Binary ionic compound oxidation number

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. 

In ionic binary compounds, the oxidation number is the charge per atom. 

In ionic binary compounds, the oxidation numbers are the charges per ion. [For example, CdCl2 is an ionic compound, as indicated more clearly by Cd +(C1 )2. Thus, the oxidation number of the cadmium ion is +2, and the oxidation number of each of the two chloride ions is -1.] The algebraic sum of the oxidation numbers of the atoms in an ion is equal to the charge on the ion (e.g., zero charge for CdCl2). 

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... 

The nature of the binary hydride is related to the characteristics of the parent element (Fig. 14.8). Strongly electropositive metallic elements form ionic compounds with hydrogen in which the latter is present as a hydride ion, H, and has oxidation number —1. These ionic compounds are called saline hydrides (or saltlike hydrides ). They are formed by all members of the s block with the exception of beryllium and are made by heating the metal in hydrogen ... 

The oxidation number, or oxidation state, of an element in a simple binary ionic compound is the number of electrons gained or lost by an atom of that element when it forms the compound. In the case of a single-atom ion, it corresponds to the actual charge... 

The preceding method is sufficient for naming binary ionic compounds containing metals that exhibit only one oxidation number other than zero (Section 4-4). Most transition metals and the metals of Groups IIIA (except Al), IVA, and VA, exhibit more than one oxidation number. These metals may form two or more binary compounds with the same nonmetal. Ta distinguish among all the possibilities, the oxidation number of the metal is indicated by a Roman numeral in parentheses following its name. This method can be applied to any binary compound of a metal and a nonmetal. 

The concept of oxidation states (sometimes called oxidation numbers) lets us keep track of electrons in oxidation-reduction reactions by assigning charges to the various atoms in a compound. Sometimes these charges are quite apparent. For example, in a binary ionic compound the ions have easily identified charges in sodium chloride, sodium is +1 and chlorine is -1 in magnesium oxide, magnesium is +2 and oxygen is -2 and so on. 

Formulas for binary ionic compounds In the chemical formula for any ionic compound, the symbol of the cation is always written first, followed by the symbol of the anion. Subscripts, which are small numbers to the lower right of a symbol, represent the number of ions of each element in an ionic compound. If no subscript is written, it is assumed to be one. You can use oxidation numbers to write formulas for ionic compounds. Recall that ionic compounds have no charge. If you add the oxidation number of each ion multiplied by the number of these ions in a formula unit, the total must be zero. 

Many ionic compounds, such as NaCl, KBr, Znlj, and AI2O3, are binary compounds, that is, compounds formed from just two elements. For binary compounds, the first element named is the metal cation, followed by the nonmetallic anion. Thus, NaCl is sodium chloride. We name the anion by taking the first part of the element name (the chlor- of chlorine) and adding -ide. The names of KBr, Znh, and AI2O3 are potassium bromide, zinc iodide, and aluminum oxide, respectively. The -ide ending is also used in the names of some simple polyatomic anions, such as hydroxide (OH ) and cyanide (CN ). Thus, the compounds LiOH and KCN are named Uthium hydroxide and potassium cyanide, respectively. These and a number of other such ionic substances are ternary compounds, meaning compounds consisting of three elements. Table 0.3 lists the names of some common cations and anions. 

Each element in a binary ionic compound has a full charge because the atom transferred its electron(s), and so the atom s oxidation number equals the ionic charge. But, each element in a covalent compound (or in a polyatomic ion) has a partial charge because the electrons shifted away from one atom and toward the other. For these cases, we determine oxidation number by a set of rules (Table 4.3, next page you ll learn the atomic basis of the rules in Chapters 8 and 9). 

CHECK FOR UNDERSTANDING Describe In your own words, describe how to determine the subscripts of a binary ionic compound by using the oxidation numbers of the ions. 

Unlike ionic compounds, molecular compounds are composed of individual covalently bonded units, or molecules. Chemists use two nomenclature systems to name binary molecules. The newer system is the Stock system for naming molecular compounds, which requires an understanding of oxidation numbers. This system will be discussed in Section 2. 

As shown in Figure 2.1, many nonmetals can have more than one oxidation number. (A more extensive list of oxidation numbers is given in Appendix Table B-15.) These numbers can sometimes be used in the same manner as ionic charges to determine formulas. Suppose, for example, you want to know the formula of a binary compound formed between sulfur and oxygen. From the common +4 and +6 oxidation states of sulfur, you could expect that sulfiu might form SO2 or SO3. Both are known compounds. Of course, a formula must represent facts. Oxidation numbers alone cannot be used to prove the existence of a compound. 

In Section 1, we introduced the use of Roman numerals to denote ionic charges in the Stock system of naming ionic compounds. The Stock system is actually based on oxidation numbers, and it can be used as an alternative to the prefix system for naming binary molecular compounds. In the prefix system, for example, SO2 and SO3 are named sulfur dioxide and sulfur trioxide, respectively. Their names according to the Stock system are sulfur(IV) oxide and sulfur(VI) oxide. The international body that governs nomenclature has endorsed the Stock system, which is more practical for complicated compounds. Prefix-based names and Stock-system names are still used interchangeably for many simple compounds, however. 

An important aspect of empirical potential parameterization is the question of transferability. Are, for example, models derived in the study of binary oxides, transferable to ternary oxides Considerable attention has been paid to this problem by Cormack et al., who have examined the use of potentials in spinel oxides, for example, MgAl204, NiCr204, and so on in addition Parker and Price have made a very careful study of silicates especially Mg2Si04. These studies conclude that transferability works well in many cases. However, systematic modifications are needed when potentials are transferred to compounds with different coordination numbers. For example, the correct modeling of MgAl204 requires that the potential developed for MgO, in which the magnesium has octahedral coordination, be modified in view of the tetrahedral coordination of Mg in the ternary oxide. The correction factor is based on the difference Ar between the effective ionic radii for the different coordination numbers. If an exponential, Bom-Mayer, repulsive term is used, the preexponential factor is modified as follows ... 

As indicated above, the covalent radius of an element depends on its oxidation state. In a binary ionic compound, MX, containing the positive ion, M. and the negative ion. X, the minimum distance between them is measurable with considerable accuracy by the method of X-ray diffraction. The problem is to divide such a distance into the ionic radii for the individual ions. That ions behave like hard spheres with a constant radius whatever their environment might be is an approximation to the real situation. In compounds which do not exhibit much covalency the approximation is reasonable, and led Shannon and Prewitt to assign radii to O- and F of 140 and 133 pm respectively after their study of many oxides and fluorides. Ionic radii are not assignable to every element, and the generalizations described apply only to those elements which do form ions in compounds, and are subject to their oxidation states (discussed in Chapter 5) and coordination numbers i.e. the number of nearest neighbours they have in the ionic compound). 

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