What is ammonium or ammonium ion?


Ammonium or ammonium ion is a positively charges polyatomic cation with chemical formula, NH4+. The systematic IUPAC name of ammonium ion is azanium.

It is formed by the protonation of ammonia molecule and hence it is known as conjugate acid of ammonia. The pKa value of ammonium ion is 9.25.

The central ’N’ atom in ammonium ion contains one lone pair of electron which is available for protonation. The nitrogen atom in ammonium ion is sp3 hybridized.

The ion has a tetrahedral geometry and is isoelectronic with methane and borohydride. In terms of size, the ammonium cation (rionic =175 pm) resembles the Cs+ cation (rionic =183 pm).

Ammonium ion formula and Lewis dot structure

Ammonium ion is the conjugate acid of ammonia. Ammonium ion is generated by the interaction of between weak base ammonia and proton donors Bronsted acids. The chemical formula of ammonium ion is NH4+. The Lewis dot structure of ammonium ion is shown below.

Ammonium ion contains which type of bond?

Ammonium ion is formed from Lewis base ammonia. The nitrogen atom in ammonia contains one lone pair of electron which is available for donation.

So ’N’ atom donates its lone pair to proton (H+) and forms a new N — H bond resulting in the formation of ammonium ion (NH4+).

Hence after protonation, ammonium ion contains four polar N — H bond which are equivalent. Three out of four N –H bonds are pure covalent and one N — H bond is co-ordinate in nature. Ammonium ion has tetrahedral structure.

Properties of ammonium ion

The ammonium ion is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia molecule:

NH4+ + B− → HB + NH3

Thus, treatment of concentrated solutions of ammonium salts with strong base gives ammonia. When ammonia is dissolved in water, a tiny amount of it converts to ammonium ions:

H2O + NH3 ⇌ OH− + NH+4

Salt formation

Ammonium cation is found in a variety of salts such as ammonium carbonate, ammonium chloride and ammonium nitrate. Most simple ammonium salts are very soluble in water.

The ammonium salts of nitrate and especially perchlorate are highly explosive, in these cases ammonium is the reducing agent.

In an unusual process, ammonium ions form an amalgam. Such species are prepared by the addition of sodium amalgam to a solution of ammonium chloride.

Identification of ammonium ion

To find whether the ammonium ion is present in the salt, first the salt is heated in presence of alkali hydroxide releasing a gas with characteristic smell which of course is ammonia.

NH4+ + OH- + heat à NH3 + H2O

To further confirm ammonia it passed through glass rod dipped in HCl solution (hydrochloric acid) creating white dense fumes of ammonium chloride.

NH3(gas) + HCl(aq) à NH4Cl(gas)

Ammonia when passed through CuSO4 solution turns from blue to deep blue color forming Schweizer’s reagent.

CuSO4(aq) + 4NH3(aq) + 4H2O à [Cu(NH3)4(H2O)2](OH)2(aq) + H2SO4(aq)

Ammonia or ammonium ion when added to alkaline solution of potassium mercuric iodide, that is, Nessler’s reagent, it gives brown color precipitate known as iodide of Million’s base in basic medium.

Ammonium ion when added to chloroplatinic acid and sodium cobaltinitrite indivisually gives a yellow precipitate. Again with potassium bitartrate, ammonium ion gives a white precipitate.