Polyatomic ion oxidation numbers

Hydrogen in a molecule. Hydrogen in a polyatomic ion, oxidation number +1 oxidation number +1... 

In a sense, each atom s oxidation number makes a contribution to the overall charge on the species. Note that the oxidation numbers in both HF [(+1) -h (-1) = 0] and NH3 [(—3) + 3(+1) = 0] sum to zero. Because compounds are electrically neutral, the oxidation numbers in any compound will sum to zero. For a polyatomic ion, oxidation numbers must sum to the chaige on the ion. (The oxidation number of a monatomic ion is equal to its chaige.)... 

In molecular compounds and polyatomic ions, oxidation numbers are assigned using a set of rules. 

The concept of oxidation number is used to simplify the electron bookkeeping in redox reactions. For a monatomic ion (e.g., Na+, S2 ), the oxidation number is, quite simply, the charge of the ion (+1, —2). In a molecule or polyatomic ion, the oxidation number of an element is a pseudo-charge obtained in a rather arbitrary way, assigning bonding electrons to the atom with the greater attraction for electrons. 

The sum of the oxidation numbers in a neutral species is 0 in a polyatomic ion, it is equal to the charge of that ion. The application of this very useful principle is illustrated in Example 4.8. 

Although most of the molecules and polyatomic ions referred to in general chemistry follow the octet rule, there are some familiar species that do not. Among these are molecules containing an odd number of valence electrons. Nitric oxide, NO, and nitrogen dioxide, N02, fall in this category ... 

When an element is part of a compound or a polyatomic ion, we assign its oxidation number by using the procedure in Toolbox K.l. 

The formula above represents the polyatomic permanganate ion. Which of these is the proper oxidation number to assign to manganese ... 

The sum of the oxidation numbers in a polyatomic ion equals the oxidation number of the ions incorporated e.g. consider Mn04 ion. Overall, its oxidation number is —I (because the ion s charge is —1). Each oxide contributes —II to this sum, so the oxidation number of the central manganese must be +V1I. 

Oxidation numbers are just a bookkeeping method used to keep track of electron transfers. In a covalent molecule or a polyatomic ion, the oxidation number of each element does not represent an ionic charge, because the elements are not present as ions. However, to assign oxidation numbers to the elements in a covalent molecule or polyatomic ion, you can pretendthe bonds are ionic. 

You have seen examples of how Lewis structures can be used to assign oxidation numbers for polar molecules such as water, non-polar molecules such as chlorine, and polar polyatomic ions such as the cyanide ion. 

Use Lewis structures to assign an oxidation number to each element in the following polyatomic ions. 

For each polyatomic ion in question 2 of the procedure, find the sum of the oxidation numbers of all the atoms present. Describe and explain any pattern you see. 

Drawing Lewis structures to assign oxidation numbers can be a very time-consuming process for large molecules or large polyatomic ions. Instead, the results from Lewis structures have been summarized to produce a more convenient set of rules, which can be applied more quickly. Table 10.1 summarizes the rules used to assign oxidation numbers. You may have discovered some of these rules for yourself in the ThoughtLab you just completed. 

Identify a polyatomic ion in which chlorine has an oxidation number of -i-3. 

An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have if it were a monatomic ion. In the hydroxide ion (OH-), for example, the oxygen atom has an oxidation number of -2, as if it were a monatomic O2- ion, and the hydrogen atom has an oxidation number of +1, as if it were H+. 

In neutral atoms of the first transition series, the 4s orbital is usually filled with 2 electrons, and the remaining electrons occupy the 3d orbitals. In transition metal ions, all the valence electrons occupy the d orbitals. For polyatomic ions, first determine the oxidation number of the transition metal, and then assign the valence electrons to the d orbitals as you would if the metal were a simple ion. 

The first reaction is much faster than the second. As Mn04 ion is common in both the reactions, the difference clearly lies in the nature of ferrous and oxalate ions. Fe2+ ion is a simple ion, whereas C2042- ion is a polyatomic ion and contains a number of covalent bonds which have to be broken in the oxidation reaction. 

The oxidation numbers in a compound must add up to zero, and the sum of oxidation numbers in a polyatomic ion must equal the overall charge of the ion. 

The charge on a polyatomic ion is equal to the sum of the oxidation numbers for the species present in the ion. For example, the sulfate ion, SO42, has a total charge of -2. This comes from adding the -2 oxidation number for 4 oxygen (total -8) and the +6 oxidation number for sulfur. 

There will be times when you will need to determine the oxidation number of an element in a polyatomic ion, for example, Mn in Mn04 1. The only change in strategy is that the sum of the oxidation numbers must equal the charge of the polyatomic ion rather than the zero used previously. 

An oxoacid is an acid formed from a polyatomic ion that contains oxygen, hydrogen, and another element. (Oxoacids are called oxyacids in some chemistry textbooks). In Chapter 3, you learned the names of common polyatomic ions and their valences (oxidation numbers). The names of oxoacids are similar to the names of their polyatomic oxoanions. Only the suffix is different. Study the three rules and examples for naming oxoacids below. Then try the Practice Problems that follow. 

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