Barium Hydroxide (\(\text{Ba}(\text{OH})_2\)) is classified as an ionic compound, although its structure involves a combination of bonding types. The compound is held together by a strong electrostatic attraction between a positively charged metal ion and a negatively charged polyatomic ion. This dominant interaction between the two distinct charged units determines its primary classification as ionic. However, the atoms within the polyatomic ion are linked by a different type of chemical bond.
The Fundamentals of Chemical Bonding
Chemical bonds form when atoms interact to achieve a more stable electron configuration, typically resembling that of a noble gas. This process results in two primary categories of bonding: ionic and covalent. The distinction between these types is determined by how electrons are distributed between the participating atoms, which is often predicted by their difference in electronegativity.
Ionic bonds form through the complete transfer of valence electrons from one atom to another, creating charged particles called ions. This typically occurs between a metal (forming a positively charged cation) and a nonmetal (forming a negatively charged anion). The resulting bond is the strong electrostatic force of attraction between these oppositely charged ions.
Covalent bonds, in contrast, form when atoms share valence electrons to complete their outer shells. This sharing is most common between two nonmetal atoms with relatively similar electronegativity values. If the sharing is unequal, it creates a polar covalent bond where electrons spend more time near the more electronegative atom. However, unlike ionic bonds, the electrons are not fully transferred.
Determining the Primary Bond Type in Barium Hydroxide (\(\text{Ba}(\text{OH})_2\))
The overall classification of Barium Hydroxide as an ionic compound stems from the interaction between its two main components: the metal cation and the polyatomic anion. Barium (\(\text{Ba}\)) is an alkaline earth metal found in Group 2 of the periodic table. As a metal, it tends to lose its two outermost valence electrons to achieve stability, forming the dipositive cation, \(\text{Ba}^{2+}\).
The other component is the hydroxide group, which is a polyatomic ion with the formula \(\text{OH}^-\). This group acts as a single unit with an overall charge of negative one. The complete compound, \(\text{Ba}(\text{OH})_2\), requires two of the singly charged hydroxide anions to balance the \(\text{Ba}^{2+}\) cation, resulting in a neutral compound.
The bond holding the \(\text{Ba}^{2+}\) ion and the two \(\text{OH}^-\) ions together is a powerful electrostatic attraction. This attraction between a metal cation and a nonmetal-containing polyatomic anion is the defining characteristic of an ionic bond. Because this strong ionic force is the main structural element, Barium Hydroxide is classified as an ionic salt.
The Internal Covalent Bond within the Hydroxide Group
While the bond between the Barium ion and the Hydroxide ion is ionic, the full picture of Barium Hydroxide’s structure reveals a mixed-bonding nature. The hydroxide ion (\(\text{OH}^-\)) itself is a tightly bound group of two different nonmetal atoms: Oxygen (\(\text{O}\)) and Hydrogen (\(\text{H}\)). The bond that links the oxygen and hydrogen atoms together within the \(\text{OH}^-\) ion is a covalent bond.
Oxygen and Hydrogen are both nonmetals, and they share electrons to form the oxygen-hydrogen (O-H) bond within the polyatomic unit. This internal covalent bond keeps the hydroxide group intact as it interacts as a single, charged unit. The atoms inside are connected by shared electron pairs, which is consistent with the definition of covalent bonding.
The result is a compound where the primary interaction between the charged parts is ionic, but the internal structure of the anion is maintained by a covalent bond. This dual-bonding characteristic is common for compounds containing polyatomic ions, meaning \(\text{Ba}(\text{OH})_2\) is best described as an ionic compound containing internal covalent bonds.