(Polar molecules, Non-polar molecules, etc.)
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It means that the VSEPR model can give you the quantitative shapes of the molecules like tetrahedral or trigonal planar or something. However, when there are lone pairs involved, the VSEPR model cannot give you an exact measurement of the bond angles due to the higher electron repulsion, thus it cannot give you a good quantitative model. You can only guess the approximate value of the bond angle measurement.
Last edited by spark99 on Thu Nov 15, 2018 11:35 pm, edited 1 time in total.
It means that when lone pair electrons are attached to the central atom of a molecule, that VSEPR has no specific formulas that will give you the exact numerical (quantitative) angle values for that specific molecule. The exact angle values in this case can only be determined experimentally. VSEPR in this scenario can only provide basic trends for the bond angles (like whether or not the angle will be "less than X degrees") which, by definition, are qualitative and not quantitative. For example, VSEPR says that a molecule where the central atom has one lone pair and two atoms attached to it will have bonds angles that are "some value less than 120 degrees" but will not give you the exact numerical value in that range.
Qualitatively (like visually) as in a VSEPR model can predict a molecule's general shape using lone pairs and bonding pairs, but it can't predict the qualitative (numerical) aspects of a molecule that has distortions, such as bond angles. That's why, for example, a VSEPR model predicts a bent or angular molecule with 3 areas of electron density as having a bond angle less than 120 degrees, not an exact number for a bond angle.
I believe bent and angular are different names for the same shape. The VSEPR model predicting things qualitatively but not quantitatively means that the VSEPR model can't predict the exact bond angles between atoms; when the atoms are different or there are lone pairs, it can only predict the relative sizes of the bond angles.
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