Meek99
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Hi,

Can someone explain why (CH3)2CO has permanent dipole dipole forces please?

And why does CCL4, C2F2 and CO2 not have dipole dipole forces?

Thanks.
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christineedaley
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Hi!

I'm doing Chemistry as well and might get this wrong but I think it's because (CH3)2CO is not a symmetrical molecule unlike the others. If the molecule's symmetrical then the dipoles on each side cancel each other out so the overall dipole is zero.

Feel free to correct me if I'm wrong!
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thefatone
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(Original post by Meek99)
Hi,

Can someone explain why (CH3)2CO has permanent dipole dipole forces please?

And why does CCL4, C2F2 and CO2 not have dipole dipole forces?

Thanks.
All those molecules are symmetrical in some way or another so charges cancel out.

(CH3)2CO isn't symmetrical and there's a difference in electronegativity thus it's a polar molecule. If it's a polar molecule then it has dipole dipole forces
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Marrow
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(Original post by Meek99)
Hi,

Can someone explain why (CH3)2CO has permanent dipole dipole forces please?

And why does CCL4, C2F2 and CO2 not have dipole dipole forces?

Thanks.
Hi, the oxygen (in red) that is double bonded to the carbon (in black) is electronegative and the carbon is electropositive. This is due to a significant difference in electronegativity between the two atoms.

The C-H bonds "aren't polar" (they are technically but not for the sake of A-Levels).

So you have to look at the shape of the molecule in order to determine states of polarisation. Here, we have an electronegative end (the top of the molecule) and a neutral end (bottom of the molecule), giving rise to a polar molecule! As a result, it forms Van der Waals.

Image

Look at the shapes of the first two: tetrahedral, so these cancel out if all the bonds are polar due to the symmetry of the molecule.
O=C=O is linear with polar bonds, so also cancels out.
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Meek99
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How would you determine the shape of (CH3)2CO?

Is it that one C is the central atom which has 4 electrons and then add the number of atoms that it's bonded to and divide that by 2 to find the no. Of electron pairs?

So 4+9/2 gives an uneven number so is that why it is not symmetrical and has dipole forces?
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Meek99
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How would you determine the shape of (CH3)2CO?Is it that one C is the central atom which has 4 electrons and then add the number of atoms that it's bonded to and divide that by 2 to find the no. Of electron pairs?So 4+9/2 gives an uneven number so is that why it is not symmetrical and has dipole forces?


(Original post by thefatone)
All those molecules are symmetrical in some way or another so charges cancel out.

(CH3)2CO isn't symmetrical and there's a difference in electronegativity thus it's a polar molecule. If it's a polar molecule then it has dipole dipole forces
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thefatone
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(Original post by Meek99)
How would you determine the shape of (CH3)2CO?Is it that one C is the central atom which has 4 electrons and then add the number of atoms that it's bonded to and divide that by 2 to find the no. Of electron pairs?So 4+9/2 gives an uneven number so is that why it is not symmetrical and has dipole forces?
I haven't done enough research as to why but this here i guess sorta shows everything

Image

so it looks like that because it somehow has 3 electron pairs.... dunno why but it does, so you know it's a trigonal planer.

it's not symmetrical. You can have more than one line of symmetry.
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Meek99
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Oh okay and also,

Can you help with this as we'll pleas?

Element q forms a sulfate with formula qso4. It's electronic configuration is (Ar)3s2.

Why is it AR(3s2)?
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gingeee123
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bit late but its bc its a shorthand version of qso4. It is the electronic configuration of Ar and then 3s2. They use a noble gas for the shorthand bc it always has a complete outer shell
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charco
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(Original post by thefatone)
I haven't done enough research as to why but this here i guess sorta shows everything

Image

so it looks like that because it somehow has 3 electron pairs.... dunno why but it does, so you know it's a trigonal planer.

it's not symmetrical. You can have more than one line of symmetry.
There are not three electron pairs, there are three electron domains, one of which has four electrons (the double bond)
Three domains gives a trigonal planar arrangement.
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