ABSTRACT
A pure covalent bond can exist only between identical nuclei, e.g., two carbon nuclei bonded together. If the nuclei are different, the center of charge in the bond always moves closer to one of the two nuclei, conferring electrical polarity on the bond; this polarity gives the bond extra stability. The bond has an ionic character in addition to its covalent character. The most extreme case the electron from one nucleus becomes completely "captured" by the other nucleus; when this happens the "sharing of an electron pair" is no longer a reasonable explanation for the bond – rather, the bonding is better describes as the attraction between positive and negative ions. The strengths of these ionic bonds are about the same as those of pure covalent bonds. Chemists have coined the word "electronegativity" to describes the degree to which a given atom seems to attract the valence electrons toward itself.
