Molecular shapes

[nacho]

Hi,
so i was going over molecular shapes and stuff, just when I thought i had it nailed,
i came across CS2.
From what i've seen previously, molecules with more than 2 atoms, in which there are several atoms attached to just one (eg. H2S)
The molecular shape is always tetrahedral (including the bonding pairs and lone pairs, otherwise, it is: 5 atoms - tetrahedral, 4 atoms, triangular pyramid, 3 atoms bent or v-shaped, 2 atoms - linear [please correct me if i'm mistaken])..
So I drew the molecular shapes for 4 molecules: H2S, HI, CCl4 and PH3 - all of which, were correct.
I then came across CS2, which of course, has a double bond - this is what differed this molecule from the others.
What struck me as odd, was that there were 3 atoms in this molecule, yet it maintained that Linear shape,
and now thinking about it, I've come to the realisation that i overlooked the fact that the lone pair electrons help determine the shape (once again, correct if i'm wrong)
So does CS2 have a Linear shape, due to the fact that CS2 has no lone pair electrons?
If a certain molecule had double bonds as well as lone pair electrons, would it still follow the thing i stated before ?(in green)
Is there anything different about molecules with triple bonds?
Please guide me.

Edit: I would like to rephrase what I said in the green and before, so that it is clearer.
As long as there are 4 pairs of electrons, the general shape of the molecule, including lone pairs, is tetrahedral, EXCEPT for a molecule with only 2 atoms, where the shape is tetrahedral, but arranged horizontally rather than vertically.
That is all.


Edit again: This is what my book has to say for CO2(which is essentially the same as CS2):
In a carbon dioxide molecule, the four electron pairs around the central carbon atom are concentrated in two regions of negative charge only, each one consisting of a double bond. These two regions repel and so assume positions as far from one another as possible on opposite sides of the central carbon.
The two oxygen atoms and the carbon lie in a straight line and so the molecule has a linear shape.
This sort of clears things(anyone have to add, rephrase anything?) however, in the bold - is this a statement or conclusion?

Thanks.

[luken93]

In the end all molecules' atoms/electrons are spread in such a way that they are as far apart from each other.

Have a look at the attached electron dot diagrams of H2O and CO2, and see if you pick up anything...

[nacho]

In the end all molecules' atoms/electrons are spread in such a way that they are as far apart from each other.

Have a look at the attached electron dot diagrams of H2O and CO2, and see if you pick up anything...

True enough, I see how this works now, thanks,
however, i still seem to be getting some of them wrong(i think) I'll post them up in a sec

[luken93]

In the end all molecules' atoms/electrons are spread in such a way that they are as far apart from each other.

Have a look at the attached electron dot diagrams of H2O and CO2, and see if you pick up anything...

True enough, I see how this works now, thanks,
however, i still seem to be getting some of them wrong(i think) I'll post them up in a sec

no worries, it takes a while to work them out properly.

I'll try and find the link to the website I used to learn it..

[luken93]

http://www.dynamicscience.com.au/tester/solutions/chemistry/molecules/molecularshapes.htm

Try that, it's really good for most Chem topics

[nacho]

Sorry it took so long to reply,
attached is two attempts of drawing..
Also, thats an awesome site!
Looks butt-ugly though.

[Mao]

Each atom has some electrons in the outer shell (valence). some participate in bonding, some become lone pairs. Each lone pair and bonding region (double bond is one region, single bond is also one region) occupies space and like to be far away from other regions. The regions thus form configurations furthest away from each other: 1-linear, 2-linear, 3-trigonal, 4-tetrahedral. these are the parent geometry or central geometry.

However, the actual geometry is different, as lone pairs don't occupy space. H2O for example has 4 regions (2 single OH bonds, two line pairs on O), its parent geometry is tetrahedral, but since two regions are invisible (lone pairs are invisible), it appears bent. HF has 4 regions (single bond, 3 lone pairs), parent geometry is tetrahedral, apparent geometry is linear. CO2 has two regions (2 double bonds), parent geometry is linear, apparent geometry is linear.

the name of this theory is called VSEPER theory.

Try CH3Cl, NH3, CH2O, BH3, FH, H-O-O-H (hydrogen peroxide, H2O2), N2H2 (H-N=N-H), S8 (crown sulfur), HCOOH (formic acid), CS2/CO2

[luken93]

Haha I know the website looks pretty bad, but the animations are very helpful compared to the fixed textbook pics

[nacho]

Haha I know the website looks pretty bad, but the animations are very helpful compared to the fixed textbook pics

very true, the animations are hideously awesome.
could anyone help me in identifying my errors in the one I drew  ?

[Russ]

Haha I know the website looks pretty bad

Haha this is what my class said (our teacher was one of the guys who developed it)

[nacho]

^

hideously awesome.

Haha is this oxymoron meant to be sarcasm then?

Na, those diagrams are extremely hideous and hurt my eyes, but they are definitely helpful
was my attempt of drawing the molecule wrong though? The book didn't seem to include the lone pair electrons for the oxygen?

[nacho]

I'm a little lost as to why the bonding pair is ommitted, but your shape does seem wrong. Technically speaking, the O atoms are supposed to 'repel' each other as far as possible (resulting in that out-spread shape => linear/planar) - dipole-dipole forces

yea i just went over that, so fair enough
what about the one underneath? still so confused as to how you are supposed to know the correct order left to right (are you?)

[AskQuestions]

Sorry it took so long to reply,
attached is two attempts of drawing..
Also, thats an awesome site!
Looks butt-ugly though.

Wait, how would you even know in the first place whether there is a double bond or not?
Like in HCO2, how would you know there is a double bond between the C and the O rather than there being an extra lone pair?

[vexx]

H-C=O

Oxygen has 6 electrons in its outer shell, and it needs to make an octet. If C-O was a single bond, then it would not make the octet, rather then if it makes a double bond an octet is formed for both carbon and oxygen.