112

THINK ABOUT IT

For thousands of years, human beings have expressed themselves through painting. Over time, people have discovered pigments with a wide variety of brilliant colors. Almost all paint pigments contain a transition metal cation.

What types of compounds are made from transition metals?

To answer this question, you will explore

EXPLORING THE TOPIC

Transition Metal Compounds

The transition metals are named for their location in the middle of the periodic table. Many of the metals that we use in our daily life, such as copper, iron, nickel, silver, and gold, are located in this part of the periodic table.

Transition metal compounds tend to be brightly colored. Hence, they are commonly found as pigments used to color paints. For example, red ochre is a pigment made from iron (III) oxide, Fe₂O₃. It is thought to be the first pigment ever used by human beings for creating art.

The ancient alchemists worked with the transition metals a great deal, mostly because these metals were closer in their properties to gold than to other metals. As a result, alchemists occasionally discovered paint pigments while trying to create gold.

The table on the next page lists various paint pigments that you can buy at an art store. Examine the data. Pay particular attention to the different charges on the transition metal cations.

Notice that the chemical names are a bit unusual. In the middle of each chemical name for a transition metal compound is a Roman numeral: I, II, III, or IV (meaning 1, 2, 3, or 4). This Roman numeral indicates what the charge is on the transition metal cation. Thus, cobalt (II) oxide has a +2 charge on the cobalt ion, and manganese (IV) carbonate has a +4 charge on the manganese ion.

113

Note that copper, Cu, has a charge of +1 in copper (I) oxide, Cu₂O, and a charge of +2 in copper (II) carbonate, CuCO₃.

There is an older naming system for the transition metal ionic compounds. In this system, Fe₂O₃ and FeO were called ferric oxide and ferrous oxide. The “-ic” ending indicated the higher ion charge and the “-ous” ending referred to the lower ion charge. So Co²⁺ was cobaltous and Co³⁺ was cobaltic.

Charges on Transition Metal Cations

Recall that you can determine the charges on the main group metals and nonmetals from their positions on the periodic table. For example, all of the alkali metals, Group 1A, form cations with +1 charges. All of the halogens, Group 7A, form anions with –1 charges. However, you cannot simply determine the charge of a transition metal ion from its location on the periodic table. Instead, it is necessary to use chemical formulas to determine the charge of the ion.

To determine the charges on transition metal cations, you use the charges on ions that you do know and apply the rule of zero charge. For example, iron combines with oxygen to form both Fe₂O₃ and FeO. Oxygen is a main group anion, so you can use the periodic table to determine that oxygen atoms form ions with a –2 charge. The rule of zero charge states that the charges on the ions in the compound should add up to zero. Working backward, you can determine the charge on the iron cations in each compound.

These two different iron compounds are formally named iron (III) oxide and iron (II) oxide.

LESSON SUMMARY

What types of compounds are made from transition metals?

Transition metals bond with nonmetal atoms to form ionic compounds. Unlike the main group atoms, most transition metals can have more than one ion charge. The best way to determine the charge on a transition metal cation is to work backward from the anion, whose charge is known. When naming ionic compounds, a Roman numeral is used to indicate the charge on the transition metal cation. Colorful paint pigments are frequently composed of transition metal compounds.

115