POLYATOMIC IONS Polyatomic ions are made up of two or more atoms that are stuck together and will often travel as a group during a chemical reaction. They are created in such a way that the atoms that make up the polyatomic ion are in need of one, two or three more electrons in order to satisfy the desire of each atom in the polyatomic ion to reach a stable state. The number of electrons that a polyatomic ion group needs to ensure all of its atoms reach a stable state is shown as a negative symbol at the upper right of the compound formula of the ion (it is the opposite in the case of the ammonium polyatomic ion which has one extra electron to give). A list of compound formulas for the common polyatomic ions is shown in the table below: Table of Common Polyatomic Ions Ion Name Ion Name + 2– NH4 ammonium CO3 carbonate – 2– NO2 nitrite SO3 sulfite – 2– NO3 nitrate SO4 sulfate – – ClO hypochlorite ClO2 chlorite – – ClO3 chlorate ClO4 perchlorate – – OH hydroxide C2H3O2 acetate – – CN cyanide MnO4 permanganate 3– 2– PO4 phosphate Cr2O7 dichromate 2– 2– HPO4 hydrogen phosphate CrO4 chromate – 2– H2PO4 dihydrogenphosphate O2 peroxide – – HCO3 hydrogen carbonate HSO4 hydrogen sulfate (bicarbonate is a widely (bisulfate is a widely used common name) used common name) In the table above, the polyatomic ion group known as cyanide (CN–) has a single negative sign in the upper right corner of the compound formula. This indicates that the cyanide polyatomic ion is looking for one more electron to ensure that both the carbon and the nitrogen atoms that make up the polyatomic ion will reach a stable state. In the case of carbonate (CO32–), this polyatomic ion group needs two more electrons to ensure that the carbon and the three oxygen atoms reach a stable state. COMBINING AND NAMING POLYATOMIC IONS Combining and naming ionic compounds that contain polyatomic ions is very similar to naming binary ionic compounds (which you have mastered by now). The most important thing to remember is that the polyatomic ion stays intact and that the charge in the upper right corner of the polyatomic ion describes how many electrons the ion needs to obtain in order to reach a stable state for each of its atoms; it also describes what charge the polyatomic ion would take if the ionic compound containing the polyatomic ion were to dissociate. For example, the polyatomic ion sulfate (SO42-), needs two electrons to ensure that each atom that makes up the polyatomic ion will reach a stable state. If it obtains these electrons from a magnesium atom, then the resulting ionic compound is as follows: • 2– Mg • + SO4 MgSO4 Magnesium sulfate If the magnesium sulfate were to dissociate for any reason, the following would be the results: 2+ 2– MgSO4 Mg + SO4 When naming polyatomic ionic compounds, use the entire name of both the polyatomic ion and whatever donated the electron(s). When combining polyatomic ions with transitional metals, it is important to remember that transitional metals can form multiple cations (positive ions). To illustrate this concept, the transition metal iron (Fe), can form two types of cations (Fe2+ and Fe3+). When a transition metal is present that forms more than one cation, a Roman numeral is required to specify the cation charge used. In this example, the compound FeSO4 is called iron II sulfate, because it contains the Fe2+ cation and not the Fe3+ cation. In order to do this, you must understand that the sulfate ion has a charge of 2–. The following are general rules for naming polyatomic compounds: If you look at the table of polyatomic ions above, you will notice that several series of polyatomic anions exist that contain an atom of a given element and different numbers of oxygen atoms. These anions are called oxyanions. When there are two members in such a series, the name of the one with the smaller number of oxygen atoms ends in –ite and the name of the one with the larger number ends in –ate. When more than two oxyanions make up a series, hypo– (less than) and per– (more than) are used as prefixes to name the members of the series with the fewest and the most oxygen atoms, respectively. The best example involves the oxyanions containing chlorine: ClO– hypochlorite, ClO2– chlorite ClO3– chlorate, and ClO4– perchlorate The following are general rules for naming polyatomic compounds: • Rule 1. The cation is written first in the name; the anion (takes electrons) is written second in the name. • Rule 2. When the formula unit contains two or more of the same polyatomic ion, that ion is written in parentheses with the subscript written outside the parentheses. Note: parentheses and a subscript are not used unless more than one of a polyatomic ion is present in the formula unit (e.g., the formula unit for calcium sulfate is "CaSO4" not "Ca(SO4)"). •Rule 3. If the cation is a metal ion with a fixed charge, the name of the cation is the same as the (neutral) element from which it is derived (e.g., Na+ = "sodium"). If the cation is a metal ion with a variable charge, the charge on the cation is indicated using a Roman numeral, in parentheses, immediately following the name of the cation (e.g., Fe3+ = "iron III"). POLYATOMIC ION PRACTICE 1. Refer to the list of polyatomic ions and write the ionic formula for the following ionic compounds containing polyatomic ions: a. Potassium hydroxide ____________________ b. Ammonium chloride ____________________ c. Strontium sulfate ____________________ d. Sodium perchlorate ____________________ e. Aluminum hydroxide ____________________ f. Barium acetate ____________________ g. Silver sulfite ____________________ h. Calcium phosphate ____________________ i. Beryllium dichromate ____________________ j. Cesium sulfite ____________________ k. Lithium chromate ____________________ l. Potassium acetate ____________________ m. Barium hydroxide ____________________ n. Iron II phosphate ____________________ 2. Name the following ionic compounds containing polyatomic ions: a. Na2SO4 __________________________________________ b. Ca(OH)2 __________________________________________ c. Na3PO4 __________________________________________ d. Ni(SO4)2 __________________________________________ e. Cd(ClO4)2 __________________________________________ f. KClO3 __________________________________________ g. Cu(NO2)2 __________________________________________ h. Na2SO3 __________________________________________ i. Na2CO3 __________________________________________ j. AgClO4 __________________________________________ k. KClO __________________________________________ l. BaSO4 __________________________________________ m. NH4HCO3 __________________________________________ n. Zn(NO3)2 __________________________________________ o. Al(MnO4)3 __________________________________________ p. Li2Cr2O7 __________________________________________ q. H2O2 __________________________________________ r. NH4HSO4 __________________________________________ 3. The ionic formula for the compound magnesium dihydrogenphosphate is Mg(H2PO4)2. For this molecule, what is the anion (what takes electrons) and the cation (what gives the electrons)? .