The Ammonium Ion NH

When the ammonium ion NH is formed the lone pair becomes a bonding pair and the shape becomes a regular tetrahedron. 

The ions dealt with to this point (e.g., Na+, Cl-) are monatomic that is, they are derived from a single atom by the loss or gain of electrons. Many of the most important ions in chemistry are polyatomic, containing more than one atom. Examples include the hydroxide ion (OH-) and the ammonium ion (NH ). In these and other polyatomic ions, the atoms are held together by covalent bonds, for example,... 

Cations. The ammonium ion, NH,+, behaves as a weak acid in water a 0.10 M solution of NH4CI has a pH of about 5. The process by which the NH4+ ion lowers the pH of water can be represented by the (Bransted-Lowry) equation ... 

Even the ammonium ion, NH 4+, is acidic enough to react with magnesium metal. Therefore, ammonium perchlorate and other ammonium salts should not be used with magnesium unless the metal surface is coated with linseed oil, paraffin, or a similar material. 

The reaetion between the ammonium ion, NH , and trimethylamine, (CH3)3N, analogous to Eq. (4-16), has been studied by pulsed ICR mass spectrometry [115]. The Gibbs energy diagram in Fig. 4-2 describes what happens to the reactants on going from... 

Consider the reaction of NH3 with hydrogen ion, H, to form the ammonium ion, NH+. 

The cation exchange capacity is usually measured by leaching the soil or colloid with neutral normal ammonium acetate. Then the excess salt is removed by washing with 95% ethanol. The ammonium ion (NH +) is then determined by steam distillation with magnesium oxide in an alkaline medium. The ammonia evolved is adsorbed into a known quantity of the standard acid containing methyl red indicator and the excess acid back titrated with a standard alkali. 

The ammonium ion NH 4 is formed similarly. If the nitrogen atom loses an electron, its electronic structure will become ... 

In water, ammonia is in equilibrium with the ammonium ion, NH/. The ammonia-ammonium ion equilibrium is highly dependent on the pH and, to a lesser extent, the temperature of the medium. In acidic waters, the equilibrium favors the ammonium ion. 

These ions with their opposite charges attract each other in the same way as do the simple ions in binary ionic compounds. However, the individual polyatomic ions are held together by covalent bonds, with all of the atoms behaving as a unit. For example, in the ammonium ion, NH +, there are four N—H covalent bonds. Likewise, the nitrate ion, N03, contains three covalent N—O bonds. Thus, although ammonium nitrate is an ionic compound because it contains the NH " and N03 ions, it also contains covalent bonds in the individual polyatomic ions. When ammonium nitrate is dissolved in water, it behaves as a strong electrolyte like the binary ionic compounds sodium chloride and potassium bromide. As we saw in Chapter 8, this occurs because when an ionic solid dissolves, the ions are freed to move independently and can conduct an electric current. 

Acids can also exist in non water-based solvents such as ammonia (NH3), where the ammonium ion NH/ is generated by the acid. This concept is generally extended to define acids as those substances that ionize to generate the positive ion of the solvent in which they reside. 

Biological fixation is done by specialized microbes that convert the N2 into ammonia (NHj"), nitrate ions (NOj"), or urea ((NH2)2CO). Although ammonia can be taken up by plants through the roots, it s more often the ammonium ion (NH ), that s the conjugate acid of ammonia, that is used by biological systems. (See Chapter 5 for a quick recap on conjugate acids and bases.)... 

In the following sections we will look at how a compound s composition lets us predict whether it is a strong electrolyte, weak electrolyte, or nonelectrolyte. For the moment, you need only to remember that water-soluble ionic compounds are strong electrolytes. Ionic compounds can usually be identified by the presence of both metals and nonmetals [for example, NaCl, FeS04, and A1(N03)3]. Ionic compounds containing the ammonium ion, NH [for example, NH4Br and (NH4)2C03], are exceptions to this rule of thumb. 

T 0 help understand the influence of ammonia on adsorption and coprecipitation, it is useful to first look at the influence of ammonia on the solution chemistry of the metal ions to be removed. The concentration of the ammonia ligand is pH-depen-dent, forming the ammonium ion (NH ) at low pH. It is assumed that the ammonium ion does not, itself, complex with positively charged metal ions and serves mainly to limit the amount of free ammonia available at any given pH. 

In Section 2.5 we learned that ionic compounds are made up of cations (positive ions) and anions (negative ions). With the important exception of the ammonium ion, NH, aU cations of interest to us are derived from metal atoms. Metal cations take their names from the elements. For example. 

