Question

What are examples of dipoles?

Answer 1

Any molecule that has a nonzero vector sum of dipole moments is said to be polar and have a dipole moment

Explanation:

A dipole moment refers to slight opposite charges on opposite sides of a bond. The resulting bond is said to be polar; it has a positive pole and a negative pole, much like a bar magnet.

In order to determine if a particular bond is polar or not, one must look for the electronegativity of each atom. Pauling's electronegativity is a measure of how strong a particular atom pulls electrons towards it in a bond. The value of the difference between their electronegativities (`DeltaEN`) determines how polar a bond is.

If:
`0<= DeltaEN <=0.4`, the bond is nonpolar.

`0.4<` `DeltaEN` `<=1.8`, the bond is polar

`DeltaEN>1.8`, the bond is ionic

Consider the bonds in `H_2O`,

There is one oxygen bonded to two hydrogens in one water molecule. Based on the difference in electronegativites for the bonds, it is clearly a polar molecule
`EN_O=3.44`
`EN_H=2.20`
`DeltaEN=3.44-2.20=1.24`

In the figure above, the `delta` symbol indicates an area of partial charge on the atom. Note that they are not full charges as in ions, but partial charges due to a difference in electron density at each "pole". The arrow in the figure indicates the direction of electron density and the slight negative charge `delta^-`and the cross indicates an area of electron deficiency and the slight positive charge `delta^+`. This difference in charges is called a dipole moment and it is a vector quantity; it has magnitude and direction.

Notice that the water molecule has an overall dipole moment that points straight up towards the oxygen. This is because a dipole moment of a molecule depends on the vector sum of the bond dipoles.

Consider `CO_2`,

As you can see, the `DeltaEN` for the `C-O` bond is within the polar range. However, since `CO_2` is a linear molecule, the dipoles point in opposite directions and the vector sum of the two is equal to zero. `CO_2` is nonpolar.