Help on the VSEPR theory?

Question

how do you know when a molecule is supposed to be linear or bent, or trigonal planar or pyramid? are there specific rules? examples will be greatly appreciated. thanks.

Answer 1

I was taught that in order to figure out which structure the molecule will take, you need to calculate the steric number. The steric number is simply the sum of the number of bonds on the central atom plus the number of lone pairs on that atom. Then simply compare it to a chart such as the one illustrated here:

http://en.wikipedia.org/wiki/VSEPR

After a while of doing these you won't need the chart anymore and you'll remember what shapes are associated with what steric number and number of lone pairs.

Methane for example has 4 bonds and no lone pairs. This makes for a steric number of 4 with no lone pairs. This configuration is therefore tetrahedral.

Answer 2

http://en.wikipedia.org/wiki/VSEPR

scroll down to AXE method, this should help you a lot  

Answer 3

trust your instincts

Answer 4

Okay, so suppose you have a CH4 (methane) molecule. Now, after drawing the Lewis dot structure, you see that there are no lone pair electrons (E). Therefore, you could write methane as AX4.

A= Central atom            X= Atoms around the central atom

E= Lone electrons (not always there)

So, according to the table, the electron and molecular geometry would be tetrahedral. Now, electron and molecular geometry are not always the same.

Suppose you have a AX4E2 molecule:

Electron geometry: Octahedral

Molecular: Square Planar.

The URL for the table: http://intro.chem.okstate.edu/1314F00/Le...

Good luck! Hope that helps!!!