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Quick valence question

  1. Apr 13, 2008 #1
    I haven't taken a single Chem course for probably 3 years and I was asked a question which I couldn't answer. Basically, why can the valence of Carbon be either +4 or -4? I know it has something to do with the electron configuration.

    Thanks.
     
  2. jcsd
  3. Apr 13, 2008 #2
    Basically, due to the four valence electrons that carbon has, it can either lose electrons or gain electrons to reach a noble gas configuration.

    (Of course, carbon does have many other oxidation states than just plus/minus four--not that this answers the question)

    Edit: Though technically, carbon usually forms covalent bonds, not ions, so it is actually sharing its four valence electrons, in most cases (aside from carbides, etc.).
     
    Last edited: Apr 13, 2008
  4. Apr 13, 2008 #3

    GCT

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    You need to understand the difference between the terms of Valence and Oxidation states. Carbon bonds to four other atoms and usually this is with atoms of higher electronegativity or atoms of similar electronegativity. Carbon has been known to bond with transition metal elements although this phenomena is quite unique.

    Carbon would have a state of +4 if it were bonded to four Oyxgens, something that I don't quite know of. The same argument applies with a theoretical oxidation state of -4 for Carbon meaning that I am not aware of any tetravalent metal - Carbon bonds.

    Carbonyl carbons as esters are common, which is basically a Carbon bonded to two oxygens with one of the bonds being double bond. Not quite sure about this case with Carbons being Carbides.
     
  5. Apr 14, 2008 #4

    Gib Z

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    Carbons valency can be either +4 or -4 because it can either lose 4 electrons to gain Helium electron configuration, or gain 4 to get Neons, but this rarely ever applies because as it loses those electrons, it gains positive charge, which attracts electrons back again. And as for gaining four electrons, as it gains those electrons, it gains negative charge, repelling electrons, making it hard to get those 4. I hope im clear :(
     
  6. Apr 15, 2008 #5

    GCT

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    Carbon participates in covalent bonding thus these electrons are shared however are allocated more towards elements such as Oxygen both positive and negatively charged species with the charge on the carbon exists however, if one really wants to know whether +4 or -4 charge would be favored in theory simply observe the successive ionization and electron affinity.

    ACLerok, I suggest that you read up on the topic of Effective Nuclear Charge this relates directly to your question and should answer it.
     
  7. May 9, 2008 #6

    Hi, would you explain more "Effective Nuclear Charge" please? I am confussing this part also. Thanks.
     
  8. May 9, 2008 #7

    GCT

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  9. May 9, 2008 #8

    chemisttree

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    Would you count CO2 as a carbon bonded to four oxygens? What is the theoretical oxidation state of carbon in CO2? In CH4?
     
  10. May 27, 2008 #9
    In CO2 i think carbon is bonded to 2 oxygen atoms and has a resonating structure. By the way is oxidation state of carbon in CO2 '+4' and CH4 '-4' ?
     
  11. May 27, 2008 #10

    chemisttree

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    Correct. You will note that in both examples, there are four bonds to the other atoms (oxygen and hydrogen). The bonds are both covalent in nature but they are somewhat different based on polarity. The oxygen-containing carbon compound is indeed +4 and the hydrogen-containing one is -4. These oxidation states are merely formalities that reflect the differences in electronegativity rather than purely an oxidation state as you might have seen in some inorganic compounds.

    Things get more complicated with other carbon compounds that contain both hydrogen and oxygen. How about methanol (CH3OH)? What is the oxidation state here? Is it +2? Is formaldehyde (H2C=O) a zero valent carbon like diamond or graphite? How useful is it to use oxidation states in these examples to predict chemical reactivity? And welcome to the wild and crazy world of organic chemistry...
     
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