There are some examples of polyatomic ions whose behavior mimics that of an ion of a group of elements in the periodic table. This is the case with the ammonium ion, NH, which in some respects behaves like an alkali metal ion. On the one hand, this may be explained by the remarkable similarity between the charge densities of NH, which is 151 C/m, and at 152 C/m. Nevertheless, the chemistry of the ammonium ion more closely resembles that ofRb and Cs+. Among the similarities are reactions with the [Co(N02)anion, which give precipitates in the case of NH +, K, Rb+, and Cs. ... 

There are two categories (1) molecules containing an ionizable hydrogen atom such as nitrous acid (HNO2), and (2) cations such as the ammonium ion (NH/). Weak bases are defined as solutes that react with water molecules to acquire a H+ ion and leave an hydroxide ion (HO ) behind. Once again, all definitions relate to ionization in water. 

H3O+, and the ammonium ion, NH +. An example of a polyatomic anion is the bicarbonate ion, HCOj . It is important that be able to determine which atom or atoms in a neutral molecule or polyatomic ion bear a positive or negative charge. The charge on an atom in a molecule or polyatomic ion is called its formal charge. To derive a formal charge ... 

Most acids are weak adds, which ionize only to a limited extent in water. At equilibrium, aqueous solutions of weak acids contain a mixture of nonionized acid molecules, H30 ions, and the conjugate base. Examples of weak acids are hydrofluoric acid (HF), acetic acid (CH3COOH), and the ammonium ion (NH J). The limited ionization of weak acids is related to the equihbrium constant for ionization, which we will study in the next section. 

The presence of a pair of electrons in an sp orbital directed toward an apex of the trigonal pyramid suggests that the reactivity of ammonia might be governed largely by interactions with electron-poor species, which would thus form a bond with the nonbonding pair [e.g., as in the ammonium ion (NH, )]. 

Y. Yamaguchi and H. F. Schaefer, ID,). Chem. Phys., 73,2310 (1980). A Systematic Theoretical Study of Harmonic Vibrational Frequencies The Ammonium Ion NH and Odier Simple Molecules. 

Most of the common bases are ionic compounds consisting of a positive metal ion and the negatively charged hydroxide ion, OH. Examples include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide, Ca(OH)2. Ammonia (NH3) is a weak base, capable of reacting with H+ to form the ammonium ion, NH. ... 

In the reaction shown in Equation (2), NH3 is the base and HjO is the acid. The ammonium ion, NH, is the conjugate acid of NH3, and OH is the conjugate base of H3O. When NHg reacts with HCl in benzene solution, the HCl (acid) donates a proton to the NH3 (base). The products are Cl , the conjugate base of HCl, and NH4, the conjugate acid of NHg, which form a salt (NH4CI) insoluble in benzene. 

Rather than individual atoms that have lost or gained electrons, many ions are groups of atoms bonded together covalently and having a net charge. A connnon example of such an ion is the ammonium ion, NH, ... 

This section has been discussing monoatomic (one atom) ions. But polyatomic (many atom) ions do exist. The ammonium ion, NH, is a polyatomic ion, or specifically, polyatomic cation. The nitrate ion, NO ", is also a polyatomic ion, or specifically, a. polyatomic anion. 

An example of this is the ammonium ion, NH +, formed when ammonia combines with a hydrogen ion, Hb The hydrogen ion is electron deficient it has space for two electrons in its shell. The nitrogen atom in the ammonia molecule has a lone pair of electrons. The lone pair on the nitrogen atom provides both electrons for the bond (Figure 4.13). 

Since the NQCC depends not only on the nuclear quadrupole moment Q, but also on the chemical environment of the nucleus, quadrupole couplings can vary greatly for the same nucleus. Moreover, the NQCC tensor contains useful chemical information on the disposition of bonding and nonbonding electrons which determine the efg. Thus for N, with 7=1, the NQCC is small (a few kHz) in the symmetrical environment of the ammonium ion NH/, small also in linear groups (as in R — N s C), but larger (about 9 MHz) in very asymmetric locations, as in NHFj. 

Figure 2.2 shows what we would see when we shrink and watch ammonia dissolve in water. After splashdown we see an NH3 molecule accept a proton from a neighbouring water molecule and become the ammonium ion, NH. That ion then wriggles off through the surrounding water molecules and away from the OH left as a result of the proton transfer from H2O. We conclude that because it accepts a proton, NH3 is a base. 

The state of ionization of weak bases also can be described by acidic ionization constants. For example, ammonia is a weak base which can take up hydrogen ions to form ammonium ions. This is, of course, equivalent to thinking of the ammonium ion (NH ) as a weak acid which ionizes in water to give some hydrogen ions (H ) and some molecules of ammonia (NH3). Thus ... 

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