The question of whether a substance is ionic or covalent requires looking beyond simple labels, especially for elements like gold. Chemical bonds, the forces that hold atoms together, determine nearly every physical and chemical property of matter, from hardness to electrical conductivity. While bonding is typically categorized into distinct groups, many substances exist in a spectrum where characteristics of different bond types overlap. Understanding these categories is necessary to grasp the unique nature of gold’s atomic structure and how it interacts with other elements.
The Three Types of Chemical Bonds
Ionic bonding typically occurs between a metal and a nonmetal atom. This process involves the complete transfer of one or more electrons from the metal atom to the nonmetal atom. This transfer creates a positively charged ion (cation) and a negatively charged ion (anion). The resulting bond is a strong electrostatic attraction between these oppositely charged ions, exemplified by sodium chloride (table salt).
Covalent bonding involves atoms achieving stability by sharing one or more pairs of electrons. These bonds generally form between two nonmetal atoms, and the shared electrons orbit both atomic nuclei. Examples include the bonds found in water (\(\text{H}_2\text{O}\)) or methane (\(\text{CH}_4\)). Unlike the strong electrostatic forces of ionic bonds, covalent bonds are highly directional and result in discrete molecules that often have lower melting points.
The third primary type is metallic bonding, which is specific to metals. In this model, the valence electrons are not localized to any single atom or shared pair. Instead, these outer electrons become delocalized, forming a “sea of electrons” that moves freely throughout the solid structure. This sea of negatively charged electrons holds together a lattice of positively charged metal ions (cations) through a strong electrostatic attraction.
The Specific Case of Gold: Metallic Bonding
Pure, elemental gold (Au) is held together by metallic bonding, not ionic or covalent bonds. As a metal, gold atoms contribute their valence electrons to the communal electron sea. The attraction between this lattice of positive gold ions and the mobile electrons constitutes the metallic bond.
Gold’s characteristic properties result directly from metallic bonding and the electron sea model. Gold is an excellent conductor of electricity because the delocalized electrons are free to move when an electrical potential is applied. The metal also exhibits high thermal conductivity and a distinctive metallic luster because the free electrons easily absorb and re-emit light.
The non-directional nature of the metallic bond explains gold’s unique mechanical properties, such as its malleability and ductility. The positive gold ions can slide past one another without fracturing the material. This occurs because the electron sea shifts to accommodate the new positions of the ions, allowing gold to be hammered into thin sheets or drawn into fine wires.
Gold forms a metallic structure due to its electron configuration and relatively low number of valence electrons. Elemental gold has one electron in its outermost shell. It is energetically favorable to share this electron with the collective rather than attempting to gain seven more electrons or become a simple isolated cation. This collective sharing maximizes attractive forces throughout the solid, leading to the highly stable metallic state.
When Gold Forms Compounds: Shifting Between Bond Types
When gold reacts with other elements to form compounds, its bonding behavior changes, and the simple metallic model no longer applies. Gold is most commonly found with oxidation states of +1 (aurous) and +3 (auric), though it can range from -1 to +5. These compounds are not purely ionic, despite containing a metal and a nonmetal, because gold’s chemical nature pushes the bond toward covalent character.
In gold(III) chloride (\(\text{AuCl}_3\)), the compound is not a simple ionic salt like sodium chloride. Gold’s relatively large size and high electronegativity allow it to exert a stronger pull on bonding electrons than many other metals. This results in significant electron sharing between the gold and chlorine atoms, giving the bond a strong covalent character. The covalent nature of \(\text{AuCl}_3\) is so pronounced that it exists as a dimer (\(\text{Au}_2\text{Cl}_6\)) and has a lower melting point than its gold(I) counterpart, \(\text{AuCl}\).
The tendency toward covalent bonding is also evident in the complex ions gold forms, which are common in industrial applications like gold plating. A prime example is the dicyanoaurate ion, \([\text{Au}(\text{CN})_2]^-\), a highly stable complex where the gold atom is covalently bonded to the cyanide groups. The bonds formed in these structures are best described as having a mix of ionic and covalent qualities, with covalent characteristics often dominating.
While elemental gold is metallic, gold in a compound exists within a spectrum of bonding. The bonds are rarely purely ionic, even with highly electronegative elements. Instead, they are polar covalent, reflecting gold’s unique position among transition metals and its resistance to forming simple ionic salts.