Chemical bonds may be classified as being either polar or nonpolar. The difference is how the electrons in the bond are arranged.
Polar Bond Definition
A polar bond is a covalent bond between two atoms where the electrons forming the bond are unequally distributed. This causes the molecule to have a slight electrical dipole moment where one end is slightly positive and the other is slightly negative. The charge of the electric dipoles is less than a full unit charge, so they are considered partial charges and denoted by delta plus (δ+) and delta minus (δ-). Because positive and negative charges are separated in the bond, molecules with polar covalent bonds interact with dipoles in other molecules. This produces dipole-dipole intermolecular forces between the molecules.
Polar bonds are the dividing line between pure covalent bonding and pure ionic bonding. Pure covalent bonds (nonpolar covalent bonds) share electron pairs equally between atoms. Technically, nonpolar bonding only occurs when the atoms are identical to each other (e.g., H2 gas), but chemists consider any bond between atoms with a difference in electronegativity less than 0.4 to be a nonpolar covalent bond. Carbon dioxide (CO2) and methane (CH4) are nonpolar molecules.
In ionic bonds, the electrons in the bond are essentially donated to one atom by the other (e.g., NaCl). Ionic bonds form between atoms when the electronegativity difference between them is greater than 1.7. Technically ionic bonds are completely polar bonds, so the terminology can be confusing.
Just remember a polar bond refers to a type of covalent bond where electrons aren't equally shared and electronegativity values are slightly different. Polar covalent bonds form between atoms with an electronegativity difference between 0.4 and 1.7.
Examples of Molecules with Polar Covalent Bonds
Water (H2O) is a polar bonded molecule. The electronegativity value of oxygen is 3.44, while the electronegativity of hydrogen is 2.20. The inequality in electron distribution accounts for the bent shape of the molecule. The oxygen "side" of the molecule has a net negative charge, while the two hydrogen atoms (on the other "side") have a net positive charge.
Hydrogen fluoride (HF) is another example of a molecule that has a polar covalent bond. Fluorine is the more electronegative atom, so the electrons in the bond are more closely associated with the fluorine atom than with the hydrogen atom. A dipole forms with the fluorine side having a net negative charge and the hydrogen side having a net positive charge. Hydrogen fluoride is a linear molecule because there are only two atoms, so no other geometry is possible.
The ammonia molecule (NH3) has polar covalent bonds between the nitrogen and hydrogen atoms. The dipole is such that the nitrogen atom is more negatively charged, with the three hydrogen atoms all on one side of the nitrogen atom with a positive charge.
Which Elements Form Polar Bonds?
Polar covalent bonds form between two nonmetal atoms that have sufficiently different electronegativities from each other. Because the electronegativity values are slightly different, the bonding electron pair isn't equally shared between the atoms. For example, polar covalent bonds typically form between hydrogen and any other nonmetal.
The electronegativity value between metals and nonmetals is large, so they form ionic bonds with each other.