Ammonia, represented by the chemical formula \(\text{NH}_3\), is not an ionic compound; it is a molecule held together by covalent bonds. Understanding the distinction between ionic and covalent bonds is necessary to accurately determine the properties of any chemical substance. This classification depends on how electrons are distributed or shared between the atoms.
Distinguishing Ionic and Covalent Bonds
The primary criterion for classifying a chemical bond is the difference in electronegativity between the two atoms. Electronegativity is an atom’s ability to attract shared electrons toward itself in a chemical bond. This difference determines the nature of the bond, dictating whether electrons are transferred or shared.
Ionic bonds form when there is a large difference in electronegativity, typically greater than 1.7 on the Pauling scale. This significant difference causes one atom, usually a metal, to completely transfer electrons to a nonmetal atom. This transfer results in the formation of a positively charged ion (cation) and a negatively charged ion (anion), which are held together by electrostatic attraction.
Covalent bonds occur when atoms share electrons, a process that usually happens between two nonmetal atoms. When the electronegativity difference is small (less than 0.4), the sharing is nearly equal, resulting in a nonpolar covalent bond. If the difference is between approximately 0.4 and 1.7, the sharing is unequal, creating a polar covalent bond. This unequal sharing creates partial charges on the atoms, but no full ion formation occurs.
Analysis of Ammonia’s Molecular Structure
We examine the bonding between the central nitrogen (N) atom and the three surrounding hydrogen (H) atoms in ammonia. Since both nitrogen and hydrogen are nonmetal elements, this combination initially indicates that the bonds holding the \(\text{NH}_3\) molecule together are covalent.
To confirm the bond type, we consider the electronegativity values. Nitrogen has a value of approximately 3.04, and hydrogen’s value is around 2.2 on the Pauling scale. The difference is approximately 0.84 (3.04 – 2.2), which falls within the range for a polar covalent bond.
The bonds in ammonia are polar covalent, meaning the electrons are shared, but the nitrogen atom pulls the electron density closer to itself. This unequal sharing gives nitrogen a partial negative charge and the hydrogen atoms partial positive charges. The molecule adopts a trigonal pyramidal shape due to the nitrogen atom possessing a non-bonding lone pair of electrons.
The Source of Confusion: The Ammonium Ion
The persistent confusion regarding ammonia’s classification stems from the existence of the ammonium ion, \(\text{NH}_4^+\). While the neutral ammonia molecule (\(\text{NH}_3\)) is purely covalent, it can participate in the formation of ionic compounds. Ammonia readily accepts a proton, or hydrogen ion (\(\text{H}^+\)), due to the lone pair of electrons on its nitrogen atom.
When \(\text{NH}_3\) reacts with an acid, it becomes protonated, forming the polyatomic ammonium cation, \(\text{NH}_4^+\). The four N-H bonds within this \(\text{NH}_4^+\) structure remain covalent in nature. However, this newly formed cation carries a full positive charge and behaves like a metal ion.
Compounds like ammonium chloride (\(\text{NH}_4\text{Cl}\)) are considered ionic compounds. This is because the positively charged ammonium ion (\(\text{NH}_4^+\)) bonds with a separate, negatively charged anion, such as the chloride ion (\(\text{Cl}^-\)). The bond between the \(\text{NH}_4^+\) group and the \(\text{Cl}^-\) ion is an ionic bond, even though the internal bonds within the \(\text{NH}_4^+\) ion remain covalent.