VSEPR, please help?
for molecular geometry, you do not take into account the lone pairs
but what is the official term for when you DO count the lone pairs?
Answer:
Molecular geometry does take into account the lone pairs, but only from the standpoint that the lone pairs affect the geometry of the molecule.
VSEPR "counts" lone pairs and any type of bond equally. VSEPR is concerned with bonding domains. For example, let's say you have a molecule that has 4 bonds. 4 bonds = tetrahedral VESPR geometry
Molecular is ONLY going to report the shape of the molecule where there are bond - there aren't any lone pairs on this guy so the molecular is also tetrahedral (when there are no lone pairs, VSEPR and molecular are the same).
Ok - next: now we have 3 bonds and 1 lone pair. So, VSEPR counts everything up and we get 4 electron domains again (3 bonds + 1 lone pair = 4) and so VSEPR says, tetrahedral
Molecular says - wait a minute, there isn't a bond there. BUT, those lone pairs doo affect where the other real atoms are in space (think of a bee. If there was a bee buzzing near your head, what would you do - move away - and if someone took a picture of you, you would be alll cockeyed in space but the bee would not appear in the picture - too small - the bee affected your location in space). Lone pairs do the same thing - they prevent atoms from moving into their space because electrons repel one another.
So: 3 bonds, 1 lone pair: molecular only reports the shapes of the atoms in the molecule. 3 bonds, 1 lone pair will be a shape BASED off of tetrahedral, but now we have a l.p. sticking up. The shape of the remaining atoms in the structure is trigonal pyramid. The lone pair prevents the atoms from moving so that is why the molecular shapes are based off VSEPR.
You might consider searching on the term VSEPR.
Here is mine:
http://dbhs.wvusd.k12.ca.us/webdocs/vsep...
www.ochem.com
its a pretty good website if you want help in organic chem. :)
try
http://www.chem.purdue.edu/gchelp/vsepr/...
Stephen,
Here's the simple answer to your question: The term used for the geometry that counts lone pairs as well as bonded pairs is the "electron pair geometry".
However, "believer" is right to point out your assumption about molecular geometry. You DO in fact need to take into account the lone pairs when finding a molecular geometry. Thinking about it as "molecular geometry does not include lone pairs but electron pair geometry does" is too much of a simplification. Taking into account the lone pairs to find the electron pair geometry is a necessary PRECURSOR to finding the molecular geometry. The next step is to look at how many bonded pairs there are compared to the total number of electron pairs, and decide where, if any, the lone pairs will be.
So a better way to think about it is that electron pair geometry takes into account all electron pairs equally, then the molecular geometry distiguishes between the bonded pairs and the lone pairs, but both are taken into account.
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