Difference Between Polar Bonds and Polar Molecules

Polar Bonds vs Polar Molecules
 

Polarity arises due to the differences in electronegativity. Electronegativity gives a measurement of an atom to attract electrons in a bond. Usually Pauling scale is used to indicate the electronegativity values. In the periodic table, there is a pattern as to how the electronegativity values are changing. Fluorine has the highest electronegativity value, which is 4 according to the Pauling scale. From left to right through a period, the electronegativity value increases. Therefore, halogens have larger electronegativity values in a period, and group 1 elements have comparatively low electronegativity values. Down the group, the electronegativity values decrease. When two of the same atom or atoms having the same electronegativity form a bond between them, those atoms pull the electron pair in a similar way. Therefore, they tend to share the electrons and this kind of bonds is known as covalent bonds.

What are Polar Bonds?

However, when the two atoms are different, their electronegativities are often different. But the degree of difference may be higher or lower. Therefore, the bonded electron pair is pulled more by one atom compared to the other atom, which is participating in making the bond. This will result in an unequal distribution of electrons between the two atoms. And these types of covalent bonds are known as polar bonds. Because of the uneven sharing of electrons, one atom will have a slightly negative charge, whereas the other atom will have a slightly positive charge. At this instance, we say that the atoms have obtained a partial negative or positive charge. The atom with a higher electronegativity gets the slight negative charge, and the atom with a lower electronegativity will get the slight positive charge. Polarity means the separation of the charges. These molecules have a dipole moment. Dipole moment measures the polarity of a bond, and it is commonly measured in debyes (it also has a direction).

What are Polar Molecules?

In a molecule, there can be at least one bond or more than that. Some bonds are polar, and some bonds are non-polar. For a molecule to be polar, all the bonds collectively should produce an uneven charge distribution within the molecule. Further, molecules have different geometries, so the distribution of bonds also determines the polarity of the molecule. For example, hydrogen chloride is a polar molecule with only one bond. Water molecule is a polar molecule with two bonds. And ammonia is another polar molecule. The dipole moment in these molecules is permanent because they have arisen due to the electronegativity differences, but there are other molecules, which can be polar only in certain occasions. A molecule with a permanent dipole can induce a dipole in another non polar molecule and then that will also become temporary polar molecules. Even within a molecule certain changes may cause a temporally dipole moment.