0

As I understand, polarity in molecules comes from the difference in electronegativity or the ability of the atoms to attract eletrons so the electrons spend more time or are more probable to gravitate more around the atom with stronger electronegativity right? So I wonder why polar molecules must necessarily have an asymetrical structure? (as I've read on multiple sites). Shouldn't a symetrical atom with strong electronegativity with 2 atoms bonded linearly 180° on both sides which are significantly less electronegative, be polar since the electrons would gravitate more towards the center and the sides would then be more positively charged? What am I missing here?

Drawing of my question

Why is the first molecule (1) not possible? Is it becuase of the less polar atom on the left side repells electrons just as much as the one on the right? But then the electrons would still orbit the middle atom more because its more elctronegative, right?

Žan Spehonja
  • 27
  • 3
  • I meant gravitate not orbit in the last sentence – Žan Spehonja Jun 02 '19 at 19:39
  • You ain't missing anything. The first molecule is surely possible, and indeed does exist (think CO2), and is quite definitely very polar. Some people would claim it isn't polar because it has zero dipole moment. They are irrelevant. – Ivan Neretin Jun 02 '19 at 19:41
  • So symetrical molecules can be polar and the sources that say otherwise are wrong? Thank you. – Žan Spehonja Jun 02 '19 at 19:43
  • Or https://chemistry.stackexchange.com/questions/14641/why-is-carbon-dioxide-nonpolar – Mithoron Jun 02 '19 at 19:44
  • 3
    Zero net dipole moment means zero net dipole, just that. Molecules shouldn't be call polar, it only leads to misunderstandings. Solvents can and should be, but still with some measure of caution. – Mithoron Jun 02 '19 at 19:47
  • @IvanNeretin I have a feeling OP might just not get how dipoles work. – Mithoron Jun 02 '19 at 19:50
  • @IvanNeretin and @ Mithoron, I think you are touching on something important. Most courses that introduce the concept of polarity/dipoles in the context of molecular structure use vectors to describe the dipoles of the individual bonds, and then the sum of those vectors to determine the dipole moment (at least qualitatively) of the molecule. In that case, a molecule like CO2 would be non-polar. Perhaps you should elaborate for the OP and other readers. – Michael Lautman Jun 02 '19 at 20:26
  • @MichaelLautman We did, on a number of occasions. – Ivan Neretin Jun 02 '19 at 20:30
  • @IvanNeretin, I only saw the link that was posted after my comment. I can retract my comment if you like. – Michael Lautman Jun 02 '19 at 20:36
  • Thank you for answers and for this link whoever posted it https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_(Zumdahl_and_Decoste)/08%3A_Bonding_General_Concepts/13.03_Bond_Polarity_and_Dipole_Moments I thought the dipole came from the individual parts of the molecule now I understand that its the whole molecule's +/- that is Directionaly not negated or that the symetrical polar parts of the atom cancel eachother out directionaly so the molecule doesn't seek equilibrium in others because it alredy has it – Žan Spehonja Jun 02 '19 at 20:40
  • @MichaelLautman No, leave it, because it is still useful. – Ivan Neretin Jun 02 '19 at 21:18
  • Zan what you say is certainly true as far you look at parts of the molecule. Those are polar bonds. The various electrical moments can sum up to cancel each other and this requires a symmetrical arrangement. – Alchimista Jun 03 '19 at 11:07

1 Answers1

0

If you start with the wikipedia definition of chemical polarity, which is standard (if not IUPAC) usage:

In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end.

then it's not hard to understand why a molecule such as $\ce{CO2}$ containing a center of inversion lacks a dipole moment and is therefore "nonpolar", as are, by this definition, other molecules with high symmetry properties and lacking a net dipole moment:

The only groups compatible with a dipole moment are Cn, Cnv and Cs. In molecules belonging to Cn or Cnv the dipole must lie along the axis of rotation.

Buck Thorn
  • 21,621
  • 5
  • 38
  • 89
  • It's not " bound to be canonical". IUPAC def. only applies polarity to solvents, which this article is missing almost entirely. – Mithoron Jun 02 '19 at 21:22
  • @Mithoron I used "canonical" to mean "standard" in this case, but you have a point, I was a bit loose with language. – Buck Thorn Jun 03 '19 at 07:30