The EN's of carbon and hydrogen are similar: 2.5 and 2.1. Thus in a molecule containing carbon and hydrogen the electron pair will be shared almost equally between them in an electron-pair, or covalent, bond. In contrast, Li and F have radically different electronegativities: 1.0 and 4.0. In the compound LiF, the electrons will be seized almost entirely by the F atoms, forming Li and F ions in a salt, as described previously.

LiF is said to be held together by ionic bonds. Pure ionic and pure covalent bonds are extreme types of bonds; most real bonds are somewhere in between. The type of bond that will be formed between atoms depends entirely on their relative electronegativities, or electron-pulling power, as in the HF bond on the next page. For this reason, electronegativity is the most important single concept in predicting the nature of chemical bonds.