Bonding between atoms involves the transfer, or sharing, of electron density such that each atom is left with a stable outer shell of electrons. For atoms with significantly different electronegativity, the process of "bonding" generally involves complete electron transfer to form two species having net positive and negative charges. This type of bonding is termed ionic and simple salts are the classic examples (the electronegativities of sodium and chlorine are 0.93 and 3.16, respectively, and NaCl forms a fully ionized compound with distinct and separate sodium and chloride ions). For atoms with similar or identical electronegativities (such as chlorine, Cl₂) bonding involves the sharing of electron density between the two atoms, in a type of bonding which is termed covalent. While fully ionic and symmetrical covalent bonds represent the limits of bonding, the bonding in most organic (carbon-containing) molecules falls somewhere between these two extremes. For example, carbon and chlorine have electronegativities of 2.55 and 3.96, respectively. These values are close enough that the carbon-chlorine bond is covalent, but the bond is not symmetrical and the bulk of the electron density is associated with the more electronegative chlorine. This polarization leaves a partial positive charge on the carbon and a matching partial negative charge on the chlorine. It is this polarization which gives organic compounds their chemical reactivity and allows simple structural units to be combined to form more complex molecules, i.e., the science of organic synthesis.

Ionic structures are often represented using Lewis dot formulas, in which the electrons in the outer shell are shown as paired or unpaired "dots" surrounding the atomic symbol. Covalent bonds can likewise be shown using dot formulas, and these are often useful. The large number of bonds in a typical organic molecule, however, would make drawing Lewis dot structures somewhat tedious, and covalent bonds are typically represented as a single line connecting the two atoms. When it is desirable, polarization along a covalent bond can be shown in these structures by using the symbols "d+" and "d-" over the atoms in question, as shown below.