Dipoles/Polarity of molecules

[luken93]

Ok, so apparently I'm meant to be explaining to my class about dipoles, and how to determine the polarity of molecules.

So basically, my understanding is:
- A polar covalent molecule is a molecule of different elements which have different electronegativities.

- A dipole is a charge separation between the two sides of a polar covalent molecule

Please correct me if I'm wrong

So for example:

Hydrogen Chloride = (Slightly +ve end of the dipole) + H - Cl (Slightly -ve end of the dipole)
Hydrogen is the positive because it is less electronegative than Chlorine, therefore the bonding pairs move towards Cl

And finally, I do not get this:
I understand Iodine, because the electronegativities are equal
I understand Hydrogen Bromide, because Bromine has a higher electronegativity
I understand Carbon Dioxide, because the two sides cancel out
But this is the part I don't understand
Can you please explain the difference between the CH₄ and CH₃BR, because I don't really understand how the shapes affect it for these and H₂O.

Thanks

[Twenty10]

CH4 is non-polar as the electronegativity of H's are equal so the electrons are attracted to each H by the same strength, basically meaning that the electrons arnt attracted to any H more than the other.  

Br is more electronegative than the H's and will therefore have a share of electrons for longer period of time so that makes the Br slightly negative while making C slightly positive this makes it polar....
 
or

Its because Br has more electrons in its outer shell. Lone pairs that make it electronegative.

Im not really too sure lol....

[luken93]

CH4 is non-polar as the electronegativity of H's are equal so the electrons are attracted to each H by the same strength, basically meaning that the electrons arnt attracted to any H more than the other. 

Br is more electronegative than the H's and will therefore have a share of electrons for longer period of time so that makes the Br slightly negative while making C slightly positive this makes it polar....

is it easier if you do it as a lewis diagram, because it seems easier that way

[Martoman]

don't forget the most interesting part of bonding, instantaneous dipoles, basically the reason for dispersion forces. Kinda irrelevant, but since you are discussing bonding you might as well. Also, for anything Chem, Nelson is King, jacaranda is bad. Really bad. Heinemann isn't any better. I find Heinemann treats its audience like 1st year chem students. This we are not.

[luken93]

don't forget the most interesting part of bonding, instantaneous dipoles, basically the reason for dispersion forces. Kinda irrelevant, but since you are discussing bonding you might as well. Also, for anything Chem, Nelson is King, jacaranda is bad. Really bad. Heinemann isn't any better. I find Heinemann treats its audience like 1st year chem students. This we are not.

haha studyon chemistry 1
I think I have realised that I probably should've looked at geometric shapes, because it seems this is how you solve them
any good  places to learn these?

[Martoman]

Oh the most common are the
tetrahedral: , non polar because it is symmetrical and all things cancel out.
Pyramidal: (which is polar because the N atom is a hungry mutha for electrons... that is, it is electronegative  :coolsmiley:)
V-shaped again which is polar because of a loss of symmetry (cool kids call this asymmetric FYI)
Linear in which the dipole charges balance out, so it isn't polar.

Also note that polar-polar likes to bond together, and non-polar to non-polar like to as well, but they don't particularily like to go polar-non polar, because.... they are racist to each other  :knuppel2:
Erm, scientifically, its because the bonds between polar-polar are strong, they would rather stay with that bond then break up to form a bond with an (inferior? lol) non-polar bond.
This is why oil is immiscible, or doesn't tend to bond with water and stays as a film on the top of it.

For resources there is always Khan-academy, but I don't know if he does stuff on this, he focuses more on maths.

 

[luken93]

Oh the most common are the
tetrahedral: , non polar because it is symmetrical and all things cancel out.
Pyramidal: (which is polar because the N atom is a hungry mutha for electrons... that is, it is electronegative  :coolsmiley:)
V-shaped again which is polar because of a loss of symmetry (cool kids call this asymmetric FYI)
Linear in which the dipole charges balance out, so it isn't polar.

Also note that polar-polar likes to bond together, and non-polar to non-polar like to as well, but they don't particularily like to go polar-non polar, because.... they are racist to each other  :knuppel2:
Erm, scientifically, its because the bonds between polar-polar are strong, they would rather stay with that bond then break up to form a bond with an (inferior? lol) non-polar bond.
This is why oil is immiscible, or doesn't tend to bond with water and stays as a film on the top of it.

For resources there is always Khan-academy, but I don't know if he does stuff on this, he focuses more on maths.

 

ok then......
but when i have about 20 questions asking what type of bond is it, what is the best way of workin it out?
do i draw it? lewis diagram?
?

[Martoman]

erm draw it.