A nonpolar covalent bond is a type of bond between two identical nonmetallic atoms where the two electrons are shared equally by each atom. It is different from a polar covalent bond, in that involves the electrons being shared unequally and partially transferred to one of the two atoms.
If the two atoms have equal electronegativity, they form a nonpolar covalent bond. If the electronegativity is only slightly unequal, the two form a polar covalent bond, and if there is a large difference between the two, the result is an ionic bond. Both polar and nonpolar covalent bonds can only occur between two nonmetallic atoms, unlike ionic bonds, which occur between a metal and a nonmetal atom.
Some common elements that form nonpolar covalent bonds are iodine and oxygen. An atom of iodine normally has seven electrons in its outer shell, which means it is searching for one more electron to complete the octet. When two atoms of iodine come together, they then join in a nonpolar covalent bond, sharing an electron so that each atom completes its octet. Two oxygen molecules join together in a similar way to make O2, but in this case they form a double bond where each atom shares two electrons with the other.
Virtually all identical nonmetallic compounds join together in this type of bond, with the only exceptions being hydrogen and carbon, which can form other types of bonds in organic compounds.