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Likely Formulas for Substances

  1. Jan 9, 2007 #1
    1. The problem statement, all variables and given/known data
    What is the likely formula for CH3NH?

    2. Relevant equations
    We need to find the value of ? in order to have 8 valence electrons on the outer shell of the compound.

    3. The attempt at a solution
    I could do less "complex" ones, such as GeCl? Ge has 4 valence elctrons, therefore it needs 4 more, and so the value of ? is 4.
    I am not sure I understand how to do the same for CH3NH?.

    The textbook give Ch3NH2 as the answer.

    Thank you for any explanation you can give me.
  2. jcsd
  3. Jan 9, 2007 #2


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    Find the normal valences for the elements given (C,H,N). Usually this information is just rote memorization or in a tabular form somewhere in your textbook or in a handout the Teacher provides.

    Assume the three H's of CH3 are attached to carbon (C). Assume that there is some bond between carbon and nitrogen (N). Will it be a single bond? Double? Write out the Lewis dot structure for the CH3N radical and add sufficient hydrogens to pair up with the unpaired electrons on Nitrogen (N).
    Your Lewis dot structure should have complete octets (8 valence electrons) for both Carbon and Nitrogen. Note that this is a little different than saying that there should be 8 valence electrons for the outer shell of the compound. Compounds should be written so that the elements that make them up have complete octets. Hydrogen only needs two electrons, by the way....

    CH3NH2 is the correct answer.

    Hint: use different symbols to represent the electrons from each of the different elements to help you keep track of where the electrons are coming from. eg. little dots for hydrogen, little open circles for carbon and little X's from Nitrogen...
  4. Jan 9, 2007 #3
    Thank you for your explanation. Doing a Lewis diagram makes it a lot easier to understand.

    There is something I still don't understand though (same original problem):

    We have C surrounded by 3H and 1N; all the bonds will be single.
    We need to find how many H surround N in order to complete the octet.

    How do we know that N is surrounded by 2 H and has a lone pair of electron? Why can't it be surrounded by 3H and complete the octet this way?

    Thank you,

  5. Jan 10, 2007 #4
    Actually, I read today that N prefers to have 1 lone pair of electrons. Is this the reason why we only have 2H?
  6. Jan 17, 2007 #5


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    Try using three hydrogens and see for yourself if you can achieve an octet around nitrogen...
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