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### determining VSEPR models

Posted: Sun Nov 11, 2018 8:51 pm
I understand that regions of high electron concentrations repel one another but, how do you know how much they repel one another? For example, the seesaw, tetrahedral, and square planar shapes have the same number of bonds, yet they have very different models.

### Re: determining VSEPR models

Posted: Sun Nov 11, 2018 9:54 pm
The seesaw, tetrahedral, and square planar models differ in their number of lone pair of electrons. The tetrahedral model has 4 bonding pairs of electrons around the central atom, the seesaw has 4 bonding pairs and 1 lone pair of electrons around the central atom, and the square planar model has 4 bonding pairs and 2 lone pairs of electrons around the central atom.

### Re: determining VSEPR models

Posted: Mon Nov 12, 2018 12:49 pm
I think one example that showcases how high electron densities repel each other is looking at molecules that have a tetrahedral shape: molecules with 4 bonded pairs, 3 bonded pairs and 1 lone pair or 2 bonded pairs and 2 lone pairs.

For instance, the bond angles for CH4 is 109.5 where as for NH3 it is 107.5 and for water it is 104.5. As the number of lone pairs increase, the electrons occupy more space and push the bonded atoms to be closer, decreasing the bond angles between them.

### Re: determining VSEPR models

Posted: Mon Nov 12, 2018 2:36 pm
In addition to the responses above: what helps to form our VSEPR models is to be aware of the lone pairs in the model, which will cause repulsion and push the atoms into specific shapes. Even if our number of atoms are the same across different models, it is very possible to have different VSEPR structures due to the fact lone pairs may or may not be present.

### Re: determining VSEPR models

Posted: Mon Nov 12, 2018 11:11 pm
While determining VSEPR formulas, does the E represent the total number of electrons not bonded or a collective lone pair? For example if the central atom had 2 electrons that aren't bonded would the E in the formula be E1 or E2?

### Re: determining VSEPR models

Posted: Mon Nov 12, 2018 11:27 pm
At this point when they all look similar, it is important to locate and count the amount of lone pairs. This is because of repulsion and attraction rules. The fact that one molecule may have lone pairs affects the connection between elements inside the molecule.

### Re: determining VSEPR models

Posted: Tue Nov 13, 2018 12:50 pm
When we are determining the shape of and bond angles in a molecule, we use the VSEPR theory. By figuring out the steric number of the molecule and drawing out the Lewis structure, you can use the number of bonds and lone pairs (i.e. the steric number) to figure out the shape of the molecule. After you know the shape, there are certain bond angles that correspond to each shape so you'll have to remember those too!

Basically, remember to count the number of lone pairs of electrons as well as the number of bonded atoms to figure out the shape of the molecule.

I don't think we're given the shape or bond angles in the tests so we'll have to remember those.