VSEPR theory and Molecular Geometry

[navneetdh]

I am getting really confused about this whole molecular geometry vs electronic geometry mixed with hybridizations etc. Here are a few questions if anyone can plz clarify for me

1. In molecular geometry, when the central atom is making say a double or triple bond woth a terminal atom then do we count each bond to figure out the shape or just count the terminal atoms? And I guess in electronic geometry it would work the same way too, except that if there are lone pairs then the shape would be diff. RIght?

2. I know how to do hybridization states when a central atom is bonded to many DIFFERENT terminal atoms. But how do you figure out the hybridizations when there are double or triple bonds involved? For example CO2 what is the hybridization of C? I think the molecule looks like O=C=O.

3. I get totally thrown off when to the molecular geometry and electronic geometry questions they add on if it is polar or non polar. Is there a quick way of determining that? I read that if the atoms are cancelling each others dipole then its non polar. But how can trigonal planar be non polar then? I understand how a linear molecule cancels each other dipole but how does trigonal planar? or does it have to do with if the terminal atoms are equally distributed around the central atom? How do lone pairs and double or triple bonds affect this?

Sorry I know its a lot to ask but I have consulted a few books and I just need it to be explained to me like I am a 1st grader i guess!!

I would really really really appreciate some help on this. Thank you in advance

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[nze82]

I am getting really confused about this whole molecular geometry vs electronic geometry mixed with hybridizations etc. Here are a few questions if anyone can plz clarify for me

1. In molecular geometry, when the central atom is making say a double or triple bond woth a terminal atom then do we count each bond to figure out the shape or just count the terminal atoms? And I guess in electronic geometry it would work the same way too, except that if there are lone pairs then the shape would be diff. RIght?

2. I know how to do hybridization states when a central atom is bonded to many DIFFERENT terminal atoms. But how do you figure out the hybridizations when there are double or triple bonds involved? For example CO2 what is the hybridization of C? I think the molecule looks like O=C=O.
Always count the number of surrounding atoms rather than focusing on the nature of the connecting bonds. Here you have 2 Oxygen atoms connected to C, so the hybridization state is SP. This leaves two empty p orbitals on C. It's is the overlap between these p orbitals and that of the surrounding Oxygens that results in the double bond. SP hybridization corresponds to a linear molecule (provided that there are no lone pairs bending this configuration) and this is indeed how CO2 looks like:

After knowing the hybridization of the central atom, you can figure out the shape. Google this, and there's a table that relates the hybridization of the central atom to the molecular geometry (Or check here: http://peninsula.swiftclassroom.com/phs/dstitt/documents/geometry.jpg). For example, if the central atom is SP2 (3 surrounding atoms) hybridized, then the molecular geometry is Trigonal planar. If the central atom is SP3 (4 surrounding atoms) hybridized then the molecular geometry is tetrahydral. If the central atom is SP3 (3 atoms and 1 lone pair) then the molecular geomtry is Trigonal Pyramidal. The list goes on.

3. I get totally thrown off when to the molecular geometry and electronic geometry questions they add on if it is polar or non polar. Is there a quick way of determining that? I read that if the atoms are cancelling each others dipole then its non polar. But how can trigonal planar be non polar then? I understand how a linear molecule cancels each other dipole but how does trigonal planar? or does it have to do with if the terminal atoms are equally distributed around the central atom? How do lone pairs and double or triple bonds affect this?
The easiest way is to know the 3D representation of these molecules and then be able to look at the dipole moments and see whether or not they'll cancel out. For example, think of NH3. What's the hybridization of N. Well you have 4 surrounding species (3 H atoms and one lone pair of e-), so it's sp3 hybdrizied. sp3 hybridized with a lone pair of electron corresponds to trigonal pyramidal molecular geometry. Let's look at the 3D representation:

Obviously, due to presence of the lone pairs at the top, you'll find a greater electron density at this end of the molecule, making the molecule polar. It actually looks like:

So, the easiest way is to:
-Look to see whether or not dipole moments cancel out.
-Are there any lone pairs of electrons to shift the electron density to one side of the molecule.


Sorry I know its a lot to ask but I have consulted a few books and I just need it to be explained to me like I am a 1st grader i guess!!

I would really really really appreciate some help on this. Thank you in advance

Hope this helps!

 

[navneetdh]

Thanks, yes it is making a little more sense now. So just like for molecular geometry you only count the number of surrounding atoms, same way for hybridization you only count the surrounding atoms and not the number of bonds, right?

So say something like G=H-B with a lone pair of electrons on H is still sp2 hybridized?

the link was very helpful, Thank You so much

 

[nze82]

Thanks, yes it is making a little more sense now. So just like for molecular geometry you only count the number of surrounding atoms, same way for hybridization you only count the surrounding atoms and not the number of bonds, right?
This is how I've always done it.
So say something like G=H-B with a lone pair of electrons on H is still sp2 hybridized?
How on earth is H ever going to acquire a lone pair? Hydrogen has only 1 electron!
the link was very helpful, Thank You so much

You're welcome

 

[dentrilla]

pretty sure it was a hypothetical question! but yah the atom you labelled "H" (definetly not hydrogen.. or atleast I hope thats what your thinking! hah) is sp2.

 

[nze82]

pretty sure it was a hypothetical question! but yah the atom you labelled "H" (definetly not hydrogen.. or atleast I hope thats what your thinking! hah) is sp2.

Oh! Ok.
I see H and I automatically think Hydrogen.

 

[dentrilla]

Oh yah, its sp2 due to the 3 e- domains, NOT the 3 bonds attached to "H", 1 domain is the lone pair, the next is the double bond and the final e- domain is the single bond.

 

[navneetdh]

Thanks guys it makes a sense finally. Yayyyy.

Oh and yeah "H" was hypothetical. I totally forgot it could be hydrogen too lol. sorry!!