Naming compounds using Roman numerals

Name the following compounds using Roman numerals to indicate the oxidation number of the metal ... 

Some metals, such as iron, can form more than one type of ion. Iron ions can either have a +2 or a +3 charge. Chemists use Roman numerals to distinguish between the two different types of iron ions. The Roman numeral II in the name of the chemical compound iron(II) sulfide means that the iron ion in this compound has a +2 charge. 

Name the following compounds that contain elements with variable charge. Don t forget to use Roman numerals ... 

Some elements, such as copper and iron, possess two ions with different valencies. Copper can form the Cu+ ion and the Cu2+ ion, with valencies 1 and 2 respectively. Therefore it can form two different compounds with chlorine, CuCl and CuCl2. We can also distinguish the difference by using Roman numerals in their names CuCl is copper(i) chloride and CuCl2 is copper(n) chloride. Similarly, iron forms the Fe2+ and Fe3+ ions and so can also form two different compounds with, for example, chlorine FeCl2 (iron(n) chloride) andFeCl3 (iron(m) chloride). 

All of the examples used here involve elements with only one possible oxidation state. When we are required to name compounds that contain elements with more than one oxidation state, we use the stock system, which uses Roman numerals to indicate the oxidation number of the less electronegative element. We can usually determine the oxidation number of the less electronegative element by considering the rules for oxidation numbers presented in Lesson 5-1. It would probably be a good idea to review those rules before you try to follow these next examples. 

Many metals (usually transition metals) may form cations of more than one charge. In this case, a Roman numeral in parenthesis after the name of the element is used to indicate the ion s charge in a particular compound. This Roman numeral method is known as the Stock system. An older nomenclature used the suffix -ous for the lower charge and -ic for the higher charge and is still used occasionally. 

In naming binary compounds, you sometimes simply name the ions. In other cases, you need to use Roman numerals, or prefixes. 

Iron forms both 2+ and 3+ cations. Write formulas for the oxide, sulfide, and chloride compound of each iron cation, and give the name of each compound in both the nomenclature method that uses Roman numerals to specify the charge of the cation and the -ousl-ic notation. 

Note that the use of a Roman numeral in a systematic name is required only in cases where more than one ionic componnd forms between a given pair of elements. This case most commonly occnrs for componnds containing transition metals, which often form more than one cation. Elements that form only one cation do not need to be identified by a Roman nnmeral. Common metals that do not require Roman numerals are the (jroup 1A elements, which form only 1 + ions the Group 2A elements, which form only 2+ ions and alnminnm, which forms only Al +. The element silver deserves special mention at this point. In virtnally all its compounds, silver is found as the Ag+ ion. Therefore, althongh silver is a transition metal (and can potentially form ions other than Ag+), silver componnds are nsnally named without a Roman numeral. Thus AgCl is typically called silver chloride rather than silver(I) chloride, although the latter name is technically correct. Also, a Roman nnmeral is not used for zinc compounds, since zinc forms only the Zn + ion. 

The basic form for the names of Type II ionic compounds is to have the name of the metal cation first, followed by the charge of the metal cation (in parentheses, using Roman numerals), and finally the base name of the non-metal anion with -ide attached to the end. 

Knowing when to use Roman numerals and when not to use them is a tricky aspect of naming ionic compounds. Because the metals of groups 1 and 2 have only one ionic form (one oxidation state), Roman numerals are not used in naming their compounds. 

The name of a monatomic cation is the same as the name of the element forming it, with the addition of the word ion, as in sodium ion for Na+. When an element can form more than one kind of cation, such as Cu+ and Cu2+ from copper, we use the oxidation number, the charge of the cation, written as a Roman numeral in parentheses following the name of the element. Thus, Cu+ is a copper(I) ion and Cu2+ is a copper(II) ion. Similarly, Fe2+ is an iron(II) ion and Fe3" is an iron(III) ion. As shown in Fig. C.6, most transition metals form more than one kind of ion so unless we are given other information we need to include the oxidation number in the names of their compounds. 

Oxidation state is a frequently used (and indeed misused) concept which apportions charges and electrons within complex molecules and ions. We stress that oxidation state is a formal concept, rather than an accurate statement of the charge distributions within compounds. The oxidation state of a metal is defined as the formal charge which would be placed upon that metal in a purely ionic description. For example, the metals in the gas phase ions Mn + and Cu are assigned oxidation states of +3 and +1 respectively. These are usually denoted by placing the formal oxidation state in Roman numerals in parentheses after the element name the ions Mn- " and Cu+ are examples of manganese(iii) and copper(i). 

Cr03 As a transition metal, chromium forms more than one stable cation. Name the metal first, using a Roman numeral to designate chromium s charge. Each of the three oxide anions has a -2 charge. To maintain net charge neutrality, Cr must be +6, so the name of the compound is chromium(VI) oxide. 

