Chemical bonding is the fundamental force that holds atoms together to form molecules and compounds. This force results from interactions between valence electrons, which are the outermost electrons of an atom. The properties of the atoms involved, particularly their tendency to attract or give up electrons, determine the type of bond that forms. When considering two nonmetal atoms, they cannot form an ionic bond because the intrinsic chemical nature of nonmetals favors a different mechanism for achieving stability.
The Defining Characteristics of Ionic Bonds
A standard ionic bond is defined by the complete transfer of one or more valence electrons from one atom to another. This transfer typically occurs between a metal atom and a nonmetal atom. The atom that loses electrons becomes a positively charged ion (a cation), while the atom that gains them becomes a negatively charged ion (an anion).
The resulting bond is a powerful electrostatic attraction between these two oppositely charged ions. This attraction is non-directional and acts equally in all directions. This strong force leads to the characteristic formation of a rigid, three-dimensional crystal lattice structure, such as sodium chloride (\(\text{NaCl}\)). A true ionic bond requires one atom to give up electrons easily (low ionization energy) and the other to readily accept them (high electron affinity).
The Nature of Bonding Between Two Nonmetals
The interaction between two nonmetal atoms results in a different type of linkage called a covalent bond. In a covalent bond, atoms achieve a stable electron configuration by sharing valence electrons instead of transferring them. These shared electron pairs are simultaneously attracted to the nuclei of both atoms, binding the atoms together into a molecule.
This shared arrangement allows each participating nonmetal to complete its outermost electron shell. Simple molecules like oxygen gas (\(\text{O}_2\)) or water (\(\text{H}_2\text{O}\)) are classic examples of this bonding type. Since both nonmetals have a relatively equal hold on the shared electrons, a full electron transfer required for an ionic bond cannot occur.
Why Electronegativity Prevents Ionic Bonding Between Nonmetals
The determining factor for bond type is electronegativity, which measures an atom’s ability to attract electrons within a chemical bond. Nonmetals are characterized by high electronegativity, meaning they exert a strong pull on electrons. When two nonmetals interact, both atoms strongly attempt to gain or attract electrons, and neither is willing to fully relinquish them.
The bond type is quantified by the difference in electronegativity (\(\Delta EN\)) between the two bonded atoms. For a bond to be considered predominantly ionic, the difference in electronegativity must be very large, often cited as a value greater than 1.7 on the Pauling scale. Because two nonmetals have similar and high electronegativity values, their difference is rarely large enough to meet this threshold. This small \(\Delta EN\) makes the electron transfer required for ion formation energetically unfavorable, resulting in an electron-sharing arrangement instead.
The Spectrum of Chemical Bonds
Bonding exists along a continuous spectrum rather than being a simple binary choice between purely ionic or purely covalent. At one end is the nonpolar covalent bond, where electrons are shared equally, typically occurring between two identical atoms like \(\text{H}_2\). The other extreme is the ionic bond, which represents the complete transfer of electrons due to a maximum difference in electronegativity.
In the middle lies the polar covalent bond, which forms when two different nonmetals share electrons unequally. The atom with the greater electronegativity attracts the shared electron pair more strongly, creating a partial negative charge (\(\delta^{-}\)), while the other atom develops a partial positive charge (\(\delta^{+}\)). Even with this unequal sharing, the electrons are still shared and not fully transferred, meaning the bond remains covalent.