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Lewis Dot Drawings

  1. Dec 21, 2005 #1
    Give each group a shape name based on geometry. This one does not have to be "correct," but I want you to remember what the molecules look like for application later in the unit.

    H2O
    NH3
    CH4
    H2S
    CCl2F2


    I've drawn both a Lewis dot drawing and a stereo projection for each but I still can't come up with any names.

    H2O and H2S could both be called triangles because, if you were to connect the two hydrogen atoms on a stereo projection, you'd have a triangle. CH4 and CCl2F2 could be squares...but what on Earth would NH3 be? If I were to connect it the same way, I'd get another triangle.:bugeye:
     
  2. jcsd
  3. Dec 21, 2005 #2
    well, both [itex] H_2O [/itex] and [itex] H_2S [/itex] are "triangles", but they are flat. [itex] NH_3 [/itex] would be more like a pyramid (because it's 3d)
     
  4. Dec 21, 2005 #3

    chroot

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    What leads you to think that methane (CH4) is planar? Can you think of a 3D shape which would keep the hydrogen atoms even further from each other than they would be on a plane?

    - Warren
     
  5. Dec 21, 2005 #4
    Uh...what leads me to think methane is planar? Planar? I think you're going a bit over my head without knowing it.:rofl:

    So...CCl2F2 and CH4 are both pyramids? That confuses me even more though...what in the hell would the others be?:confused: I don't know if my teacher is asking for their 3D geometric shape...

    I could say that both H2O and H2S could be flat triangles while NH3 is a 3D triangle...and that CH4 and CCl2F2 are 3D pyramids...right?
     
    Last edited: Dec 21, 2005
  6. Dec 21, 2005 #5
    [QUOTE="AngelShare]Give each group a shape name based on geometry. This one does not have to be "correct," but I want you to remember what the molecules look like for application later in the unit.
    H2O
    NH3
    CH4
    H2S
    CCl2F2
    I've drawn both a Lewis dot drawing and a stereo projection for each but I still can't come up with any names.
    H2O and H2S could both be called triangles because, if you were to connect the two hydrogen atoms on a stereo projection, you'd have a triangle. CH4 and CCl2F2 could be squares...but what on Earth would NH3 be? If I were to connect it the same way, I'd get another triangle.[/QUOTE]
    It's a bit unfair being asked for these molecules' shapes without being taught how to do that in the first place, isn't it?

    Perhaps you could check out
    http://www.chemguide.co.uk/atoms/bonding/shapes.html

    Scroll down to
    "Two electron pairs around the central atom"
    and read down to H2O, to see how the types of lewis diagrams you have drawn translate into 3-dimensional shapes.

    Don't worry too much about bond angles at the moment.
     
  7. Dec 22, 2005 #6
    Unco is right, Lewis dot structures say nothing about the three dimensional shape of a molecule. What you are looking for is something called "VESPR" or "VSEPR" theory. The above website is a start, but if you are allergic to online learning sources (like me) you may want to thumb through your text for a bit as you might get a more detailed explanation of it there.
     
  8. Dec 23, 2005 #7
    Did I get this right? Did I word it correctly?

    According to the VSEPR theory, H2O and H2S are linear molecules. CH4 and CCl2F2 are Tetrahedral shaped and NH3 is Trigonal Planar.
     
  9. Dec 23, 2005 #8

    siddharth

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    No, that's not correct.
    H20 and H2S are not linear. NH3 is not planar.

    AngelShare, are you reading this topic from a book or are you learning this from an online course?
     
  10. Dec 23, 2005 #9
    My class is online but, in order to keep my grade up, I was given a book.

    Now I'm confused because the example in the book shows CO2 and it looks exactly like H2O and H2S...doesn't it?

    Clearly, all molecules that contain only 2 atoms, such as molecular oxygen and hydrochloric acid, are linear. But many molecules that contain three atoms are also linear.
     
  11. Dec 23, 2005 #10

    siddharth

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    Nope, there are differences between C02 and H20. How many lone pair of electrons are there in the central atom in C02? On H20?

    That does not mean all molecules with three atoms are linear!

    Here is another link which might help.
    http://misterguch.brinkster.net/VSEPR.html
     
    Last edited: Dec 23, 2005
  12. Dec 23, 2005 #11
    In terms of Lewis structures they are almost exactly the same shape (because again, Lewis structures do not include any information on the 3D shape of the molecule). Think of the problem you are trying to solve as really being a *two* step process:

    Step 1. Use Lewis theory to draw the electron dot structures of the compound you are considering (you seem to be doing this step OK).
    Step 2. Look at the central atom and use the VSEPR theory to figure out the 3D shape of the molecule (you seem to be stuck at this step).

    For example, look at your lewis structures of H20 and CO2. Notice that CO2 has two double bonds on the central Carbon atom? This means there are no free *electrons* on the central Carbon which means that it's shape is considered linear. Now look at your lewis structure for H20. Yes, there are two molecules bonded to the central carbon as well here, but in this case these are single bonds. This means that you have two bonds on the carbon AND two free electron pairs attached to the central carbon. What the electron pairs do is "push" the Hydrogens closer together and this is why you see that "bent" shape of H20 molecules in your book (and also why the molecule ends up being slightly polar).

    There is no shortcut, the Lewis structures are insufficient in that regard as the above example shows, you have to read up on the VSEPR theory or you won't have the tools you need to make the distinction as shown above (many links have been provided on this for you so from here it is up to you if you want to put in some extra work and get this right or not).
     
  13. Dec 23, 2005 #12
    blehh

    I'm doing the same exact assignment right now and I'm having trouble giving Lewis dots to my molecules!! oh and just curious who is your chem teacher?
     
  14. Dec 23, 2005 #13
    Try chemguide as suggested above. Its great!
    Just so as its in words, lewis structures are not too hard once you get a good grasp of concepts :)
     
  15. Dec 23, 2005 #14
    Thanks, I'll look over everything and post my answer again once I think I have it.

    Oh and, xxemeraldsxx, I had Mr. Lanier but he was just replaced by Mr. Burns.
     
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