Phosphorus trichloride, a compound with the chemical formula \(\text{PCl}_3\), is an important chemical used widely in industry for producing various other substances, including insecticides and organophosphorus compounds. The definitive answer is that \(\text{PCl}_3\) is a covalent molecule, a classification that dictates its structure, reactivity, and physical behavior. Understanding this requires looking at how scientists determine the bond character between atoms.
Understanding the Two Main Bond Types
Chemical compounds are generally categorized by one of two main types of atomic connections: ionic or covalent. Ionic bonds form when a complete transfer of one or more electrons occurs between atoms, typically involving a metal and a nonmetal. This transfer results in the formation of oppositely charged ions, which are then held together by a strong electrostatic attraction, creating a crystal lattice structure.
Covalent bonds, in contrast, involve the sharing of electrons between atoms, which commonly occurs between two nonmetals. Instead of forming charged ions, the atoms join to create a distinct, neutral unit known as a molecule. This results in a molecular compound with properties very different from those of an ionic solid.
The Role of Electronegativity in Bond Classification
To precisely determine a bond’s character, chemists rely on the concept of electronegativity, which is a measure of an atom’s tendency to attract a shared pair of electrons toward itself in a chemical bond. The bond type exists on a continuum, ranging from purely covalent to highly ionic, and is quantified by calculating the difference in electronegativity (\(\Delta\)EN) between the two bonded atoms. A difference close to zero indicates a nonpolar covalent bond, where electrons are shared almost equally.
When this difference becomes greater, the bond is considered polar covalent, meaning the electron sharing is unequal. This results in a slight negative charge on the more electronegative atom and a slight positive charge on the less electronegative one. A \(\Delta\)EN value generally below \(1.7\) is characteristic of a covalent bond. Values greater than this threshold indicate that the bond is considered ionic, characterized by the full transfer of electrons.
Analyzing Phosphorus Trichloride (\(\text{PCl}_3\))
The atoms involved in phosphorus trichloride are phosphorus (\(\text{P}\)) and chlorine (\(\text{Cl}\)), both of which are nonmetals, a strong initial indicator of covalent bonding. Using the Pauling scale, the electronegativity value for phosphorus is approximately \(2.19\), and for chlorine, it is approximately \(3.16\). Calculating the difference in electronegativity (\(\Delta\)EN) for the \(\text{P-Cl}\) bond yields a value of about \(0.97\).
This value falls firmly within the range designated for polar covalent bonds, confirming that the electrons are shared between the phosphorus and chlorine atoms, but not equally. The chlorine atom, being more electronegative, pulls the shared electrons closer to itself, giving each \(\text{Cl}\) atom a partial negative charge, while the central \(\text{P}\) atom carries a partial positive charge.
The overall \(\text{PCl}_3\) molecule is formed by one central phosphorus atom bonded to three chlorine atoms, resulting in a trigonal pyramidal molecular geometry. The presence of a lone pair of electrons on the phosphorus atom contributes to this specific shape. Since the compound is fundamentally a molecular substance formed entirely from nonmetals, the formation of a distinct molecule with shared electrons is the reason \(\text{PCl}_3\) is classified as a covalent compound.
How Covalent Bonding Shapes Physical Properties
The covalent nature of \(\text{PCl}_3\) directly influences its physical properties, setting it apart from ionic substances like table salt. Because atoms are linked by shared electrons to form discrete molecules, the forces holding one \(\text{PCl}_3\) molecule to another are relatively weak intermolecular forces. These weak forces require little energy to overcome, resulting in low melting and boiling points.
Phosphorus trichloride is a liquid at room temperature, freezing at a low \(-93.6^\circ\text{C}\) and boiling at \(76.1^\circ\text{C}\). These values are typical of small, covalent molecules. Ionic compounds, conversely, possess extremely strong electrostatic attractions, leading to high melting points and a solid state at room temperature. Furthermore, \(\text{PCl}_3\) does not conduct electricity effectively because it lacks mobile ions.