Notices
Results 1 to 9 of 9

Thread: Atomic Weight

  1. #1 Atomic Weight 
    Forum Freshman
    Join Date
    Jul 2009
    Posts
    45
    I am just wondering why atoms are compared to 1/12 of the mass of an atom of carbon? Why carbon? Why is there a need to compare?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Reptile Dysfunction drowsy turtle's Avatar
    Join Date
    Mar 2009
    Location
    UK
    Posts
    2,255
    The weight corresponds to the number of nucleons the atom has, where 1/12 the mass of an atom of carbon-12 is roughly the weight of any single nucleon.

    The number of nucleons is useful to identify isotopes, which have different properties, mainly relating to stability/how likely it is to decay and emit ionising radiation.


    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
    Reply With Quote  
     

  4. #3  
    Forum Freshman
    Join Date
    Jul 2009
    Posts
    21
    Quote Originally Posted by drowsy turtle
    The weight corresponds to the number of nucleons the atom has, where 1/12 the mass of an atom of carbon-12 is roughly the weight of any single nucleon.

    The number of nucleons is useful to identify isotopes, which have different properties, mainly relating to stability/how likely it is to decay and emit ionising radiation.
    I think he was wondering why 1/12 of the mass of carbon-12 is stated as the weight of a nucleon instead of the mass of a hydrogen, 1/39 of the mass of potassium-39, etc. What makes carbon so special?
    Reply With Quote  
     

  5. #4  
    Reptile Dysfunction drowsy turtle's Avatar
    Join Date
    Mar 2009
    Location
    UK
    Posts
    2,255
    Quote Originally Posted by Shadow
    Quote Originally Posted by drowsy turtle
    The weight corresponds to the number of nucleons the atom has, where 1/12 the mass of an atom of carbon-12 is roughly the weight of any single nucleon.

    The number of nucleons is useful to identify isotopes, which have different properties, mainly relating to stability/how likely it is to decay and emit ionising radiation.
    I think he was wondering why 1/12 of the mass of carbon-12 is stated as the weight of a nucleon instead of the mass of a hydrogen, 1/39 of the mass of potassium-39, etc. What makes carbon so special?
    Simpler to purify into known masses of 100% carbon-12, probably.

    But it's just the same question as 'why do we base our mass unit (kilogram) on a piece of metal in a safe somewhere?'
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
    Reply With Quote  
     

  6. #5  
    Forum Freshman
    Join Date
    Jul 2009
    Posts
    21
    Quote Originally Posted by drowsy turtle
    Quote Originally Posted by Shadow
    Quote Originally Posted by drowsy turtle
    The weight corresponds to the number of nucleons the atom has, where 1/12 the mass of an atom of carbon-12 is roughly the weight of any single nucleon.

    The number of nucleons is useful to identify isotopes, which have different properties, mainly relating to stability/how likely it is to decay and emit ionising radiation.
    I think he was wondering why 1/12 of the mass of carbon-12 is stated as the weight of a nucleon instead of the mass of a hydrogen, 1/39 of the mass of potassium-39, etc. What makes carbon so special?
    Simpler to purify into known masses of 100% carbon-12, probably.

    But it's just the same question as 'why do we base our mass unit (kilogram) on a piece of metal in a safe somewhere?'
    Haha, this is true. I think I recall my teacher saying that the piece of metal that represents the kilogram was radioactive or something, and as a result is actually losing mass and is slightly less than a kilogram now? Is that true or is my memory of something he might have said over six months ago just fuzzy?
    Reply With Quote  
     

  7. #6  
    Reptile Dysfunction drowsy turtle's Avatar
    Join Date
    Mar 2009
    Location
    UK
    Posts
    2,255
    No, I heard that too. I think since they realised that, they stopped using it as a measure of the kilogram, and started to rely on digitally saved versions of the kilogram instead.

    After all, if the kilogram changes, the rest of the SI units no longer fit together properly.

    We'll get to the bizaar situation where the kilogram weighs less than a kilogram
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
    Reply With Quote  
     

  8. #7  
    Forum Freshman
    Join Date
    Jul 2009
    Posts
    21
    Quote Originally Posted by drowsy turtle
    No, I heard that too. I think since they realised that, they stopped using it as a measure of the kilogram, and started to rely on digitally saved versions of the kilogram instead.

    After all, if the kilogram changes, the rest of the SI units no longer fit together properly.

    We'll get to the bizaar situation where the kilogram weighs less than a kilogram
    Haha, why use a radioactive mass? Well, then again, I guess all samples of matter almost inevitably have a few radioisotopes in them.
    Reply With Quote  
     

  9. #8  
    Reptile Dysfunction drowsy turtle's Avatar
    Join Date
    Mar 2009
    Location
    UK
    Posts
    2,255
    The only isotope that can ever be 100% stable is , as the neutron is unstable and decays into the protons over time.

    Then again, it is theorised that even the proton is unstable, and decays into mesons and positrons.

    And that is why, invariably, we are all going to die
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
    Reply With Quote  
     

  10. #9  
    Forum Freshman
    Join Date
    Jul 2009
    Posts
    21
    Quote Originally Posted by drowsy turtle
    The only isotope that can ever be 100% stable is , as the neutron is unstable and decays into the protons over time.

    Then again, it is theorised that even the proton is unstable, and decays into mesons and positrons.

    And that is why, invariably, we are all going to die
    Yeah, but... "In other words, proton decay has never been witnessed and the experimental lower bound on the mean proton lifetime (2.110^29 years) is put by the Sudbury Neutrino Observatory" - Wikipedia

    We won't even be around that long to witness the protons decay! We could just have a kilogram of hydrogen
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •