The Polyatomic Ions

Up to this point, only ions derived from single atoms have been used since the focus has been on binary ionic compounds. But there are many ions, almost all of them anions, that contain two or more atoms, and they are the polyatomic ions. Polyatomic ions contain two or more atoms chemically bound into a single unit that bears an electrical charge. The most common polyatomic ions are listed in the following table. 

Single Charged Ions Double Charged Ions Triple Charged Ions 

The number of oxygen atoms in an oxyion of the same element has an affect on the name of the ion. The most common oxyion of a given element ends in -ate. One less oxygen than the most common ion ends in -ite. 

N03 nitrate S042 sulfate C103 chlorate (the most common oxyions of N, S, and Cl) 

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Table 2.2 lists some of the polyatomic ions that you will need to know, along with their names... 

Consider the polyatomic ion I065-. How many pairs of electrons are around the central iodine atom What is its hybridization Describe foe geometry of foe ion. 

Step 2 If the compound is an oxoacid, derive the name of the acid from the name of the polyatomic ion that it produces, as in Toolbox D.1. In general,... 

One form of the polyatomic ion I, has an unusual V-shaped structure one I atom lies at the point of the V, with a linear chain of two I atoms extending on each side. The bond angles are 88° at the central atom and 180° at the two atoms in the side chains. Draw a Lewis structure for L that explains its shape and indicate the hybridization you would assign to each nonterminal atom. 

Because aqueous solutions containing ions play a central role in the world around us, the chemistry of such solutions is discussed in depth in several chapters of this book. To understand the chemishy of aqueous solutions containing ions, it is essential that you leant to recognize the common ions at a glance. This is especially true of the polyatomic ions. 

In the polyatomic ion the formation of a coordinate covalent bond between nitrogen and hydrogen involves —... 

Many bases contain the polyatomic ion hydroxide (OH ). To name a base containing this ion, name the metal first, then list the... 

Not all bases contain hydroxide, however. For example, Na2C03 is a base, but its name is sodium carbonate. Baking soda, which has a chemical formula of NaHC03, is also a base. The scientific name for baking soda is sodium hydrogen carbonate (hydrogen carbonate is the name of the polyatomic ion). 

The presence of a polyatomic ion in a compound requires substituting the name of the polyatomic ion for the name of the cation or anion in a comparable mctal/nonmetal compound. We will use no prefixes. 

Ionic bonding may also involve polyatomic ions. The polyatomic ion(s) simply replace(s) one or both of the monoatomic ions. 

Keep in mind that the need for an expanded valence level for the central atom may not always be as obvious as in the previous Sample Problem. For example, what if you were asked to predict the molecular shape of the polyatomic ion, BrCU" Drawing the Lewis structure enables you to determine that the central atom has an expanded valence level. 

As discussed in Section VI, the benzene ion is the polyatomic ion that has received the most careful attention regarding the extrapolation of rate-energy data to find the true thermochemical threshold [Equation (11)]. The TRPD results over... 

Lithium nitrate. Because lithium is an alkali metal and the charge of alkali metals in Group I is always -i-l, you don t need to use Roman numerals to indicate the charge of lithium. You simply write lithium and then the name of the polyatomic ion, which is nitrate. You don t change the ending of the polyatomic ion ncime. 

If the polyatomic ion name ends in -ate, change the ending to -ic if the polyatomic ion name ends in -ite, change the ending to -ous. 

Pb(S04)2 The cation lead is a metal with a variable charge, as indicated by the Roman numeral IV in parentheses. This classification means that lead has a charge of +A. SO is the polyatomic ion sulfate with a charge of -2. To balance out the charges, you need two sulfate ions for each lead ion. Thus, the formula has one Pb ion with a total charge of -1-4 and two sulfate ions with a total charge of -A. To indicate the need for two polyatomic ions, you put pcirentheses around the sulfate ion and write the 2 as a subscript outside the parentheses. 

Look up (or recall) the polyatomic ions in each compound, and specify the charge of the cation if it s a metal that can take on different charges. 

First, look up (or better, recall from memory) the charge of the polyatomic ion or ions, and then use subscripts as necessary to balance chcirges. 

