Sodium chloride (NaCl), known widely as common table salt, is a classic example of an ionic compound. Chemical bonds are the forces that hold atoms together, categorized by how atoms interact with their outermost electrons. The type of bond that forms is dependent on the nature of the elements involved and their individual tendencies toward electron interaction. The strong attraction that holds sodium and chlorine together results from a complete transfer of an electron, which is the defining characteristic of an ionic bond.
Distinguishing Covalent and Ionic Bonds
The two primary types of chemical linkage, ionic and covalent bonds, are distinguished by the fate of the valence electrons. Covalent bonding occurs when two atoms share electrons between them to fill their outermost electron shells. This sharing mechanism typically takes place between two nonmetal atoms that have a similar pull on the shared electrons. The resulting shared electron pairs hold the atoms together in a discrete molecule.
Ionic bonding involves the complete transfer of one or more electrons from one atom to another, usually between a metal atom and a nonmetal atom. The metal atom readily gives up an electron to become a positively charged ion, or cation. The nonmetal atom accepts that electron to become a negatively charged ion, or anion. The resulting bond is the powerful electrostatic attraction between these oppositely charged ions, which does not result in a discrete molecule but a continuous lattice structure.
Electronegativity: The Deciding Factor
The specific mechanism of electron interaction is determined by a property called electronegativity, which is a measure of an atom’s ability to attract electrons toward itself within a chemical bond. This property is measured using a scale, and the difference in electronegativity (Delta EN) between two bonding atoms is what predicts the bond type. When two atoms have a small or zero difference in their Delta EN values, neither atom can overpower the other, leading to the electrons being shared in a covalent bond.
A large difference in electronegativity signals a major imbalance in electron attraction between the two atoms. Scientists use general thresholds to classify the resulting bond type based on this difference. When the Delta EN is large, generally exceeding a value of 1.7 or 2.0 on the Pauling scale, the pull is so uneven that one atom completely strips the electron from the other. This significant difference establishes the conditions necessary for the formation of an ionic bond.
How Sodium Chloride Forms its Bond
The formation of sodium chloride perfectly illustrates the principles of ionic bonding driven by a large electronegativity difference. Sodium (Na) is an alkali metal with a very low attraction for its single outermost valence electron. Chlorine (Cl) is a halogen nonmetal with a high attraction for electrons, needing only one electron to complete its stable outer shell. The calculated electronegativity difference between sodium and chlorine is approximately 2.1, which falls well into the range defined for ionic compounds.
This extreme difference causes the sodium atom to fully surrender its single valence electron to the chlorine atom. By losing an electron, the sodium atom transforms into a positively charged sodium cation (Na+). The chlorine atom, upon gaining that electron, becomes a negatively charged chloride anion (Cl-). The resulting bond is the strong, non-directional electrostatic force of attraction between the positive Na+ ion and the negative Cl- ion. This attraction organizes countless ions into the highly ordered, repeating structure known as a crystal lattice, which is the characteristic arrangement of solid table salt.