Dipole, vectors and polarity

[prsndwg]

Can someone help me understand how to deal with the vectors and when do they cancel each other? I know that when they do, then they aren't polar anymore. I just can not connect the dots of canceling each other!

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[SmilezZz]

Can someone help me understand how to deal with the vectors and when do they cancel each other? I know that when they do, then they aren't polar anymore. I just can not connect the dots of canceling each other!

Any time you have two different atoms bound together, you will have a polar bond with the dipole moment pointing in the direction of the more electronegative atom.
When you have a molecule with more than two polar bonds, the molecule as a whole can either be polar or nonpolar, depending on the different atoms attached and the molecular geometry.
Molecules are 3D structures. To determine the polarity of the molecule, you have to think of each individual bond as a vector along the X,Y, and Z axis, and add them up. Vectors that point in opposite directions and have the same magnitude(bound to the same atom) will cancel each other out.

Does this help?

 

[prsndwg]

Any time you have two different atoms bound together, you will have a polar bond with the dipole moment pointing in the direction of the more electronegative atom.
When you have a molecule with more than two polar bonds, the molecule as a whole can either be polar or nonpolar, depending on the different atoms attached and the molecular geometry.
Molecules are 3D structures. To determine the polarity of the molecule, you have to think of each individual bond as a vector along the X,Y, and Z axis, and add them up. Vectors that point in opposite directions and have the same magnitude(bound to the same atom) will cancel each other out.

Does this help?

yes, what about if electronegative atoms are odd numbers, like 3 chlorine attached to Boron, or some other center atom that has lone pairs?

 

[Mtd0588]

If all the dipoles point in the same direction, they cancel each other out. BF3, for example, is is a trigonal planar molecule with three B-F dipoles pointing out, with the negative end on the three F's. The dipoles are symmetrical and equal in magnitude, therefore cancel each other out.

 

[prsndwg]

If all the dipoles point in the same direction, they cancel each other out. BF3, for example, is is a trigonal planar molecule with three B-F dipoles pointing out, with the negative end on the three F's. The dipoles are symmetrical and equal in magnitude, therefore cancel each other out.

what do you mean by same direction?

 

[SmilezZz]

what do you mean by same direction?

I believe MTD meant opposing directions. BF3 takes on a trigonal planar shape. So the central atom is B and the 3 F atoms are bound to B on the same plane. It's very difficult to explain through text without any visual aid, but I will try my best.



First, imagine that you have two atoms, lets say X, attached to a central atom, A (AX2). The molecule will take on a linear shape (X-A-X). Let's say that X is the more electronegative atom of the two. Each bond has a dipole moment pointing toward X. Because each bond is exactly opposite of each other (180 degrees), they will cancel each other out.

Now, add another X atom to the molecule (AX3). The 3rd X atom will come in from the top of the molecule and attach itself to the central atom, A. The addition of the 3rd X atom pushes on, or repels, the other 2 X atoms. The molecule as a whole has shifted from the linear arrangement to a flat trigonal planar arrangement with 120 degrees between each X atom.

Each bond will still have the same magnitude of dipole moment. The two X atoms at the bottom can be thought of as vectors. They both point down, but one is to the left and one is to the right. If you add this up, the left and right will cancel each other out and the net dipole moment will be pointing downward. This downward dipole moment is cancelled out by the 3rd X atom, which has a net dipole moment pointing upward.

Does that make sense?

 

[prsndwg]

I believe MTD meant opposing directions. BF3 takes on a trigonal planar shape. So the central atom is B and the 3 F atoms are bound to B on the same plane. It's very difficult to explain through text without any visual aid, but I will try my best.



First, imagine that you have two atoms, lets say X, attached to a central atom, A (AX2). The molecule will take on a linear shape (X-A-X). Let's say that X is the more electronegative atom of the two. Each bond has a dipole moment pointing toward X. Because each bond is exactly opposite of each other (180 degrees), they will cancel each other out.

Now, add another X atom to the molecule (AX3). The 3rd X atom will come in from the top of the molecule and attach itself to the central atom, A. The addition of the 3rd X atom pushes on, or repels, the other 2 X atoms. The molecule as a whole has shifted from the linear arrangement to a flat trigonal planar arrangement with 120 degrees between each X atom.

Each bond will still have the same magnitude of dipole moment. The two X atoms at the bottom can be thought of as vectors. They both point down, but one is to the left and one is to the right. If you add this up, the left and right will cancel each other out and the net dipole moment will be pointing downward. This downward dipole moment is cancelled out by the 3rd X atom, which has a net dipole moment pointing upward.

Does that make sense?

yup, got it. Thanks

 

[299678]

If all the dipoles point in the same direction, they cancel each other out. BF3, for example, is is a trigonal planar molecule with three B-F dipoles pointing out, with the negative end on the three F's. The dipoles are symmetrical and equal in magnitude, therefore cancel each other out.

BF3 has a dipole, they cancel each other, but there is one left

 

[SmilezZz]

yup, got it. Thanks

You're very welcome! Glad I could help! 🙂

 

[Mtd0588]

BF3 has a dipole, they cancel each other, but there is one left

BF3 has three dipoles; the sum of all three is zero. The three dipoles all cancel each other out becuase BF3 is trigonal planar (its valence doesn't have a full octet, so it has 3 bonding pairs). In order for two dipoles to cancel each other out, they would have to be linear, as in CO2

 

[prsndwg]

what if there is a carbon with 3 F's? how does that one unpaired electron do? what if there was unpaired e-? I say it would cause dipole!

 

[Mtd0588]

well, first of all, CF3 would be very unstable because carbon needs a full octet in its valence. However, something like NH3, which is trigonal pyramidal (kinda like a tripod with the N at the top) has 3 bonding e- pairs, and an unpaired election pair as you were saying, would be polar. Because the Nitrogen is more electronegative, all the electrons pairs, (including the unpaired electron pair) would hang out toward the central atom (Nitrogen), therefore, making the molecule polar. This molecule has a dipole moment because the dipoles do not oppose each other.

Does that answer your question?