TPRH Shape vs Geometry

[labqi]

Hey All!
I hope studying is going well. I have been reading through a dozen of the past posts and can't seem to decipher what the actual difference between shapes and geometry. The past posts seem to contradict each other and if anyone had a clear answer that would be great.
From my current understanding:
Shape ---> does not consider electrons thus is one of: Trigonal Planar, Tetrahedral, Trigonal Bipyramidal, Octahedral
Molecular Geometry ---> includes the electrons and falls under the subcategory of shape. Ie. AX3 = trigonal planar, but if its AX2E1 = bent.

Could someone please clear this up, and what term is the MCAT expecting us to use.

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[DrknoSDN]

The MCAT will use the same terms you did in your post. Atoms position constitute shape while total bonding to central atom determines geometry. [One correction. Molecular geometry cannot be bent. Molecular Geometries are the normal shapes you would expect from 2,3,4,5,6 atoms bonded to a central carbon.] To further clarify, shape and geometry will be the same if there are no unpaired electrons.

If you use the table found on Wiki: http://en.wikipedia.org/wiki/Molecular_geometry#VSEPR_table
The Electron domains is the "geometry" based on number of bonds formed (e.g. 6 is always octahedral). The "Shape" column is self explanatory.

There are more shapes then geometries because the geometry is determined by the number of 'things' (atoms or electrons) bonded to the central atom. So a Geometry containing 5 things is always trigonal bipyramidal, but the shape can be linear, trigonal bipyramidal, seesaw or t-shaped.

Hope that helps.

 

[labqi]

Thanks. I was confused! But now that seems to clear it up.

 

[labqi]

With that explanation. Doesn't the shapes then take into consideration electrons/pairs whereas geometries do not?

 

[DrknoSDN]

With that explanation. Doesn't the shapes then take into consideration electrons/pairs whereas geometries do not?

Geometries take into consideration everything bonded, atoms or electron pairs.
I suppose you could say shape takes into consideration the repulsion force of the electrons on the atoms.

It would be easier to know that if you have sp2 hybridization and only 2 atoms around the central atom that it is bent. Also the names of the shapes are describing the atom configuration... Look at the Seesaw shape.. It atomic configuration actually looks like a seesaw with the 120* bonds forming the legs. You can think about it however works best as long as it's clear on MCAT day. =D

 

[Blueprint MCAT Tutor]

Electronic geometry = the overall shape of all the electrons, including lone pairs and bonds.

CH4
NH3
H2O

All have electronic geometry that is tetrahedral. They all have four things around the central atom.

Molecular geometry = shape = the overall shape of the molecule only looking at the bonds, ignore the lone pairs

CH4 = shape is tetrahedral
NH3 = shape is trig pyr
H2O = shape is bent

 

[Blueprint MCAT Tutor]

Molecular geometry cannot be bent.

Where did you read that? That strikes me as a really odd use of the term "molecular geometry".

I'd bet dollars to doughnuts that if you walked up to 100 chemistry professors and said, "Quick! What's the molecular geometry of water?!" that you'd get 99 people immediately saying, "BENT!" (the last guy would be startled and keel over from a heart attack)

 

[inasensegone]

Where did you read that? That strikes me as a really odd use of the term "molecular geometry".

I'd bet dollars to doughnuts that if you walked up to 100 chemistry professors and said, "Quick! What's the molecular geometry of water?!" that you'd get 99 people immediately saying, "BENT!" (the last guy would be startled and keel over from a heart attack)

I think he may have meant electronic geometry, because even in the link he provided (and this one) it shows bent molecular geometry

Since a picture is worth a thousand words:

 

[manohman]

Hey guys sorry to bump this back up, but i ran across a question that asked:

What is the shape of the ozone molecule?

IT said the geometry was Trigonal Planar but its shape is bent. They dont specify whether were talking about electron pair shape or not.

Or is this just beyond the scope/specificity of the mcat. Getting real tired of finding random little rules i never knew about Princeton -__-

 

[popopopop]

http://www.elmhurst.edu/~chm/vchembook/206bent.html

^Perhaps that will help? When I think of shapes, I don't think of electrons since they are a cloud. I think of the electron repulsing the atoms away, causing it to bend to a certain shape.

 

[manohman]

I see thank you that link is really helpful.

Ithink part of what's getting me caught up is the resonance that exists. How do you factor in resonance into determining structure? is it just by choosing the best resonance structure?

 

[Teleologist]

I see thank you that link is really helpful.

Ithink part of what's getting me caught up is the resonance that exists. How do you factor in resonance into determining structure? is it just by choosing the best resonance structure?

Are there parallel p-orbitals into which electrons from one atom can be delocalized? If there exist such p-orbitals of the correct mutual orientation (and size) then you have to take into account the delocalization of electrons. You can draw an structure of ozone that makes it look as if one oxygen is sp3 hybridized. But in reality it's not sp3; it's definitely ~sp2 as one p-orbital on each oxygen atom is used for delocalization.