Phosphorous acid (\(\text{H}_3\text{PO}_3\)) is a compound whose classification often leads to confusion. Based on its chemical bonds, it is definitively a molecular compound. The misconception that it might be ionic stems from its behavior when dissolved in water, where it acts as an acid. Understanding the rules chemists use clarifies why \(\text{H}_3\text{PO}_3\) is considered molecular, despite exhibiting ionic characteristics in solution.
Defining Chemical Bond Types
Chemists classify compounds based on the nature of the bond that forms between their constituent atoms, separating them into ionic and molecular (covalent) groups. Ionic compounds form through the complete transfer of valence electrons, typically between a metal and a nonmetal, creating oppositely charged ions held together by electrostatic attraction. Molecular compounds are formed when two or more nonmetal atoms share electrons, creating a covalent bond. The classification rule is straightforward: if a metal is present, the compound is typically ionic; if the compound consists exclusively of nonmetal atoms, it is classified as molecular.
The Specific Structure of \(\text{H}_3\text{PO}_3\)
To classify \(\text{H}_3\text{PO}_3\), we analyze its elemental composition: hydrogen (\(\text{H}\)), phosphorus (\(\text{P}\)), and oxygen (\(\text{O}\)). All three elements are nonmetals, meaning the compound is formed exclusively through covalent bonding, which confirms its molecular classification.
The phosphorus atom acts as the central atom, forming a double bond with one oxygen atom (\(\text{P}=\text{O}\)). It is also bonded to two hydroxyl groups (\(\text{OH}\)) and directly to one hydrogen atom (\(\text{P}-\text{H}\)). This configuration is accurately represented by the structural formula \(\text{HPO}(\text{OH})_2\).
All the bonds within this structure involve the sharing of electrons between nonmetals. The solid form of phosphorous acid exists as discrete \(\text{H}_3\text{PO}_3\) molecules held together by weaker intermolecular forces, not as a continuous network of ions.
Why Phosphorous Acid Ionizes
The confusion about \(\text{H}_3\text{PO}_3\)‘s classification arises because of its behavior in an aqueous solution. When a molecular compound dissolves in water, if its bonds are highly polarized, it may undergo ionization. Phosphorous acid is an acid, meaning it releases hydrogen ions (\(\text{H}^+\)) when dissolved in water.
The two hydrogen atoms in the \(\text{P}-\text{OH}\) groups are bonded to the highly electronegative oxygen atoms. This difference in electronegativity creates a polar covalent bond, making the \(\text{H}\) atoms susceptible to being pulled away by water molecules. This reaction results in the formation of hydronium ions (\(\text{H}_3\text{O}^+\)) and the \(\text{H}_2\text{PO}_3^-\) anion, which are charged particles.
Phosphorous acid is a diprotic acid, meaning only the two hydrogen atoms bonded to oxygen are acidic and can ionize. The third hydrogen atom, bonded directly to the phosphorus atom (\(\text{P}-\text{H}\)), is non-acidic because the bond polarity is not high enough for water to break it. The creation of ions makes the solution electrically conductive, a property often associated with ionic compounds. However, this acidic behavior is a property of the solution, while the molecular classification describes the pure, undissolved substance.