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Carbon & Carbon-12 clarification

  1. Sep 2, 2010 #1
    A Carbon atom has 6 protons, 6 neutrons and 6 electrons orbiting. If an atom gains or loses an electron it becomes an ion with a negative or positive charge. If it gains or loses a neutron, then the atom becomes an isotope.

    From what I read, Carbon-12 is an isotope of Carbon. Yet Carbon-12 has 6 protons, 6 neutrons and 6 electrons, which is exactly the same as Carbon. So how can Carbon-12 be an isotope?

    Thanks to anyone who can clear this up! I'm a chemistry virgin. :)
     
  2. jcsd
  3. Sep 2, 2010 #2

    CompuChip

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    Yes, C-12 is an isotope of the element carbon, and so are C-13 and C-14.
    The issue here arises because you think that there is 'an' element called carbon, which refers to C-12, and that only the ones with more or less neutrons are isotopes of this element.
    However, the word 'isotope' refers to any such "variation".
    Since for carbon C-12 is the most common isotope, it is sometimes referred to simply as "carbon", which probably adds to your confusion.
     
  4. Sep 2, 2010 #3
    Thanks for the reply.

    So, an isotope is basically any neutron variation of an element? Meaning that even the 'default' element shown on the periodic table is an isotype, because it's just another variation of that element?

    When trying to understand it I thought about that, but I read on wikipedia that C-12, C-13 and C-14 were isotopes of and different from Carbon. This made me think they were different variations until I found out C-12 has the same atomic structure as Carbon... which confused me.
     
  5. Sep 2, 2010 #4

    phyzguy

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    Naturally occurring carbon is a mixture. It is 98.9% C-12, 1.1% C-13, and a tiny amount of C-14. Since chemical reactions are controlled by the electronic structure, they all react the same chemically, so they are all "carbon". It is only in nuclear reactions that you see any real difference (not quite true - the slight mass difference causes the chemical reaction rates to be slightly different). When you look up the atomic weight of carbon in the periodic table, it gives you the average atomic weight of the naturally occurring mixture. That is why the atomic weight of carbon is 12.01 g/mole and not 12.00 g/mole.
     
  6. Sep 2, 2010 #5

    Ygggdrasil

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    The identity of an element is defined solely by the number of protons in the nucleus. So, any species with six protons, any number of neutrons, and any number of electrons can be considered to be carbon. Carbon atoms have the additional criteria of having six electrons (to be electrically neutral) whereas carbon ions will have more than six electrons or less than six electrons. The number of neutrons defines the exact isotope of carbon (e.g. carbon-12, carbon-13, etc.)
     
  7. Sep 2, 2010 #6
    Kaldanis you are totally correct in your last post. The number of protons is what defines the element (Carbon has 6 protons, no matter how many neutrons or electrons it has). Isotopes are just any of the neutron variations that Carbon exists in. The ones in the periodic table are just the most common/stable forms.

    Now reading everyone elses responses, guess I didn't really add anything new here.
     
  8. Sep 2, 2010 #7

    DaveC426913

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    Which incidentally makes carbon-dating possible...
     
  9. Sep 3, 2010 #8

    Borek

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    Oops, you're going to hate me. I am not sure what you are referring to, but I have a gut feeling you can be wrong. Carbon dating has nothing to do with the chemical differences between C-14 and C-12, quite the opposite - the differences are so small, that C-14 happily replaces C-12 in all tissues, so its fraction can be assumed to be identical in all living organisms. Lack of difference is what makes carbon dating possible, but you have also quoted part of phyzguy post that referred to so called kinetic isotope effect. This has nothing to do with carbon dating. It is most pronounced in the case of very light isotopes, like hydrogen and deuterium. The difference is so large that drinking heavy water is lethal. As far as I know kinetic isotope effect in the case of carbon is so low it can be ignored in living organisms.
     
  10. Sep 3, 2010 #9

    DaveC426913

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    I thought that the key to carbon-dating is that plants (and thus the things that consome them) take up C-12 and C-14 at slightly different but highly predictable rates. Thousands of years later, when we measure the differences, we know how fast the C-14 has decayed.

    I'll look it up.

    [EDIT] Well well well. You learn something new every day. Carbon-dating is only relative to known ages.

     
  11. Sep 3, 2010 #10

    phyzguy

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    I think living things take up C12 and C14 at basically the same rate. The C14 is decaying, but they are constantly taking in new C14, so there is an equilibrium concentration of C14 in their tissues. After they die, they stop taking in new C14, so the C14 decays away, with a half-life of ~5700 years. By measuring the C14/C12 ratio, you can tell how long ago they died.
     
  12. Sep 3, 2010 #11

    Borek

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    Even if they are taken at different rate, the difference is most likely the same for most organisms, so all living things have the same ratio of C12/C14, not necessarily identical with the ratio present in the atmosphere.

    Hm, that could mean there can be a measurable difference between organic and inorganic deposits (by inorganic I mean natural chemical processes, not things like reefs).
     
  13. Sep 3, 2010 #12
    Nah, it's all a bit more complex. Not in the least place the ratio in which the isotopes are processed by the biota. There are a lot of subtle differences but a very big one is the difference in C3 and C4 photosynthesis. Dave is right, things are rather predictable though, thanks to the 13C. The fractination of radiocarbon (delta14C) is rougly twice that of heavy stable carbon (d13C). So the d13C of the sample can say something about the original d14C at the time of the assimilation, assuming that the d13C and d14C rates have been constant,

    Unfortunately they weren't, many processes, especially the oceanic interaction and solar activity have made these values highly variable.

    However there have been many records from countable annual layers(lake sediments, coral, tree rings etc) that could also be carbon dated. In a big combined effort this has given a carbon dating calibration table, INTCAL04 that can be assumed to be reasonably accurate.
     
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