Phosphorus trichloride (\(\text{PCl}_3\)) is a common industrial compound. Understanding the nature of the forces that hold its atoms together is fundamental to predicting its behavior. The primary question concerning \(\text{PCl}_3\) is whether the bonds are ionic (involving the complete transfer of electrons) or covalent (involving the sharing of electrons). The classification is determined by differences in atomic properties and reflected in the compound’s observable physical characteristics.
Understanding Ionic and Covalent Bonds
Chemical bonds are generally categorized into two main types based on how electrons are distributed. Ionic bonds are formed when there is a full transfer of valence electrons, typically between a metal and a nonmetal. The metal forms a positively charged ion (cation), and the nonmetal forms a negatively charged ion (anion). The resulting strong electrostatic attraction holds the compound together, often leading to solid crystal lattice structures.
In contrast, covalent bonds involve the sharing of electrons between atoms, most commonly between two nonmetal atoms. Instead of a transfer creating charged ions, the atoms are bound together as a neutral molecule by the shared electron pair. Covalent compounds generally exhibit much lower melting and boiling points compared to their ionic counterparts due to the less intense, localized forces within the molecule.
How Electronegativity Determines Bond Type
To classify the nature of a bond, chemists use electronegativity, a quantitative measure of an atom’s tendency to attract a shared pair of electrons toward itself. The most widely accepted scale for this measurement is the Pauling scale.
The difference in electronegativity (\(\Delta EN\)) between the two bonded atoms serves as the primary tool for bond classification, revealing where the bond falls along the ionic-covalent spectrum.
When the electronegativity difference is small (less than 0.4), the electrons are shared almost equally, resulting in a nonpolar covalent bond. If the difference is moderate (between 0.4 and 1.7), the sharing of electrons becomes unequal, creating a polar covalent bond. In a polar bond, the electrons spend more time closer to the atom with the higher electronegativity, giving it a partial negative charge (\(\delta^-\)) and the other atom a partial positive charge (\(\delta^+\)). If the difference is large (greater than 1.7), the electron sharing is considered a full electron transfer, classifying the bond as ionic.
The Covalent Nature of Phosphorus Trichloride
The bond in phosphorus trichloride (\(\text{PCl}_3\)) is formed between phosphorus (P) and chlorine (Cl), both of which are nonmetal elements. Phosphorus has an electronegativity value of approximately 2.19, while chlorine is significantly more electronegative, with a value of about 3.16.
Calculating the difference in electronegativity (\(\Delta EN\)) for the P-Cl bond yields \(3.16 – 2.19 = 0.97\). Since this value falls within the range of 0.4 to 1.7, the bond in \(\text{PCl}_3\) is classified as polar covalent. The chlorine atoms pull the shared electron density closer to themselves, making the molecule polar, but the sharing is not unequal enough to qualify as a complete transfer characteristic of an ionic bond.
The physical properties of \(\text{PCl}_3\) offer strong supporting evidence for its covalent nature. \(\text{PCl}_3\) exists as a colorless to pale yellow liquid at standard room temperature. This is characteristic of covalent molecular compounds, while ionic compounds are hard, brittle solids. Furthermore, the compound has a low melting point of \(-93.6^\circ \text{C}\) and a low boiling point of \(76.1^\circ \text{C}\). These low phase-change temperatures are typical for substances held together by relatively weak intermolecular forces, unlike the strong electrostatic forces found in ionic crystal lattices.