Identify the anion for each oxy-acid from the polyatomic ion chart in Table 6-1. If the polyatomic ion ends in -ate, change the ending to -ic. If the polyatomic ion ends in -/Ye, change the ending to -ous. Write acid at the end of the name. (Don t begin with hydro- These aren t binary acids.)... 

Sorry, it s true. Not only are polyatomic ions annoying because you have to memorize them, but they pop up everywhere. If you don t memorize the polyatomic ions, you ll waste time trying to figure out weird (and incorrect) covalent bonding cirrangements when what you re really dealing with is a straightforwcird ionic compound. Here are the common polyatomic ions in Table 22-1 (see Chapter 6 for more information on these ions). 

In ICP-MS a multi-element tuning solution is applied for the mass calibration of mass spectra. Figure 6.3 shows the mass spectrum for phosphorus determination at m/z = 31. In this case, the mass calibration was performed with the aid of a solution of a phos-phorus/sulphur mixture. From the known masses of the isotopes of the atomic ions, the masses of the polyatomic ions occurring were determined and identified in accordance with the isobaric polyatomic ions (15N160+ and 14N16OH+). Mass accuracy is the deviation of experimental determined mass of an atomic, polyatomic, cluster, molecular or fragment ion from the exact mass of species expected. Polyatomic, cluster and molecular ions were calculated from the atomic masses (see Appendix I), the mass accuracy is usually presented in ppm. 

Phosphorus pentachloride is an ionic solid consisting of PC14+ cations and PC16 anions, and it sublimes at 160°C to a gas of PCI, molecules. The Lewis structures of the polyatomic ions and the molecule are shown in (33). Although the cation is a polyatomic ion in which the P atom does not need to expand its valence shell, in the anion the P atom has expanded its valence shell to 12 electrons, by making use of two of its 3d-orbitals. In PC15, the P atom has expanded its valence shell to 10 electrons by using one 3d-orbital. 

Ionic compounds containing polyatomic ions (Section 2.8) are named in the same way as binary ionic compounds First the cation is identified and then the anion. For example, Ba(N03)2 is called barium nitrate because Ba2+ is the cation and the N03 polyatomic anion has the name nitrate. Unfortunately, there is no systematic way of naming the polyatomic ions themselves, so it s necessary to memorize the names, formulas, and charges of the most common ones listed in Table 2.3. The ammonium ion (NH4+) is the only cation on the list all the others are anions. 

Find the names of N02 and NO3- and the names of S032- and SO42- in the Polyatomic Ions activity (eChapter 2.8). Based on these names, predict the names of the ions C103 and CIO4-. Use the activity to check your names. 

The table below lists some common spectral interferences that are encountered in inductively coupled plasma mass spectrometry (ICP-MS), as well as the resolution that is necessary to analyze them.1 The resolution is presented as a dimensionless ratio. As an example, the relative molecular mass (RMM) of the polyatomic ion 15N160+would be 15.000108 + 15.994915 = 30.995023. This would interfere with 31P at a mass of 30.973762. The required resolution would be RMM/8RMM, or 30.973762/0.021261 = 1457. One should bear in mind that as resolution increases, the sensitivity decreases with subsequent effects on the price of the instrument. Note that small differences exist in the published exact masses of isotopes, but for the calculation of the required resolution, these differences are trivial. Moreover, recent instrumentation has provided rapid, high-resolution mass spectra with an uncertainty of less than 0.01%. 

The rules for naming chemical compounds depend on whether the substance is a molecular substance or an ionic substance. Molecular compounds are comprised only of nonmetals. Ionic compounds are almost always comprised of a metal and a nonmetal. If a compound contains one of the polyatomic ions listed in Table 2.1, it is an ionic compound. 

When you write the formula for a compound involving a polyatomic ion, you go about it in the same general way as for a binary compound. There are two differences. The name of the polyatomic ion is used, and the polyatomic ion needs to have parentheses around it. Study the following examples of polyatomic ions to see how this works. 

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. 

Unfortunately, the severity of the molecular ion overlaps can be more difficult to predict because the molecular ion signal magnitude can be very dependent on the sample composition (including solvent and acid), the extent of solvent loading, and instrument parameters. Drift in the polyatomic ion signals is often more severe than for elemental ions. 

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