The $\text{NH}_4^+$ species, known as the ammonium ion, is a classic example of a positively charged polyatomic ion. Ions are atoms or groups of atoms that carry an overall net electrical charge because they have gained or lost electrons. The ammonium ion is a cluster of five atoms working together that possesses an electrical charge, placing it squarely within a specific category of charged particles in chemistry.
What Makes an Ion Polyatomic
A polyatomic ion is defined as a covalently bonded group of two or more atoms that functions as a single, electrically charged unit. These ions are distinct from monatomic ions, which are ions formed from a single atom, such as sodium ($\text{Na}^+$) or chloride ($\text{Cl}^-$). The atoms within a polyatomic ion are held together by strong covalent bonds, meaning they share electrons.
This cluster of atoms, despite its internal covalent structure, carries an overall net charge due to an imbalance between the total number of protons and the total number of electrons across the entire group. The resulting charge can be positive, in which case the ion is called a cation, or negative, in which case it is an anion. This net charge allows the entire unit to participate in ionic bonding with other ions of the opposite charge.
The Structure and Charge of Ammonium
The ammonium ion ($\text{NH}_4^+$) is a cation composed of one nitrogen atom and four hydrogen atoms. Its structure is derived from the neutral ammonia molecule ($\text{NH}_3$), which has a nitrogen atom bonded to three hydrogen atoms and a single non-bonding pair of electrons. The $\text{NH}_4^+$ ion is formed when the ammonia molecule reacts with a proton ($\text{H}^+$).
The nitrogen atom in ammonia uses its lone pair of electrons to form a fourth covalent bond with the incoming proton. This new bond, where both shared electrons originate from the nitrogen atom, is a type of covalent bond known as a coordinate or dative bond. Once formed, all four nitrogen-hydrogen bonds within the $\text{NH}_4^+$ ion become equivalent in strength and length.
The resulting structure is highly symmetric, adopting a tetrahedral shape with the nitrogen atom at the center. The positive charge of $+1$ comes from the fact that the group has one less electron than the total number of protons in its five nuclei.
How Ammonium Forms Compounds
As a positively charged cation, the ammonium ion behaves chemically like the ions of alkali metals, combining readily with negatively charged anions. This combination occurs through ionic bonding, where the electrostatic attraction between the positive $\text{NH}_4^+$ ion and a negative ion creates a neutral compound known as an ammonium salt. Compound formation requires that the positive and negative charges balance out to zero.
For instance, the $\text{NH}_4^+$ ion combines in a one-to-one ratio with a $-1$ charged ion like chloride ($\text{Cl}^-$) to form ammonium chloride ($\text{NH}_4\text{Cl}$). When combining with a $-2$ charged ion, such as sulfate ($\text{SO}_4^{2-}$), two ammonium ions are required to achieve neutrality, resulting in the formula $(\text{NH}_4)_2\text{SO}_4$. Naming these compounds involves replacing the name of the metal cation with “ammonium” followed by the name of the anion, such as ammonium nitrate ($\text{NH}_4\text{NO}_3$).