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In the advanced chemistry textbook we are using, Chemistry, 7th edition, Zumdahl and Zumdahl, the electric dipole moment is defined to point from the positive charge center toward the negative charge center. (page 335) Can you tell me why this would be the definition, since they also state that the dipole moment will align with the electric field. (page 336) It is well-known that the electric field vector between a pair of metal plates points from the positive plate toward the negative plate. And in physics classes, I always learned that the electric dipole points from negative toward the positive.
Question Date: 2011-01-24
Answer 1:

Your question raises a very subtle point about the way the to define the direction of electric fields in dipoles. You are correct that the electric field vector between the plates of a capacitor points from the positive plate to the negative plate, and I believe you are asking if this is different for the dipole moment in polar molecules. Before reading below, keep in mind that the direction of an electric field vector is purely convention, and the field vectors always point "out" of positive charges and "into" negative charges (I believe Ben Franklin defined this convention because he initially thought charge carriers were positive).

For a capacitor, most calculations deal with internal fields (i.e. between the plates) and your capacitor fields observations is drawn from this perspective. However think about the field lines that would be drawn outside of the two capacitor plates. Viewing the plates externally (i..e. ignoring the field between the plates), a field vector would be drawn leaving the positive plate and another vector would be drawn entering the negative plate, and this field would be opposite in direction to the internal field drawn between the two plates.

Next, consider what is important in molecular systems. Since nothing will enter the space between chemically bound atoms (i.e. consider a simple dipolar 2 atom molecule, such as HF), we are not so concerned with the internal field that develops within the dipole. What is most important is the external field (since this is the field that actually acts on surrounding particles/molecules/surfaces/etc.). Therefore, the conventional dipole moment that is drawn for molecules actually corresponds to the external field direction (however an internal field of opposite direction is also present and can be drawn analogous to the space in between capacitor plates).

Thanks for the great question, and have a great day!

Answer 2:

Wikipedia has a good answer for you:
[Bond dipole moment] is a vector, parallel to the bond axis, pointing from minus to plus, as is conventional[1] for electric dipole moment vectors. (Some chemists draw the vector the other way around, pointing from plus to minus, but only in situations where the direction doesn't matter.)[1]

Your textbook says the dipole moment is 'often represented' by an arrow pointing from + to -, so you could point out that it is More Often represented by an arrow pointing in the other direction!

I think I learned that electricity was originally visualized as a flow of positive charges, which made for confusion when it was discovered that the moving charges are actually negative.

This is a good lesson, too, for students to learn to read critically.Errors in written info are such a pain to one who is trying to learn!

Best wishes,

Answer 3:

This must just be a typo in the textbook. Those do happen occasionally? Good on you for paying attention!

Answer 4:

This is one of the cases in science where two different fields define the same thing differently. (Another example is that mathematicians and physicists define theta and phi differently in spherical coordinates). I do not know the exact historical reason that chemists define the electric dipole moment in the opposite direction from physicists, but that is how it is in many (but not all) chemistry books. When they say that it aligns with the electric field, they mean that it does exactly what you would expect it to do from your physicsdefinition. It seems that you have a good understanding of dipoles andelectric fields, so I will not go into detail about that.

If you are reading a chemistry book and you are not sure how they havedefined the dipole moment, the easiest way to figure it out is to finda molecule in the book that has a highly electronegative atom in it(O, F, Cl) and assume that that atom is the partial negative end ofthe dipole. From that, you can figure out how they are defining theirdipoles based on which way they have the arrows drawn.

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