How do i work out if a molecule has an overall dipole moment?

For any given compound, how do i work out if the compound has an overall dipole moment or not? I know how to draw all the lewis dot structure or the shape of any compound, e.g drawing a tetrahedral compound or a trigonal planner compound (A-lvl student) but i m have no idea as to how to work out, if a compound has a net overall dipole moment or not.

Can someone pleassssse tell me how to work it out? my exam is in 2days and this is the only bit i dont get in my whole book :frown:

Reply 1

Plato's Trousers

17

Original post by arnab

For any given compound, how do i work out if the compound has an overall dipole moment or not? I know how to draw all the lewis dot structure or the shape of any compound, e.g drawing a tetrahedral compound or a trigonal planner compound (A-lvl student) but i m have no idea as to how to work out, if a compound has a net overall dipole moment or not.

Can someone pleassssse tell me how to work it out? my exam is in 2days and this is the only bit i dont get in my whole book :frown:

A dipole moment is caused by the presence of one or more polar bonds (ie bonds between two different elements) that is(are) not balanced by other bonds. So HCl has a dipole moment because the H-Cl bond is polar (H and Cl have different electronegativity). But H₂ has no dipole moment because the bond is between two atoms of the same element (ie no difference in electronegativity). CCl₄ has no dipole moment as all the C-Cl bonds (whilst individually polar) are exactly balanced out in space (tetrahedral symmetry). So it's about symmetry and balance really. It's pretty easy to spot them.

SF₆ - no dipole (all bonds the same, octahedral symmetry)
CH₃Cl - dipole (because of the unbalanced C-Cl bond)
NH₃ - dipole (all the bonds are the same, but they are directed towards three of the verticies of a tetrahedron. The other has a lone pair. So the molecule is unbalanced)
CO₂ - no dipole, linear molecule, each bond counterbalances the other

(edited 12 years ago)

Reply 2

arnab

OP

11

Original post by Plato's Trousers

x

i am sorry but i still dont understand it. How do i work out the symmetry of a shape?

Reply 3

username913907

10

VSEPR, look up how use it to predict shapes of simple molecules..... Once you know the shape, you can consider the distribution of the electrons. If there is a difference in electronegativity, then there will be a dipole, eg SF6. The only reason this doesn't have an overall dipole is because it is symmetric around the center, so you could consider each mini dipole (s-f bond) cancelling eachother out

Reply 4

Plato's Trousers

17

Original post by arnab

i am sorry but i still dont understand it. How do i work out the symmetry of a shape?

Well yes, you do need to know the shape. Sorry, I was assuming you had that info.

Then you just have to take a look and see if everything is symmetrical about the centre. If not = dipole

:smile:

This picture may help. Ignore the numbers, but do you see how things either balance themseleves out, or not?

(edited 12 years ago)

Reply 5

arnab

OP

11

ahh thanks
got it now :biggrin:

Reply 6

Plato's Trousers

17

Original post by arnab

ahh thanks
got it now :biggrin:

you're welcome :smile:

Feel free to post any molecules you're not sure of or you want checking...

Reply 7

arnab

OP

11

Original post by Plato's Trousers

Well yes, you do need to know the shape. Sorry, I was assuming you had that info.

Then you just have to take a look and see if everything is symmetrical about the centre. If not = dipole

:smile:

This picture may help. Ignore the numbers, but do you see how things either balance themseleves out, or not?

heyy

i just want to say a BIG FREAKING THANK YOU for helping with these question I had. I just got my results for the jan exams i sat and guess what? I GOT an A OVERALL IN AS CHEMISTRY!!!! so yh thanks again for helping me :biggrin:

now to get an A in A2 chemistry

(edited 12 years ago)

Reply 8

Plato's Trousers

17

Original post by arnab

heyy

i just want to say a BIG FREAKING THANK YOU for helping with these question I had. I just got my results for the jan exams i sat and guess what? I GOT an A OVERALL IN AS CHEMISTRY!!!! so yh thanks again for helping me :biggrin:

now to get an A in A2 chemistry

Great news. Thanks for feeding back with the result.

Well done. :smile:

Reply 9

Manisha g

1

SO2 has dipole or not

SO2 has dipole???

SO2 has dipole???

Original post by Plato's Trousers

you're welcome :smile:

Feel free to post any molecules you're not sure of or you want checking...

Reply 12

Manisha g

1

SO2 has dipole or not

Reply 13

anosmianAcrimony

20

Original post by Manisha g

SO2 has dipole or not

You’re about six years late to get a response out of those guys, I’m afraid - the rest of the discussion happened in 2012.

Anyway - yeah, SO₂ has a dipole for the same reason water does - the central sulfur atom has a lone pair that means the two oxygens don’t arrange linearly. The direction the oxygens are pointing in is delta negative, and the sulfur is delta positive. I had to look it up on Wikipedia and have a look at its structure to work that out, though.