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Topic: molecular geometry vs. electronic geometry  (Read 38070 times)

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briteyellowness

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molecular geometry vs. electronic geometry
« on: August 17, 2005, 05:08:37 PM »
what is the molecular geometry of XeF2?

the answer book says linear because there is a difference between molecular and electronic geometry.

is that true?  i was taught that that molecular shape depends on the geometric family to which it belongs to.  and then once you determine the geometric family, you can determine the overal shape by the number of lone pair electrons.

so i put the answer as trigonal bipyramidal.

Offline Donaldson Tan

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Re:molecular geometry vs. electronic geometry
« Reply #1 on: August 17, 2005, 05:46:46 PM »
molecular geometry is actually the electronic geometry minus the lone pairs because the lone pairs are "invisible". eg. ammonia shares the same electronic geometry as methane, ie. tetrahedral, but because of its lone pair, 1 corner becomes "invisible" and the tetrahedral shape is transformed into trigonal pyramidal. only the atoms are "visible".
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline peacefulltortoise

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Re:molecular geometry vs. electronic geometry
« Reply #2 on: October 15, 2005, 07:18:02 AM »
I have some more questions about this problem. In molecule PBr2F3  the two Br lie at equatoral places because they need bigger space than F.  Also with XeF2, this molecule has 4 lone pairs, the lone pairs must have the bigger space than the others so why the geometry of XeF2 does not has V sharp but linear? I think with V sharp the angle of two of the lone pairs would be 180, bigger than 90 when XeF2 is linear. Please explain this for me. Thank you.
 

Offline Winga

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Re:molecular geometry vs. electronic geometry
« Reply #3 on: October 15, 2005, 10:14:20 AM »
I have some more questions about this problem. In molecule PBr2F3  the two Br lie at equatoral places because they need bigger space than F.  Also with XeF2, this molecule has 4 lone pairs, the lone pairs must have the bigger space than the others so why the geometry of XeF2 does not has V sharp but linear? I think with V sharp the angle of two of the lone pairs would be 180, bigger than 90 when XeF2 is linear. Please explain this for me. Thank you.
 
I think you are familiar with VSEPR theory, right?
Electron pairs tend to minimize the repulsions.
Electron-electron pair repulsion decrease:
l.p.-l.p. > l.p.-b.p. > b.p.-b.p.   (l.p.=lone pair, b.p.=bond pair)

For XeF2, the electronic geometry is octahedral, 4 l.p. and 2 b.p..
What you need to do is to calculate their repulsions that finding the most stable configuration.

There are only 2 configurations in XeF2, either both F atoms are 180 degree away from each other or 90 degree separation.

For F-F 180 degree separation, there are:
4 x 90 degree l.p.-l.p. repulsions, 8 x 90 degree l.p.-b.p. repulsions...(no need to discuss 180 degree repulsions at this moment)

For F-F 90 degree separation, there are:
5 x 90 degree l.p.-l.p. repulsions (the main factor), 6 x 90 degree l.p.-b.p. repulsions and 1 x 90 degree b.p.-b.p. repulsion.

It is obvious that the electronic repulsion in 2nd one is higher. Therefore, the 1st one is prefer to the 2nd one.

Offline peacefulltortoise

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Re:molecular geometry vs. electronic geometry
« Reply #4 on: October 16, 2005, 12:22:22 AM »
Great! Thank you Winga.

Offline AWK

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Re:molecular geometry vs. electronic geometry
« Reply #5 on: October 16, 2005, 07:13:31 AM »
I have some more questions about this problem. In molecule PBr2F3  the two Br lie at equatoral places because they need bigger space than F.  Also with XeF2, this molecule has 4 lone pairs, the lone pairs must have the bigger space than the others so why the geometry of XeF2 does not has V sharp but linear? I think with V sharp the angle of two of the lone pairs would be 180, bigger than 90 when XeF2 is linear. Please explain this for me. Thank you.
 
This molecule has 3 lone pairs at Xe
AWK

Offline Winga

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Re:molecular geometry vs. electronic geometry
« Reply #6 on: October 16, 2005, 08:05:30 AM »
I see.


The method is the same, try it yourself, anquyanquyanquy.

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