The difference between a polar molecule and a nonpolar molecule depends on molecular geometry.
Polar result when two different elements share electrons and form a covalent bond. Each element has a characteristic electronegativity. A more electronegative element has a stronger attraction for electrons it shares in a bond.
Stronger electronegativity pulls more electron density towards the more electronegative element. Greater electron density confers a partial negative charge, -δ. This also induces a partial positive charge on the element which is less electronegative, +δ, Figure 1.
When a difference exists in relative charge between two elements, a dipole is established. One side of the dipole has a +δ and the other side has a -δ charge.
The strength of a dipole is expressed as the difference between the two poles, μ.
An arrow shows the direction and strength of a dipole. The arrow points from positive at the base of the arrow to negative at the head of the arrow.
The length of the arrow shows the strength of the dipole. The arrows represent vectors which when added, contribute to the overall dipole present on a molecule.
When vectors oppose one another and are equal and opposite, they cancel each other out. This explains why molecules may have polar bonds, but not be polar molecules, Figure 3.
In the same manner, when two dipole vectors add together and do not directly oppose each other, the result is a new dipole vector composed of individual dipole vectors, Figure 4.
The example shows two equal dipole vectors at a right angle to each other. The result is a single vector which comes from the hypotenuse: a2 + b2 = c2.
Polar Molecules: Diatomic Molecules
Diatomic molecules can be either polar or nonpolar, Figure 5. If both atoms are the same, then the molecule is nonpolar, O2. Fluorine is much more electronegative than hydrogen.
In HF, there is a strong dipole which leaves a partial positive charge on H and a partial negative charge on F.
Some cases can be deceptive, like carbon monoxide, CO. Oxygen is more electronegative than carbon. You might expect this means the O would have a partial negative charge while the carbon has a partial positive charge.
However the oxygen has three bonds and one lone pair, which gives it a formal charge of +1. Carbon has three bonds and one lone pair of electrons. That leaves carbon with a formal charge of -1. Therefore the dipole from positive to negative points from oxygen to carbon.
Polar Molecules: Triatomic Molecules
The situation turns more complicated with three atoms. More than one polar bond in the molecule becomes possible, Figure 6.
A linear molecule is polar if the molecule is not symmetric. When you compare the size of the dipoles and add them together, it results in a non-zero molecular dipole. The molecule is polar.
HCN has a central carbon with a hydrogen on one side and hydrogen on the other. Consider the C-H bond nonpolar. The remaining C-N bond has a dipole that goes from the less electronegative carbon to the more electronegative nitrogen. The molecule is polar.
H20, O3, and SO2 are all polar. Though each bond dipole is equal to the other dipole in the same molecule, the bond dipoles do not directly oppose each other. The addition of bond dipoles leaves a molecular dipole which has not cancelled out. The molecules left with an overall molecular dipole are polar.
In the case of two dipoles which are equal and exactly oppose each other, CO2, the two dipoles add together to leave the molecule with no overall molecular dipole. The molecule is nonpolar.
Polar Molecules: Four Atom Molecules
Four atom molecules follow the same pattern as three atom molecules, Figure 7.
COCl2 has dipoles which all point away from the central carbon out towards the more electronegative O and Cl. You might be tempted to think this is nonpolar.
Yet when you add the two Cl, the resulting downward vector would become smaller than either of them alone. The O which opposes them over balance the two Cl‘s. This confers an overall molecular dipole that points from C to O.
BF3 acts as an example of a four atom molecule which is nonpolar. The three atoms around boron are identical, so the dipoles exert a force from a +δ on boron to -δ on each fluorine. Because the dipoles are symmetric and oppose each other, their sum is zero. This makes BF3 nonpolar.
NH3 provides an example of a trigonal pyramidal molecule. Though the N has three identical atoms attached, the lone pair on the N causes the bonds to pucker lower than the N. This causes the dipoles to no longer directly oppose each other. (All trigonal pyramidal molecules are polar).
Pay attention to the difference between trigonal planar and trigonal pyramidal. Trigonal planar molecules are flat. They can be polar or nonpolar. Trigonal pyramidal molecules are puckered. They are always polar.
Polar Molecules: Five Atom Molecules
The finally example of small molecules, are molecules which are made of five atoms, Figure 8.
In this case, the molecule all derive from the parent compound of methane, CH4. Because carbon has four H‘s attached to it which are uniformly placed around the C, the molecule is symmetric. This makes it nonpolar.
The remaining molecules in Figure 8 are all not symmetric. This means they must be polar. In the last case, CHBrClF, even though Br and Cl have very similar electronegativities, fluorine is far more electronegative than either of them. When the resulting vector sum of Cl and Br oppose fluorine, it is not strong enough to counter act the dipole associated with F. This makes this asymmetric molecule polar.
Polar and Nonpolar Regions
More often you will encounter molecules which do not have just one single atom. The more common situation comes up when a molecule has multiple atoms, each with a unique dipole, Figure 9.
The simplest amino acid, glycine, illustrates the idea. The formula for glycine is H2NCH2COOH. The carboxyl group CO2H has two polar bonds and a permanent dipole, μ. The CH2 group has no dipole moment, and is nonpolar. Further along the molecule, the NH2 group also has a permanent dipole.
The behavior of molecules can depend not only of the entire molecule is polar or nonpolar, buy also on whether parts of a molecule have polar or nonpolar character.