VESPR question

[mjk9]

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im constantly getting confused about the questions that ask whether its linear, trigonal planar etc.

my problem is that im unsure when to include the lone pairs.

sometimes they ask about arrangement around the central atom, other times they ask what is the molecular shape.

my take on it is that in the former situation we include lone pairs but not in the latter.

Any suggestions?

Thanks

 

[klutzy1987]

For Molecular Geometry we include the lone pair always and the shape will change depending on how many electron lone pairs it has. The Electron Domain Geometry will always stay the same regarless of whether the components are electron pairs or another element. When they ask for the shape of a molecule they are referring to the Molecular Geometry unless otherwise stated.

 

[DRKNM]

is there anything online that will help with predicting the Molecular geaometry?
Thanks

 

[klutzy1987]

as far as i know just memorize them and also you can just picture what they look like in your head and usually the geometry is just like the shape you are picturing.

 

[hoss19]

yeah, visualizing them is key. remember that the total number of LP electrons plus attached atoms are repulsive and therefore they try to distance themselves as far as possible from each other.

getting the shape is then simply a function of first of visualizing how all entities (LP + attached atoms) are oriented in space; once you have this mentally remove, the bonds that contain LP electrons and voila you have the mg shape. Of course you are going to need to memorize the shape names....

 

[hoss19]

most chem texts have problems that you can practive from to hone your skills...