Ionic compounds oxidation numbers

The oxidation number, or oxidation state, is the formal charge on an atom calculated on the basis that it is in a wholly ionic compound. Oxidation numbers are assigned according to several rules. 

What is an oxidation number How is it used to identify redox reactions Explain why, except for ionic compounds, oxidation number does not have any physical significance. 

The oxidation number of an element in a monatomic ion is equal to the charge of that ion. In the ionic compound NaCl, sodium has an oxidation number of +1, chlorine an oxidation number of — 1. The oxidation numbers of aluminum and oxygen in A1203 (Al3+, O2- ions) are +3 and —2, respectively. 

An ionic compound is named with the cation name first, followed by the name of the anion the word ion is omitted in each case. The oxidation number of the cation is given if more than one charge is possible. However, if the cation comes from an element that exists in only one charge state (as listed in Fig. C.6), then the oxidation number is omitted. Typical names include potassium chloride (KC1), a compound containing K+ and Cl" ions and ammonium nitrate (NH4NOs), which contains NH4+ and NO3" ions. The cobalt chloride that contains Co2+ ions (CoCl,) is called cobalt(II) chloride C0CI3 contains Co3+ ions and is called cobalt(III) chloride. 

Ionic compounds are named by starting with the name of the cation (with its oxidation number if more than one charge is possible), followed by the name of the anion hydrates are named by adding the word hydrate, preceded by a Greek prefix indicating the number of water molecules in the formula unit. 

To determine oxidation numbers, we assign each valence electron to a specific atom in a compound. This means that the oxidation number of an atom is the charge it would have if the compound were composed of ions. Ionic FeO, for example, would contain Fe cations and 0 anions. Thus, in FeO we assign iron an oxidation number of +2 and oxygen an oxidation number of-2. 

The bonding electrons in CO and CO2 are polarized in the direction of the O atoms, as shown by the electronegativities of C (2.5) and O (3.5). Thus, if these substances were ionic, CO would consist of C cations and O anions, and CO2 would contain cations and O anions. Accordingly, we assign O an oxidation number of-2 in both these compounds, whereas C has an oxidation number of +2 in CO and +4 in CO2. ... 

In an ionic compound, each ion is an individual chemical species with its own set of oxidation numbers. For example, we treat ammonium nitrate, NH4 NO3, as NHq cations and NO3 anions. 

FeC03 This ionic compound contains the carbonate polyatomic anion, C O3. The -2 charge on carbonate requires that iron have +2 oxidation number (Guideline 2). In the carbonate anion, each oxygen atom is -2, for a total of-6. The oxidation numbers must add up to the net charge on the anion (-2), so the carbon atom must have an oxidation number of +4. 

Although naturally occurring compounds of transition metals are restricted in scope, a wide variety of compounds can be synthesized in the laboratory. Representative compounds appear in Table 20-2. These compounds fall into three general categories There are many binary halides and oxides in a range of oxidation numbers. Ionic compounds containing transition metal cations and polyatomic oxoanions also are common these include nitrates, carbonates, sulfates, phosphates, and perchlorates. Finally, there are numerous ionic compounds in which the transition metal is part of an oxoanion. 

According to Table 5, the electronegativity of tin changes with its oxidation number, the lower oxidation state being connected with a more electropositive character. It may be concluded that tin(II) compounds are more ionic than the... 

We learned to write formulas of ionic compounds in Chaps. 5 and 6. We balanced the charges to determine the number of each ion to use in the formula. We could not do the same thing for atoms of elements in covalent compounds, because in these compounds the atoms do not have charges. In order to overcome this difficulty, we define oxidation numbers, also called oxidation states. 

Since O is to the right of C in the second period of the periodic table, O is more electronegative, and we assign control of all eight shared electrons to the two O atoms. (It does not really have complete control of the electrons if it did, the compound would be ionic.) Thus, the oxidation number of each atom is calculated as follows ... 

Stable binary ionic compounds are formed from ions that have noble gas configurations. None of the compounds meet this requirement. First of all, C04 is not an ionic compound at all because it is a covalent compound, made from 2 nonmetals. Even so, C04 is not stable because with O2, C would have an oxidation number of +8, which is very unlikely. Consider the following ionic compounds composed of a metal and... 

Unfortunately, many compounds contain bonds that are a mixture of ionic and covalent. In such a case, a formal charge as written is unlikely to represent the actual number of charges gained or lost. For example, the complex ferrocyanide anion [Fe(CN)6]4- is prepared from aqueous Fe2+, but the central iron atom in the complex definitely does not bear a +2 charge (in fact, the charge is likely to be nearer +1.5). Therefore, we employ the concept of oxidation number. Oxidation numbers are cited with Roman numbers, so the oxidation number of the iron atom in the ferrocyanide complex is +11. The IUPAC name for the complex requires the oxidation number we call it hexacyanoferrate (II). 

When finding the oxidation numbers of elements in ionic compounds, you can work with the ions separately. For example, Na2Cr207 contains two Na" ions, and so sodium has an oxidation number of+1. The oxidation numbers of Cr and 0 can then be calculated as shown in part (c) of the Sample Problem. 

Determine the oxidation number of each element in each of the following ionic compounds by considering the ions separately. 

The oxides of low-valency metals (i.e., with cations in oxidation number < -i-4) are typically ionic compounds [76]. They are most frequently easily obtained in crystalline forms. In ionic metal oxides the coordination of the cations (four to eight) is generally higher than their valency (one to four) and this also occurs for the coordination of 0 oxide ions (three to six). The bulk basic nature of the ionic metal oxides is associated with the strong polarization of the metal-oxygen bond, to its tendency to be dissociated by water and to the basic nature of the products of their reaction with water (i.e., the metal hydroxides) [67]. 

KCl This is an ionic compound made up of K and Cl" ions, with respective oxidation numbers of + 1 and -1 by rule 2. 

These terms are frequently used and may lead to confusion, if used in the wrong context. As a reminder, the valency of an atom is strictly the number of bonds (including o, n and 8) in which it participates in any particular compound. In IF , the iodine atom participates in seven a bonds to the fluorine atoms. The fluorine atoms are individually monovalent. Oxidation states are more useful than valency in describing ionic compounds. In the crystalline solid CaF0, the calcium is best thought of as composed of calcium dications and fluoride anions, Ca2+(F ),. The calcium is in its +2 oxidation state, having lost its valency electrons, and the fluorine atoms are in the -1 oxidation state, both having accepted an elec-... 

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