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Ionic Charges Worksheet Ionic compounds are electrically neutral compounds. This means that the number of electrons lost by the positive ions in the compound must always equal the number of electrons gained by the negative ions in the compound. Total positive charge in an ionic compound = Total negative charge The number of electrons and element will lose of gain in the process of forming an ionic compound is known as the element’s oxidation number. Oxidation numbers for the individual elements can be found on the periodic table. Figure 1: The periodic blocks for the metallic elements sodium, magnesium, chromium, and manganese. Note that the oxidation numbers are circled. Notice in Figure 1, that some metals have more than one oxidation number. Chromium may lose 6, 3, or two electrons to nonmetals when forming an ionic compound and manganese may lose 7, 6, 4, 2 and 3. Generally the most common oxidation number is bolded. Thus chromium will most often lose three electrons and manganese will most often lose two. Metals with more than one oxidation number are called multivalent metals. Multivalent metals will always have a Roman numeral associated with their name; that will be discussed in more detail later. Figure 2: The periodic table blocks for the nonmetallic elements nitrogen, oxygen, fluorine, and neon. Note that their oxidation numbers are predominantly negative values. Nonmetals, Figure 2, have negative oxidation numbers. When they form ionic compounds they lose electrons. Nitrogen does have some positive oxidation numbers (used when it combines with other nonmetals to form covalent compounds) but when it forms an ionic compound its oxidation number is -‐3. Oxygen is a -‐2, fluorine is a -‐1, and the noble gas Ne does not lose or gain electrons so its oxidation number is zero. Additionally, when a nonmetal has a negative oxidation number its name ending is changed to –ide. It is easy to determine an elements oxidation number(s) using the periodic table. It is written on the periodic table. For example: Sodium ion = +1 Calcium ion = +2 Record the oxidation numbers for the following ions: Potassium ion _____________ Aluminum ion _________________ Iodide ion ____________ Sulfide ion _____________ Barium ion _____________ Nitride ion _____________ But how do we know what the oxidation number of a multivalent metal is? If you have to guess, pick the bolded value. But there is a better way. Multivalent metals always include a roman numeral in their name when they are part of an ionic compound, for example, Chromium (III) chloride. The Roman numeral III tells the reader that iron’s oxidation number is +3 not a +2 or a +6. While the name manganese(IV) oxide indicates that the oxidation number of manganese is +4. Roman numerals are not used with nonmetals in ionic compounds. Record the oxidation numbers for the following multivalent metallic ions: Iron (III) ion _______________ Copper (II) ion _________________ Lead (IV) ion ______________ Lead (II) ion ___________________ Explain why zinc ion does not have a Roman numeral included with its name. Polyatomic Ions Polyatomic ions are molecules that have lost or gained electrons, thus they have a charge. You will need to memorize the list given with the summer project. You will need to know the name, the formula, and the charge. There are some hints to help remember them. 1) The suffixes –ate and –ite always indicate a polyatomic ion that contains oxygen. a. Sulfate – SO42-‐ b. Sulfite – SO32-‐ 2) The –ite form of a polyatomic ion always has the same charge but one less oxygen than the –ate form. a. Nitrate – NO3-‐ b. Nitrite -‐ NO2-‐ 3) Nick the camel -‐ate Clams for supper in phoenix. (this will give you the ate form of the most common polyatomic ions. a. The first letter of each noun is the first element of the polyatomic ion. b. The number of vowels = the negative charge of the polyatomic ion. c. The number of consonants = the number of oxygens in the polyatomic ion. Nick One vowel= -‐1 charge Three consonants = 3 oxygens NO3-‐ 4) Make flash cards and practice writing and naming them as often as you can. Determining an elements oxidation number when it is part of a compound. When naming an ionic compound that contains a multivalent metal ion, you must be to determine what the oxidation number of the metal is. This is done in order to make sure the correct Roman numeral is used in the compounds name. For example, without Roman numerals the compounds FeO and Fe2O3 would have the same name: iron oxide. This would lead to problems, however, as they both have distinctly different chemical and physical properties. In order to determine the oxidation number of the metal, you need to know the oxidation number of the nonmetal, and that the total positive charge must equal the total negative charge. Lets determine the oxidation numbers for iron in FeO and Fe2O3. 1) First write the oxidation number of the nonmetal or negative ion above the symbol for the nonmetal or negative polyatomic ion or negative polyatomic ion. 𝐹𝑒𝑂!! 2) The charge of iron must be +2 to balance the -‐2 of oxygen. 𝐹𝑒 !! 𝑂!! 3) Thus the oxidation number of iron is +2 and the name of this compound would be iron (II) oxide. It is slightly more challenging for Fe2O3, but not by much. 1) First write the oxidation number of the nonmetal or negative polyatomic ion above the symbol for the nonmetal or negative polyatomic ion. 𝐹𝑒! 𝑂!!! 2) Now multiply the number of oxygen ions by the charge, this is the total negative charge. 𝐹𝑒! 𝑂!!! − 2×3 = −6 3) The total positive charge must be +6 to balance the -‐6. Since there are two iron ions, each ion must be +3. 𝐹𝑒!!! 𝑂!!! 4) Therefore the iron’s oxidation number is +3 and the name of this compound is iron (III) oxide. You may have noticed that there is another way to determine the oxidation of a multivalent metal. The oxidation number can be determined by reversing the subscripts. 𝐹𝑒!!! 𝑂!!! This works 95% of time. It will not work if both ions have different even oxidation numbers. Determine the oxidation number the metal ion in each of these compounds. FeCl3 ________________ CrO3 _______________ CuCl _________________ V2O4 _______________ PbCl2_________________ PbO2 ______________ NaCl_________________ FeP _______________ Ca2N2________________ Use the charge of the polyatomic ion when completing the following questions. K2SO4______________ Cu(OH)2___________ Ni(C2H3O2) ________ VCO3 ______________