Why is XY polar but XY2 isn't necessarily polar?

[bjoyful]

A diatomic molecule XY that contains a polar bond must be a polar molecule. A triatomic molecule XY2 that contains a polar bond does not necessarily form a polar molecule. I need some examples of real molecules to help me explain the difference.

Thanks

 

[chemisttree]

Look at CO2

 

[bjoyful]

concentration of hydrogen ions

oops...posted wrong message:(

 

[bjoyful]

so you are saying to look at CO2 as an example?

 

[Snazzy]

so you are saying to look at CO2 as an example?

That's what he/she said.

 

[bjoyful]

so I guess my question should become a little more specific...can someone tell me how (or give me a website) CO2 demonstates this?!

 

[LtStorm]

Look at CO2, compare it to H2O.

 

[bjoyful]

..
CO2 = :O::C::O:
..

H2O = H::O:

So I compared them this way, but am I using the right method for this problem? I know that H2O is a polar covalent bonds forming a polar covalent molecule. But carbon is my center in CO2, not oxygen as it is in H2O. Help:(

edit: my dots are suppose to be over the carbon to represent non bonded pairs...

 

[Kushal]

hint: CO2 is linear and H2O is bent

a molecule is polar when it has a side with a partial negative charge and the opposite side with a partial positive charge.

try to locate these partial charges on the molecules.

 

[clouded.perception]

H2O does not have any double bonds, BTW; its electron distribution is why it's a different shape to CO2 (as Kushal pointed out). Only vaguely related to the question, but an important concept.