[widemind]

Hey all,

So I'm sitting here doing my chemistry homework - and this is INTRO to chemistry.  I've always done horribly at chemistry, so I am hoping desperately I will be able to get some help here!

I missed a day a few weeks ago when they talked about Lewis Structures - which luckily for me happens to be the one thing I REALLY do not understand, so basically at this point I'm totally lost.

Here's my question... and there will be plenty more to follow I'm sure...

When doing Lewis Structures, say I'm doing something simple like O2.  When I drew it up, I drew it as :O///0:  (/// is triple bonds - I don't know how else to type it).  But when I look in the answers, it has it shown as ::O=O::  

Is there some rule of thumb that tells you when to double bond and when to triple bond?  Does it matter?

Any clarification on this is appreciated.

Thanks.
Adam

[mike]

Each oxygen only has two unpaired electrons when you do the lewis dot structure, therefore can only make two bonds and not three.

[widemind]

Thanks for the quick reply.  But I have to say I'm still a bit confused.

If O2 is ::O::O::

Then how is CO written as :C:::O: instead of ::C::O::  ?

I know this must sound like a totally stupid question for most people here - and will most likely be totally stupid to me too as soon as it clicks in my head... I'm just having a rough time getting it to do that  

Thanks again for the *delete me*

[mike]

Good question:

So there are four ways you can bond the two atoms C and O;

.:C-O::. single bond ---> octet not satisfied for either atom

::C=O:: double bond ---> 2 too many electrons giving C a formal negative charge, when CO should be neutral

:C=O: triple bond ---> here both C and O have a formal charge, - and + respectively so they cancel to leave CO neutral, so this satisfies the octet and the charges

C::::O quadruple bond ---> this leaves oxygen with a formal charge of +2 which means the overal molecule is positive, ie 2 too few electrons

[Moonshyne]

According to molecular orbital theory, O₂'s lewis structure looks like:
.::O-O::.
This makes sense because diatomic oxygen is known to be paramagnetic. There is still controversy on this subject though . Many say it's written ::O=O::