ammoniumionien

The ammonium ion, or ammoniumion, is the positively charged polyatomic ion NH4+. It forms when ammonia (NH3) accepts a proton, commonly in aqueous solution or during acid–base reactions to produce various ammonium salts such as ammonium chloride or ammonium sulfate.

Structurally, the ammonium ion is tetrahedral, with four N–H bonds and no lone pair on the nitrogen. In solid salts, NH4+ is coordinated by surrounding anions and often participates in hydrogen bonding with solvent molecules. In solution, it can interact with water and other solutes, influencing the solution's acidity and ionic strength.

Chemically, NH4+ is a weak acid. In water it has a pKa of about 9.25, establishing the equilibrium NH4+ ⇌ NH3 + H+. Thus, in more basic solutions, NH4+ tends to deprotonate to NH3, while in strongly acidic environments it remains predominantly as NH4+. This acid–base behavior underlies the role of ammonium in buffering and nitrogen cycling.

Occurrence and use are widespread. Ammonium ions appear in many salts, including ammonium chloride, ammonium sulfate, and ammonium nitrate, and are important in fertilizers as a source of nitrogen for plants. Biologically, ammonium is produced during the microbial decomposition of organic matter and is a key intermediate in nitrogen metabolism, with plants able to assimilate NH4+ directly or after conversion to nitrate.

Safety and handling are generally straightforward for ordinary ammonium salts, which are typically soluble and relatively stable. Heating certain ammonium salts can release ammonia gas, and exposure to ammonium compounds in high concentrations can irritate the respiratory tract and skin.