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Thread: DO not understand dot/cross diagrams

  1. #1 DO not understand dot/cross diagrams 
    Forum Freshman
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    Aug 2006
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    Hey guys,

    Anyways were doing this 'project in chemistry, and i am having trouble understanding the easy bits, but all the hard bits i am finding very easy.

    I just cannot do dot/cross diagrams. I have already selected what i will be doing

    1. Cl20
    2. CCl4
    3. SiCl4
    4. C2H6

    I don't know why i am finding it hard, but i will speak with my chemistry teacher when i see him. I am not having any other trouble with electronic configuration or covalent bonds etc. (I'm only 14 years old)

    Can you please help me with the dot/cross diagrams for the above?


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  3. #2  
    Forum Freshman Leukocyte's Avatar
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    Joel,

    Providing you're not having any other trouble with valence electrons, it's pretty easy to just follow a method when doing these things.

    Write out how the arrangement of the molecule should look. So for these, that's going to be:

    1. Cl-O-Cl

    2. Cl
    Cl-C-Cl
    **** Cl

    3. As above

    4. H H
    H-C-C-H
    ****H H

    Starting with the central atoms (that's what I used to do anyway), draw in the valence electrons. So for number one, you'd put six dots around the oxygen to represent the six electrons that an oxygen atom has in it's outer shell. Remember the lone pairs - if the oxygen is involved in two covalent bonds, it's going to be sharing one electron with each neighbour and have two lone pairs (at the top and bottom).

    Now, you need to fill the shell - a full outer shell needs eight electrons (apart from hydrogen, which needs two), so you need to put in a cross on each side, representing that the oxygen is sharing one electron with the chlorine on the left and one electron with the chlorine on the right.

    Now to fill up the rest of the chlorine shells - a chlorine atom has seven valence electrons and you've already marked in one, so put another six (three pairs) around each chlorine atom.

    When you think you've done it correctly, check to see that each atom has a total of eight electrons surrounding it (except hydrogen!) and that each has the correct amount of it's own valance electrons.

    It's pretty hard to show you the answers in a messageboard post though!

    When doing the others, remember that carbon has four valence electrons and number that number three is going to look the same as number two.

    Hope that helped.


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  4. #3  
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    Thanks alot, that really helped. I don't know why i was so baffled, and sorry i know it's impossible to write it out on a message board.

    Thanks
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  5. #4  
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    The central atom should be the one with the lowest electronegativty (EN) and can form many bonds in that given molecule.

    The terminal atoms are the ones surrounding the central atom.

    1. Cl20
    2. CCl4
    3. SiCl4
    4. C2H6

    Within the molecule (drawing the lewis structures):
    (1) O is chosen to be in the middle. Although it has a higher EN than Cl, it can form more bond than Cl. Therefore, the O is central atom while the 2 Cl's are the terminal atoms.
    (2) C is chosen to be the central atom since it has a lower EN and can form more bonds than Cl. The 4 Cl's are the terminal atoms.
    (3) Si is chosen to be the central atom since it has a lower EN and can form more bonds than Cl. The 4 Cl's are the terminal atoms.
    (4) The 2 C's are chosen to be the central atoms. The 6 H's are the terminal atoms. 3 H terminal atoms are given to each C.


    "Understanding is better than memorizing."
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