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Thread: Atomic weights of elements

  1. #1 Atomic weights of elements 
    Forum Freshman
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    I'm new to this field and need help on this.
    For example:
    Carbon
    -6 protons
    -atomic weight = 12.0107

    It is neutral, so where does the '.0107' come from? Someone explained to me that it's a collection of average numbers of all the Carbon known but he didn't explain it that well and it's still troubling me.


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  3. #2  
    Universal Mind John Galt's Avatar
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    Elements typically have a number of isotopes. These are atoms of the element that have the requisite number of protons, but different numbers of neutrons. For example oxygen has thirteen isotopes with three of them being stable - i.e. they are not radioactive, decaying to another element. The atomic weight is the average 'mass' of the element based upon the ratios of each isotope in a typical sample.

    Check out what wikipedia has to say on the subject.


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  4. #3  
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    On top of that, carbon has electrons too. Even though their mass is really really small, you can't just count all the protons + neutrons and then assume it's the right mass when you look into detail.
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  5. #4 Re: Atomic weights of elements 
    Forum Freshman jsloan's Avatar
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    Quote Originally Posted by bassmethod
    I'm new to this field and need help on this.
    For example:
    Carbon
    -6 protons
    -atomic weight = 12.0107

    It is neutral, so where does the '.0107' come from? Someone explained to me that it's a collection of average numbers of all the Carbon known but he didn't explain it that well and it's still troubling me.
    Here's how it's done:

    http://nobel.scas.bcit.ca/chem0010/u...alc_carbon.htm
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  6. #5  
    Forum Bachelors Degree x(x-y)'s Avatar
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    The atomic mass () of an element is taken as an average of all of the isotopes of that element.

    Note: An isotope is a different atomic form of the same element with the same number of protons and electrons, but a different number of neutrons (different mass number).

    At your level you just probably need to know that



    but as toonb says, electrons are ultimately taken into account too.
    "Nature doesn't care what we call it, she just does it anyway" - R. Feynman
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  7. #6  
    Forum Freshman Sobek52's Avatar
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    Yes, electrons do have to be taken into account when precise measurements are needed. They have virtually no mass, with nearly all of an atom's mass being contained in the nucleus.
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  8. #7  
    Forum Bachelors Degree x(x-y)'s Avatar
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    An electron has something like 1/1870 the relative mass of a proton, just so you know.
    "Nature doesn't care what we call it, she just does it anyway" - R. Feynman
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  9. #8  
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    Thanks for all the help!
    I just purchased this book and it's been working wonders for me.
    Chemistry is such a beautiful thing.
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