What are examples of dipoles?

Mar 3, 2017

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 ($\Delta E N$) determines how polar a bond is.

If:
$0 \le \Delta E N \le 0.4$, the bond is nonpolar.

$0.4 <$$\Delta E N$$\le 1.8$, the bond is polar

$\Delta E N > 1.8$, the bond is ionic

Consider the bonds in ${H}_{2} O$, 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
$E {N}_{O} = 3.44$
$E {N}_{H} = 2.20$
$\Delta E N = 3.44 - 2.20 = 1.24$

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 $C {O}_{2}$, As you can see, the $\Delta E N$ for the $C - O$ bond is within the polar range. However, since $C {O}_{2}$ is a linear molecule, the dipoles point in opposite directions and the vector sum of the two is equal to zero. $C {O}_{2}$ is nonpolar.