Zero abundance elements?

Discussion in 'Physics & Math' started by Dinosaur, Mar 2, 2004.

  1. Dinosaur Rational Skeptic Valued Senior Member

    Messages:
    4,592
    A question asked by a friend caused me to use Periodic Chart software to check on the properties of various elements. For every element, the abundance of each isotope was provided as a percentage. This data was included for both radioactive and stable isotopes. For example, the abundances for uranium were 0.0055% for U<sub>234</sub>, 0.72% for U<sub>235</sub>, and 99.27% for U<sub>238</sub>. There were a lot of other isotopes of Uranium listed with zero abundances.

    For several elements (EG: Radium and actinium), all the abundances were zero. What does zero percent abundance mean? Not enough of the element to make a measurement? No valid measurements ever made? The periodic chart software is in error?

    The two elements I noticed (Radium & actinium) have no stable isotopes and are very rare. Could zero percent abundance mean that all of the element has decayed, leaving the Earth void of said element?
     
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  3. James R Just this guy, you know? Staff Member

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    The abundance usually refers to the percentages of different isotopes found in a typical sample of the element on Earth. So, if you dig up some yellow cake and examine the uranium found in it, you will find the uranium isotopes in the ratios listed.

    I am somewhat puzzled by the zero figures for things like Radium. Perhaps it is because radium is produced mostly by radioactive decay of other elements. Therefore, if you find radium in a rock, the amount and the isotopes present will likely be determined by what other elements are present in the same rock.
     
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  5. Epsilon Prime Over Epsilon Registered Senior Member

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    yea, the abundance refers to natural abundance
    some isotopes are simply part of heavier element's decay chain and then soon undergo decay to other stuff, so they will have 0 abundance

    this is also how people calculate the "atomic mass" on the periodic table
    take carbon for example, there's carbon-12, 13, 14 and etc
    they will then have mass of 12 amu, 13 amu and 14 amu respectively
    however, Carbon's atomic mass is not simply 12 amu but 12.01 amu (even tho 1 amu is defined as 1/12 mass of C-12)
    why? because they took natural abundance into account
    by summing the isotope's mass times its relative abundance

    so, for technetium, Tc (Z=43), it does not occur naturally at all and can only be synthesized, it's natural abudance should be zero and it's atomic mass has an integral value of amu. sames goes to all that trans-uranium elements.
     
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