

https://en.wikipedia.org/wiki/Isotopes_of_zirconium

Says that 94Zr is "Believed to decay by β⁻β⁻ to ⁹⁴Mo with a half-life over 1.1×10¹⁷ years" but that it is "observationally stable".

 

All rather confusing ...

Edited November 27, 2015 by Strange

[Sorcerer]

Thanks, wiki can be annoying. I imagine a class had a science project and some asshole altered the facts, ensuring no easy source. Actually on that hunch I'm checking the edit history.