Most real chemical bonds in nature are neither truly covalent nor truly ionic. Only homonuclear bonds are truly covalent, and nearly perfect ionic bonds can form between group I and group VII elements, for example, KF. Generally, however, bonds are partially covalent and partially ionic, meaning that there is partial transfer of electrons between atoms and partial sharing of electrons.
In order to quantify how much ionic character (and how much covalent
character) a bond possesses, electronegativity differences between
the atoms in the bond can be used. We have already seen one
method for estimating atomic electronegativities, the Mulliken method.
In 1936, Linus Pauling came up with another method that forms the
basis of our understanding of electronegativity today.