Ammonium Ion
The ammonium ion (NH₄⁺) is a positively charged polyatomic ion formed when a neutral ammonia molecule (NH₃) accepts a proton (H⁺) from an acid. This process aligns with Brønsted-Lowry acid-base theory, which describes acids as proton donors and bases as proton acceptors. The structure of the ammonium ion resembles that of methane (CH₄), with four equivalent N–H bonds arranged tetrahedrally around a nitrogen atom, contributing to its stability and unique properties. The ammonium ion is significant in both industrial applications and biochemistry, as it can form various ionic compounds by combining with anions, effectively functioning as a fertilizer in agriculture. It plays a crucial role in the nitrogen cycle, where nitrogen is essential for amino acid formation, impacting protein synthesis in living organisms. Additionally, in metabolic processes, excess nitrogen is often converted into ammonia, which is then neutralized to form ammonium ions for excretion through urine. Understanding the chemistry of ammonium ions is fundamental to both environmental science and biological processes, highlighting its importance across multiple disciplines.
Ammonium Ion
FIELDS OF STUDY: Organic Chemistry; Inorganic Chemistry; Biochemistry
ABSTRACT
The basic structure of the ammonium ion is defined, and its importance in various branches of chemistry is described. An ammonium ion (NH4+) forms when a neutral ammonia molecule (NH3) gains a proton from an acidic material. The NH4+ ion behaves in the same manner as any other cation.
The Nature of the Ammonium Ion
Ammonia is a simple compound consisting of a nitrogen atom chemically bonded to three hydrogen atoms; its chemical formula is therefore NH3. The electron distribution of the nitrogen atom consists of just seven electrons, with two in the 1s orbital, two in the 2s orbital, and three in the 2p orbitals. Energy considerations for the bonds to the three hydrogen atoms make it more favorable for the 2s and 2p atomic orbitals to "hybridize" into four equivalent sp3 orbitals, with three of them each containing one electron. These sp3 orbitals arrange tetrahedrally about the nucleus of the nitrogen atom to form the bonds to the hydrogen atoms. The remaining two electrons exist as a lone pair (that is, not part of a chemical bond). This arrangement exposes a region of high negative-charge density that makes the ammonia molecule highly nucleophilic (prone to donating electrons) and a strong base. Ammonia is therefore susceptible to ionization. It readily accepts a proton (or a hydrogen cation, H+) from any compound capable of releasing one, in accordance with Brønsted-Lowry acid-base theory. The interaction of the proton’s 1s orbital with the lone pair of electrons in the sp3 orbital on the nitrogen atom effectively forms a chemical bond. This results in the formation of the ammonium ion, NH4+.
Chemical Properties of the Ammonium Ion
The ammonium ion forms when the ammonia molecule accepts a proton from a Brønsted-Lowry acid. Because of the chemical bond that forms between the proton and the lone pair of electrons on the nitrogen atom, the ammonium ion is very stable. All four N−H bonds are exactly equivalent, and the ammonium ion has the same structure and electron distribution as the methane molecule, CH4. It is also the smallest of complex ions and thus has a positive charge density similar to that of the alkali metal ions, and the ammonium ion is able to form all of the corresponding ionic compounds with anions in the same way. The ammonium ion has a unique property relative to other cations, however, since it is itself the conjugate acid of the base ammonia. In combination with various anions, the resulting compound is able to separate into a neutral ammonia molecule and the corresponding acid of the anion. Ammonium chloride (NH4Cl), for example, is able to decompose into NH3 and hydrochloric acid (HCl), as well as dissociate into NH4+ and Cl− ions. The characteristic odor of ammonia is quite noticeable over solid ammonium chloride crystals. Ammonium compounds thus tend to become "self-contaminated" over time due to loss of materials from decomposition.
The Ammonium Ion in Industry and Biochemistry
Nitrogen is an essential component of living systems, required for the formation of amino acids, which are in turn used in metabolic pathways for the synthesis of proteins according to the genetic code of all living things. In plants, amino acids are converted into various other components, such as chlorophyll and various alkaloids. Nitrogen from the air can be "fixed" by soil-borne bacteria that convert N2 into nitrate ions (NO3−), which can be absorbed through the root structure of plants. Alternatively, nitrogen can be added directly to soils as water-soluble nitrate salts, the most common of which is ammonium nitrate, commonly used as a fertilizer. In biochemistry, amino acids and other compounds are also broken down into component fragments through metabolic processes. The amino groups typically end up as highly basic ammonia molecules, which are rapidly neutralized by conversion to highly soluble ammonium ions. The kidneys isolate the ammonium ions and excrete them in urine. The amount of ammonium ions in urine is tightly controlled, and significant variations can indicate various medical conditions.
The other major nitrogen-containing compound produced as a by-product of metabolism is urea (H2N−CO−NH2), which is produced biochemically by the metabolic breakdown of purines. Urea is of historical relevance to ammonium ion chemistry as the first example of an organic compound synthesized solely from inorganic materials, accomplished in 1828 by German chemist Friedrich Wohler (1800–1882). Urea is also used as a nitrogenous fertilizer and is considered more environmentally friendly than ammonium nitrate.
PRINCIPAL TERMS
- ammonia: an inorganic compound consisting of a nitrogen atom bonded to three hydrogen atoms; exists as a colorless, pungent gas at room temperature.
- biochemistry: the chemistry of living organisms and the processes incidental to and characteristic of life.
- Brønsted-Lowry acid-base theory: definitions for acids and bases developed separately in 1923 by Danish chemist Johannes Nicolaus Brønsted and English chemist Martin Lowry; defines an acid as any compound that can release a hydrogen ion and a base as any compound that can accept a hydrogen ion.
- ionization: the process by which an atom or molecule loses or gains one or more electrons to acquire a net positive or negative electrical charge.
- reaction mechanism: the sequence of electron and orbital interactions that occurs during a chemical reaction as chemical bonds are broken, made, and rearranged.
Bibliography
Berg, Jeremy M., Lubert Stryer, and John L. Tymoczko. Biochemistry. 7th ed. New York: Freeman, 2012. Print.
Gribbin, John. Science: A History, 1543–2001. London: Lane, 2002. Print.
Herbert, R. B. The Biosynthesis of Secondary Metabolites. 2nd ed. London: Chapman, 1994. Print.
Johnson, Rebecca L. Atomic Structure. Minneapolis: Lerner, 2008. Print.
Mackay, K. M., R. Ann Mackay, and W. Henderson. Introduction to Modern Inorganic Chemistry. Cheltenham: Nelson, 2002. Print.
Miessler, Gary L., Paul Fischer, and Donald A. Tarr. Inorganic Chemistry. 5th ed. Boston: Pearson, 2014. Print.
Winter, Mark J. The Orbitron: A Gallery of Atomic Orbitals and Molecular Orbitals on the WWW. U of Sheffield, 2002. Web. 26 Mar. 2014.