Naming of the positive ion depends on whether the cation is monatomic (has one atom). If not, the special names given in Sec. 6.3.2 are used. If the cation is monatomic, the name depends on whether the element forms more than one positive ion in its compounds. For example, sodium forms only one positive ion in all its compounds—NaT Iron forms two positive ions—Fc2r and Fe,+. Cations of elements that form only one type of ion in all their compounds need not be further identified in the name. Thus, Na may simply be called the sodium ion. Cations of metals that occur with two or more different charges must be further identified. Fe(NO,)2 and Fe(NO,)3 occur with Fc2+ and Fe3 ions, respectively. If we just call the ion the iron ion, we will not know which one it is. Therefore, for monatomic cations, we use a Roman numeral in parentheses attached to the name to tell the charge on such ion. (Actually, oxidation numbers are used for this purpose, but if you have... 

For compounds or ions containing elements that have a variable oxidation number, Roman numerals are used to indicate the oxidation number of the element concerned, and so name the chemical species. This is called Stock notation, after the chemist A. Stock who devised the method. 

To avoid the necessity of memorizing a separate name for each ion, we can use the Stock system. In the Stock system, the charge of the cation appears as a Roman numeral immediately after the name of the element. Using the Stock system, we write Fe2+ as the iron(II) ion, and Cu+ as the copper(I) ion. Other than the necessity of indicating the charges, there are no differences between the naming of transition metal compounds and other compounds of the metals. So while KC1 is potassium chloride, CuCl is copper(I) chloride. 

Many transition metals and the group of six elements centered around lead on the periodic table commonly have more than one valence. The valence of these metals in a compound must be known before the compound can be named. Modern nomenclature rules indicate the valence of one of these metals with a Roman numeral suffix (Stock notation). Older nomenclature rules used different suffixes to indicate the charge. Examples ... 

FeCl2 or FeCl3 compound. Therefore iron must include a Roman numeral to specify which cation is in the compound. Iron (II) chloride is FeCl2 and iron (III) chloride is FeCl3. Two or more atoms may combine to form a polyatomic ion. Common polyatomic ions are listed in Table 1.3. The names of polyatomic ions may be used directly in compounds that contain them. Hence, NaOH is sodium hydroxide, CaC03 is calcium carbonate, and Ba(N03)2 is barium nitrate. 

Many metals may form cations of more than one charge. In this case, a Roman numeral in parenthesis after the name of the element is used to indicate the ion s charge in a particular compound. 

Another source of difficulty is that the atoms in molecules do not form ions. Therefore, the use of Roman numerals is not possible. And because nonmetals have multiple oxidation states possible (nitrogen has 8), it is too difficult to determine the possible combinations of atoms in a molecule. All of this is meant to illustrate why a different naming system is required for molecular compounds. 

Many of the common metals are transition elements that have more than one possible valence. For example, tin is able to form the ions Sn2+ and Sn4+, iron can form Fe2+ and Fe3+, and copper can form Cu+ and Cu2+. (The most common transition metals with more than one valence number are listed in Table 3.3.) The name of a compound must identify which ion is present in the compound. To do this, the element s name is used, followed by the valence in parentheses, written in Roman numerals. Therefore, Sn4+ is tin(IV), Fe3+ is iron(III), and Cu2+ is copper(II). This naming method is called the Stock system after Alfred Stock, a German chemist who first used it. Some examples of Stock system names are listed in Table 3.6. 

The names of chemical compounds may consist of one or more words, and they may include locants, descriptors, and syllabic portions. Locants and descriptors can be numerals, element symbols, small capital letters, Greek letters, Latin letters, italic words and letters, and combinations of these. Treat the word or syllabic portions of chemical names just like other common nouns use roman type, keep them lowercase in text, capitalize them at the beginnings of sentences and in titles, and hyphenate them only when they do not fit completely on one line. 

Stock Method Method for naming compounds where a roman numeral is used to indicate the amount of positive charge on the cation. 

To name an ionic compound, name the cation first and then the anion. Use just the name of the element for monatomic cations of elements that form only one cation. For monatomic cations of elements that can form more than one cation, indicated the charge on the cation by a Roman numeral in parentheses added to the name of the element. Polyatomic cations have special names, ammonium ion being the most important. 

Select all of the following compounds that should be named using the prefixes of Table 6.1, all that should be named with Roman numerals, and all that should have neither. Name each. 

In Chapter 6, we learned how to name cations. In the Stock system, the charges on monatomic ions were used to distinguish between different ions of the same element. For example, Cu and Cu are named copper ion and copper(ll) ion, respectively. The Roman numeral actually represents the oxidation number, not the charge on the ion. Of course, for monatomic ions, the charge is equal to the oxidation number, and thus we used the charge to determine which Roman numeral to use. By using oxidation numbers, however, we can extend our compound-naming ability to include compounds other than those of monatomic ions. For example, Hg2 is called the mercury(I) ion because the oxidation number of each mercury atom is +1. 

Oxidation numbers are used in the Stock system for naming compounds. Positive oxidation numbers are denoted as Roman numerals in parentheses in the names of the compounds the numbers of atoms or ions can be deduced from the oxidation numbers. (In contrast, the subscripts in formulas give the numbers of atoms or ions, from which the oxidation numbers may be deduced.) (Section 16.2). 

---