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Name: Charlene
Status: educator
Grade: K-3
Location: OH
Country: USA
Date: Fall 2009


Question:
How many unstable isotopes are in gold (AU)?



Replies:
There are about 59 isotopes of gold that range from 169 atomic mass units to 205 atomic mass units; all gold has 79 protons.

There is only one stable isotope of gold (197 atomic mass units)! The other isotopes have half lives on the order of days or shorter.

Too bad other elements do not undergo spontaneous decay to produce gold!

Les

Leslie Kanat, Ph.D.
Professor of Geology
Department of Environmental Sciences


The natural abundance of gold is 197Au and it is essentially 100%. In addition, gold has 48 synthetic radioactive isotopes ranging from 171Au to 205 Au. These have half lives ranging from a fraction of a second to a few days, so they are only observed under laboratory conditions. A few of these exist as isomers, that is, excited states of an isotope of the same mass. That is why there are more isotopes than 205 - 171 = 34. There is a compilation of nuclear isotopes called "Table of the Nuclides" that can be found in chemistry/physics handbooks, and probably even on the Internet that gives you all the detailed information.

Vince Calder


Charlene -

This is the kind of question that can never have a final answer. It depends on where you want to draw the line, on how unstable an isotope can be and still be considered an isotope, still be considered gold, or still be detected.

Wikipedia says gold has 36 unstable isotopes:

http://en.wikipedia.org/wiki/Gold#Isotopes

I believe them, and many have half-lives of more than a day, but one of them has a half-like of less than 0.0001 second. Can't do much with that. In what sense is it "Gold".

In the limit, the least stable isotope is just a low, broad bump in some curve plotted in a physicist's accelerator experiment. Half life of an unstable nucleus can be much shorter than picoseconds, much too short to even collect the electrons needed to be a neutral atom. By that standard there may well be more than those 36 isotopes.

In the other direction, suppose I do not feel like considering it gold unless I can hold a 1-pound ball of it in my (gloved) hand and see the shiny gold color. Then even a half-life of 1 month is too short, because such a ball would be glowing red-hot or hotter, which definitely masks the pleasant gold color, and of course disallows human proximity due to nuclear-reactor-level radiation. I can empathize with someone who feels that's something else, not gold.

So you see the line is a little fuzzy.

There are nuclear isomers too: different "shapes" or excited states of the same bunch of particles. They have the same atomic-weight number, so they are denoted with an "m", i.e., Au 196m2. They are unstable too. I think Wikipedia mentioned 32 of those known.

So I think the answer is: there can be a lot. Having ~200 parts gives a lot of possibilities. Only the indefinitely stable ones are a clearly countable set.

Jim Swenson


Charlene,

Gold has four unstable isotopes that I am aware of: 195 196 198 199. 197 is the stable and most common isotope. 195 decays by electron capture, 196 by positron emission, 198 and 199 decay by beta decay.

There is an ace web site to find isotope information:

http://www.ptable.com/

Best wishes,

Tom Collins



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