Determining Shape

[akroberts]

How do we distinguish between shapes of VSEPR models that have the same number of regions of high e- density? Is it based on the number of lone pairs on the central atom?

[Anita Wong 1H]

yes i believe so

[Carolina Lechuga]

Yes, because the more sets of lone pairs you have, the more the shape changes since lone pairs cause a bigger amount of repulsion which ends up affecting the degree of the angle as well.

[Julia Lindner 1I]

Yes, it is based on the lone pairs around the central atom which distort the shape. For example, trigonal planar and trigonal pyramidal both have the same number of regions of electron density (3), but we can distinguish between them because a trigonal pyramidal shape has a lone pair which pushes the binding pairs closer together. The same goes for other shapes. You just have to know which ones include lone pairs (like angular, see saw, etc) and which ones are just bonding pairs (linear, tetrahedral, etc).

[Anna O 2C]

The regions of high e- density will determine the shape but lone pairs have more repulsion energy than electrons in bonds so the lone pairs will alter the bond angles. VSEPR models only take into account where the atoms are, not where the lone pairs are, but the lone pairs need to be considered because they affect how the atoms will be placed around a center atom.

[Alma Carrera 3C]

Yes, it depends on the lone pairs since they cause more repulsion. For example, a tetrahedral and a trigonal pyramid both have 4 regions of e- density but since the trigonal pyramid has a lone pair, the high repulsion causes the bond angles to be slightly smaller than those in a tetrahedral.