Polyatomic Cations and Anions

Like their sulfur congener, selenium and tellurium can form polyatomic cations and anions in many compounds. 

Throughout this course, we deal not only with molecules but also with polyatomic cations and anions. Examples of polyatomic cations are the hydronium ion. 

When a salt containing polyatomic ions dissolves In water, the cations separate from the anions, but each polyatomic ion remains intact. An example Is ammonium nitrate, composed of NH4 polyatomic cations and NO3 polyatomic anions. Ammonium nitrate dissolves In water to give a solution containing NH4 cations and NO3 anions, as Figure 3-21 Illustrates. 

Ionic compounds consist of positive ions (cations) and negative ions (anions) hence, ionic compounds often consist of a metal and nonmetal. The electrostatic attraction between a cation and anion results in an ionic bond that results in compound formation. Binary ionic compounds form from two elements. Sodium chloride (NaCl) and sodium fluoride (NaF) are examples of binary ionic compounds. Three elements can form ternary ionic compounds. Ternary compounds result when polyatomic ions such as carbonate (C032 ), hydroxide (OH-), ammonium (NH4+), form compounds. For example, a calcium ion, Ca2+, combines with the carbonate ion to form the ternary ionic compound calcium carbonate, CaC03. Molecular compounds form discrete molecular units and often consist of a combination of two nonmetals. Compounds such as water (H20), carbon dioxide (C02), and nitric oxide (NO) represent simple binary molecular compounds. Ternary molecular compounds contain three elements. Glucose ( 12 ) is a ternary molecular compound. There are several distinct differences between ionic and molecular compounds, as summarized in Table 1.2. 

When polyatomic cations and/or anions are involved, enclosing marks should be used to avoid possible ambiguity. Thus Tl lj is thallium (triiodide) and Tl11113 is thallium tn-ioJide. Alternatively. thalliumU) triiodide and thalliumfllh triiudide would suffice. 

Polyatomic ions (as opposed to neutral molecules) may also be unstable with respect to decomposition, polymerisation or disproportionation. However, ions cannot be scrutinised in isolation. In a crystalline solid, there are always counter-ions of opposite charge to be considered, and in solution an ion is surrounded by solvent molecules. The intimacy of the chemical environment of any ion must influence its viability. For example, redox reactions involving electron transfer between cation and anion, or between ion and solvent, may find easy kinetic pathways. We look here at some examples of unstable oxoanions. 

In addition to the common monatomic halide ions, numerous polyatomic species, both cationic and anionic, have been prepared. Many readers will be familiar with the brown triiodide ion, l3, formed from I2 and I ... 

Naming polyatomic ions is not as easy as naming simple cations and anions. Even so, there are rules you can follow to help you remember how to name some of them. 

When a compound has polyatomic ions, such as those in Figure 7, look for the cations and anions. Formulas can tell you which elements make up polyatomic ions. For example, in the formula KNO3, NO3 is a nitrate ion, NOi. KNO3 does not have a KN" and an O3 ion. Similarly, the formula of ammonium nitrate is written NH4NO3, because NH4 in a formula stands for the ammonium ion, NHj, and NO3 stands for a nitrate ion, NO3. If it were written as H4N2O3, the number of atoms would be correct. However, the formula would no longer clearly show which ions were in the substance and how many there were. The formula NH4NO3 shows that ammonium nitrate is made up of ammonium and nitrate ions in a 1 1 ratio. 

All the physico-chemical properties strictly dependent upon the precise nature of the cation and anion constituting the IL and they can be changed or modulated by changing the anion or cation or modifying the nature of substituents on cation. Structurally, most of the ILs that have been investigated to date are based on imidazolium, ammonium and pyridinium cations, bearing alkyl chains, associated with polyatomic anions such as chloroaluminates, tetrafluoroborate, hexafluorophosphate and bis-triflimide. In Scheme 1 are reported the above mentioned cations and anions whose combination gives the most commonly employed ionic liquids. 

In summary, the empirical approach to ionic solvation based on the MSA is quite successful for monoatomic ions of the main group elements. It helps one to understand the important differences between the way cations and anions are solvated in water. It can also be applied to other ions, including polyatomic ions, provided the solvation is essentially electrostatic in character. Thus, one may estimate effective radii for anions such as nitrate and perchlorate from the Gibbs solvation energy using the value of 8s calculated for the halide ions. Considering the simplicity of the model, it provides an useful means of understanding the thermodynamics of solvation. 

Since the time of Berzelius, there has been a persistent argument used by inorganic chemists that, in many cases, valency as a positive integer should be replaced by negative and positive oxidation numbers adding up to zero in a molecule or solid, and to the ionic charge of a polyatomic cation or anion [9]. The same year, 1916, as the Lewis paradigm was formulated [13,14], emphasiz-... 

Name one ionic compound that contains a polyatomic cation and a polyatomic anion (see Table 2.3). 

Strontium is an alkahne earth metal. It only forms a +2 cation. The polyatomic ion chlorite, CIO2, has a -1 charge. Since the charges on the cation and anion are numerically different, the subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation. The correct formula is Sr(C102)2... 

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