Dipole moments occur when there is a separation of charge. They can occur between two ions in an ionic bond or between atoms in a covalent bond; differences in electronegativity are the cause of dipole moments. The larger the difference in electronegativity, the larger the dipole moment. The distance between the charge separation is also a deciding factor into the size of the dipole moment. The dipole moment is a measure of the polarity of the molecule.
When atoms in a molecule share electrons unequally, they create what is called a dipole moment. Usually this occurs when one atom is more electronegative than another resulting in that atom pulling more tightly on the shared pair of electrons or when one atom has a lone pair of electrons and the difference of electronegativity vector points in the same way. One of the most common examples is the water molecule, made up of one oxygen atom and two hydrogen atoms. The differences in electronegativity and lone electrons give oxygen a partial negative charge and each hydrogen a partial positive charge.
The equation to figure out the dipole moment of a molecule is
\[ \mu = q \, r\]
where \(\mu\) is the dipole moment, \(q\) is the magnitude of the charge and \(r\) is the distance between the charges. The dipole moment acts in the direction of the vector quantity. The units used for dipole moments are the Debye (D); 1D=3.34 x 10-30 C x m
Figure 1: Dipole moment of water (3) Figure 2: Electronegativity of common elements
The vector points from positive to negative, on both the molecular (net) dipole moment and the individual bond dipoles. The table above shows the electronegativity of some of the common elements. The bigger the difference in electronegativity of the two atoms, the more electronegative that bond is. To be considered a polar bond the difference in electronegativity is larger. Dipole moment points in the direction of the vector quantity of each of the bond electronegativities added together.
|Example 1: Water|
Using the water molecule picture from figure 1 we can figure out the direction dipole moment acts and the value it has. Using the electronegativities of water and hydrogen we find that the difference is 1.2 for each of the hydrogen oxygen bonds. Next we know that the oxygen is the more electronegative atom, therefore it will have a greater pull on the shared electrons; it also has two lone pairs of electrons as well. From this we can conclude that the dipole moment points from between the two hydrogens and towards the oxygen. Using the equation for the dipole moment we find that is is equal to 1.85 D. This was found by multiplying the distance between the oxygen and hydrogens by the charge difference between them and then finding the components of each that point in the direction of the net dipole moment (remember the angle of the molecule is 104.5˚).
The bond moment of O-H bond =1.5 D, so the net dipole moment =2(1.5)xcos(104.5/2)=1.84 D.
The polarity of a molecule is influenced by its structure. If a molecule is completely symmetric, then the dipole moment vector on each molecule will cancel each other out causing the molecule to be nonpolar. A molecule can only be polar if the structure of that molecule is not symmetric. A basic example of a nonpolar molecule is CO2. It is linear and completely symmetric, so the dipole moment vectors on both oxygen atoms cancel each other out. An example of a polar molecule is H2O. Because of the lone pair on oxygen, the structure of H2O is bent, which means it is not symmetric. The vectors do not cancel each other out making the molecule polar.
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