Nitrogen trichloride (NCl\(_{3}\)) is a chemical compound often encountered when studying the fundamental principles of chemical bonding. Understanding whether a compound is ionic or covalent is foundational to predicting its physical and chemical properties. To accurately classify NCl\(_{3}\), we must analyze the types of atoms involved and the way electrons are distributed between them.
Defining Chemical Bonds
The attraction that holds atoms together in a compound is known as a chemical bond, with two primary types: ionic and covalent. An ionic bond typically forms between a metal and a nonmetal atom, involving a complete transfer of electrons. The resulting atoms become charged particles, called ions, with the metal forming a positively charged cation and the nonmetal forming a negatively charged anion.
Covalent bonds generally occur when two nonmetal atoms combine. Instead of electron transfer, the atoms engage in electron sharing to achieve a stable electron configuration. These shared electrons hold the atoms together as a molecule. The nature of this sharing—whether equal or unequal—defines the specific type of covalent bond.
The Electronegativity Scale
To move beyond qualitative descriptions of bonding, chemists use a quantitative measure called electronegativity. This property is defined as an atom’s inherent tendency to attract the electrons of a bond toward itself. The most common system for measuring this tendency is the Pauling scale, which assigns a numerical value to nearly every element.
The difference in electronegativity (\(\Delta EN\)) between two bonded atoms provides a reliable method for determining the bond type. A larger difference indicates a more unequal sharing of electrons, pushing the bond toward ionic character. Conversely, a smaller difference suggests a more equal sharing, indicative of a covalent bond.
While the change from one bond type to another is a continuum, numerical thresholds are used for classification. A \(\Delta EN\) greater than approximately 1.7 generally signifies an ionic bond, as the electron attraction is so unequal it constitutes a transfer. A difference between roughly 0.4 and 1.7 typically results in a polar covalent bond. If the \(\Delta EN\) is less than about 0.4, the bond is considered nonpolar covalent.
Determining the Bond Type for NCl\(_{3}\)
We can now apply these principles to Nitrogen Trichloride, NCl\(_{3}\), which is formed from one nitrogen atom and three chlorine atoms. Nitrogen and Chlorine are both located on the right side of the periodic table, classifying them as nonmetals. The combination of two nonmetals strongly suggests the formation of a covalent compound involving electron sharing.
To confirm this, we examine the electronegativity values for the two elements on the Pauling scale. Nitrogen (N) has an electronegativity of approximately 3.04, and Chlorine (Cl) has a value of about 3.16. The difference in electronegativity (\(\Delta EN\)) between these two atoms is calculated by subtracting the smaller value from the larger one, which yields \(|3.04 – 3.16| = 0.12\).
This calculated \(\Delta EN\) of 0.12 is quite small and falls significantly below the threshold for ionic bonding. The value also places the N-Cl bond firmly within the range for a covalent bond. Although the \(\Delta EN\) is not zero, the difference is minimal, meaning the electron sharing is slightly unequal, classifying the bond as a very weak polar covalent bond. Since NCl\(_{3}\) is composed solely of nonmetal atoms held together by shared electrons, it is definitively classified as a covalent compound.