Determining Compound Dipole Moment: A Guide for Finals

[Jordan Bergeron]

Can someone explain how to determine if a compound has a dipole moment and how to determine how big or small it is (possibly numerical value)? I have a final tomorrow and I've looked this up on multiple website and cannot find any good explanation.

 

[Bystander]

Tell us what you know so far, and somebody(ies) will try to make what you know work for you.

 

[Jordan Bergeron]

Tell us what you know so far, and somebody(ies) will try to make what you know work for you.

Honestly nothing.. I know it has something to do with polarity and molecular structure.

 

[Bystander]

polarity and molecular structure.

Gets to right ballpark. "Polarity" is the separation of electric charges of opposite sign along an interatomic bond axis usually (occasionally over longer distances within a molecule), and obviously the molecular structure comes in when you're considering where these charge separations occur, and whether there is any sort of symmetry to the arrangement within a molecule that would add up to zero, no matter what magnitude individual dipoles might exhibit.

Every heteronuclear bond between two atoms (this could be within the structure of some larger polyatomic molecule) exhibits some sort of charge separation (electrons preferring to hang around one nucleus more than the other) that can be treated as if there were actually X.xxx negative charge (doesn't have to be a multiple of charge on electron) on one nucleus separated from X.xxx positive charge on another nucleus at a distance equivalent to the bond length; the product of the total charge difference and that bond length is dipole moment (units of charge x distance).

Does this make sense so far?

 

[Jordan Bergeron]

Gets to right ballpark. "Polarity" is the separation of electric charges of opposite sign along an interatomic bond axis usually (occasionally over longer distances within a molecule), and obviously the molecular structure comes in when you're considering where these charge separations occur, and whether there is any sort of symmetry to the arrangement within a molecule that would add up to zero, no matter what magnitude individual dipoles might exhibit.

Every heteronuclear bond between two atoms (this could be within the structure of some larger polyatomic molecule) exhibits some sort of charge separation (electrons preferring to hang around one nucleus more than the other) that can be treated as if there were actually X.xxx negative charge (doesn't have to be a multiple of charge on electron) on one nucleus separated from X.xxx positive charge on another nucleus at a distance equivalent to the bond length; the product of the total charge difference and that bond length is dipole moment (units of charge x distance).

Does this make sense so far?

Partially, but go on.

 

[Bystander]

Structural effects: you've seen Lewis dot structures and read about "resonance" sharing of charge in anions like carbonate and sulfate; carbonate is planar trigonal, and resonance sharing of a -2 charge by 3 oxygens can be looked at as a "formal" charge of -2/3 on each oxygen in a nice symmetric triangle, so no dipole; sulfate -2 spread over 4 oxygens arranged tetrahedrally around sulfur --- same argument, same result. The ammonium ion same argument for 4 hydrogens sharing a +1 charge in a tetrahedon. Finally thought of one, thiosulfate, same as sulfate except one oxygen has been swapped for a sulfur atom, gives us a trigonal pyramid, or tetrahedron (not a regular tetrahedron) with the majority of the -2 charge sharing among the 3 oxygens on one face; -2 charge on one face, 0 charge on the opposite apex (the second sulfur) 3, maybe 4 angstrom separation; Houston, we have a dipole.

Neutral molecules: water is shaped like a boomerang with H at either end and O at the elbow; the O attracts the electrons contributed by the hydrogens to bond formation more than do the hydrogens giving a net negative charge on the O, and net positive on both hydrogens. I can't draw a straight line through all three atoms, but I can draw a straight line with the oxygen and net negative charge on one side, and the hydrogens and net positive charge on the other and see that there is a dipole acting perpendicularly to that line. You've noticed by now that I don't use graphics at all --- time for you to digest this and tell me whether it's been properly prepared.