Chemical bonds govern how atoms connect to form compounds, which is fundamental to predicting a compound’s physical and chemical behavior. Chemists categorize compounds based on the dominant type of attraction holding their constituent atoms together. While classification is usually straightforward, some compounds present a more complex picture, exhibiting characteristics of more than one bonding type. In these cases, classification is based on the primary force linking the major components.
Distinguishing Ionic and Covalent Bonds
Chemical bonding falls into two main categories: ionic and covalent, defined by how valence electrons are handled. Ionic bonds typically form between a metal and a nonmetal, involving the complete transfer of electrons. This transfer forms oppositely charged ions (cations and anions), and the resulting electrostatic force holds the compound together in a crystal lattice structure.
Covalent bonds occur when two nonmetal atoms share electrons to achieve a stable configuration. The shared electrons are mutually attracted to both nuclei, creating a strong link. The difference in electron sharing versus electron transfer is the primary determinant for classifying a compound.
Analyzing Ammonium Sulfide’s Overall Structure
Ammonium sulfide, with the chemical formula \((\text{NH}_4)_2\text{S}\), is classified as an ionic compound. This classification is determined by the strong electrostatic forces acting between its two main components: the positively charged ammonium ion (\(\text{NH}_4^+\)) and the negatively charged sulfide ion (\(\text{S}^{2-}\)).
The ionic bond is the powerful attraction between these oppositely charged ions. Two ammonium ions (each \(+1\)) are required to balance the \(-2\) charge of one sulfide ion, resulting in a neutral compound. The presence of these distinct, fully charged ions dictates that ammonium sulfide is an ionic salt, despite the fact that all constituent atoms are nonmetals. This is a common pattern for compounds containing polyatomic ions, where classification is based on the inter-ion attraction.
Covalent Bonds Within the Ammonium Ion
Although the overall compound is ionic, the internal structure of the ammonium ion (\(\text{NH}_4^+\)) relies on covalent bonding. Since nitrogen and hydrogen are nonmetals, they share electrons to form this stable, charged unit. Specifically, the nitrogen atom is covalently bonded to four hydrogen atoms.
Three of the nitrogen-hydrogen bonds form through the usual sharing of electrons. The fourth bond is a coordinate covalent bond, formed when an ammonia molecule (\(\text{NH}_3\)) uses its lone pair to bond with a hydrogen ion (\(\text{H}^+\)). Once the ammonium ion is formed, all four nitrogen-hydrogen bonds become chemically identical, creating a single, stable, positively charged ion.
How Bonding Type Affects Compound Properties
The dual nature of bonding in ammonium sulfide—ionic between the ions and covalent within the ammonium ion—influences its macroscopic properties. As an ionic compound, it forms a crystalline solid, though it is unstable, existing as a yellowish solid only below \(-18^\circ\text{C}\). The strength of the ionic attraction between the \(\text{NH}_4^+\) and \(\text{S}^{2-}\) ions results in a relatively high energy needed to break the structure.
Ammonium sulfide exhibits high solubility in water and other polar solvents like alcohol. When dissolved, the strong ionic bonds are overcome, and the compound dissociates completely into its constituent ions, \(\text{NH}_4^+\) and \(\text{S}^{2-}\). The presence of these mobile charge carriers means that an aqueous solution of ammonium sulfide is an effective conductor of electricity. This characteristic is a hallmark of ionic compounds, even those containing polyatomic ions.