CHEMISTRY THE CENTRAL SCIENCE
7 PERIODIC PROPERTIES OF THE ELEMENTS
7.6 METALS, NONMETALS, AND METALLOIDS
Atomic radii, ionization energies, and electron affinities are properties of individual atoms. With the exception of the noble gases, however, none of the elements exist in nature as individual atoms. To get a broader understanding of the properties of elements, we must also examine periodic trends in properties that involve large collections of atoms.
The elements can be broadly grouped as metals, nonmetals, and metalloids (FIGURE 7.12). (Section 2.5) Some of the distinguishing properties of metals and nonmetals are summarized in TABLE 7.3.
In the following sections, we explore some common patterns of reactivity across the periodic table. We will examine reactivity for nonmetals and metals in more depth in later chapters.
TABLE 7.3 • Characteristic Properties of Metals and Nonmetals
Notice that germanium, Ge, is a metalloid but tin, Sn, is a metal. What changes in atomic properties do you think are important in explaining this difference?
FIGURE 7.12 Metals, metalloids, and nonmetals.
The more an element exhibits the physical and chemical properties of metals, the greater its metallic character. As indicated in Figure 7.12, metallic character generally increases as we proceed down a group of the periodic table and decreases as we proceed right across a period. Let's now examine the close relationships that exist between electron configurations and the properties of metals, nonmetals, and metalloids.
Most metallic elements exhibit the shiny luster we associate with metals (FIGURE 7.13). Metals conduct heat and electricity. In general they are malleable (can be pounded into thin sheets) and ductile (can be drawn into wires). All are solids at room temperature except mercury (melting point = −39 °C), which is a liquid at room temperature. Two metals melt at slightly above room temperature, cesium at 28.4 °C and gallium at 29.8 °C. At the other extreme, many metals melt at very high temperatures. For example, chromium melts at 1900 °C.
Metals tend to have low ionization energies (Figure 7.9) and therefore tend to form cations relatively easily. As a result, metals are oxidized (lose electrons) when they undergo chemical reactions. Among the fundamental atomic properties (radius, electron configuration, electron affinity, and so forth), first ionization energy is the best indicator of whether an element behaves as a metal or a nonmetal.
FIGURE 7.14 shows the oxidation states of representative ions of metals and nonmetals. As noted in Section 2.7, the charge on any alkali metal ion in a compound is always 1 +, and that on any alkaline earth metal is always 2 +. For atoms belonging to either of these groups, the outers electrons are easily lost, yielding a noble-gas electron configuration. For metals belonging to groups with partially occupied p orbitals (groups 3A-7A), cations are formed either by losing only the outer p electrons (such as Sn2+) or the outer s and p electrons (such as Sn4+). The charge on transition-metal ions does not follow an obvious pattern. One characteristic of the transition metals is their ability to form more than one cation. For example, iron is 2+ in some compounds and 3 + in others.
GIVE IT SOME THOUGHT
Describe a general relationship between trends in metallic character and trends in ionization energy.
FIGURE 7.13 Metals are shiny and malleable.
The red stepped line divides metals from nonmetals. How are common oxidation states divided by this line?
FIGURE 7.14 Representative oxidation states of the elements. Note that hydrogen has both positive and negative oxidation numbers, +1 and —1.
Compounds made up of a metal and a nonmetal tend to be ionic substances. For example, most metal oxides and halides are ionic solids. To illustrate, the reaction between nickel metal and oxygen produces nickel oxide, an ionic solid containing Ni2+ and O2− ions:
The oxides are particularly important because of the great abundance of oxygen in our environment.
Most metal oxides are basic. Those that dissolve in water react to form metal hydroxides, as in the following examples:
The basicity of metal oxides is due to the oxide ion, which reacts with water:
Even metal oxides that are insoluble in water demonstrate their basicity by reacting with acids to form a salt plus water, as illustrated in FIGURE 7.15:
FIGURE 7.15 Metal oxides react with acids. NIO does not dissolve in water but does react with nitric acid (HNO3) to give a green solution of Ni(NO3)2.
SAMPLE EXERCISE 7.8 Metal Oxides
(a) Would you expect scandium oxide to be a solid, liquid, or gas at room temperature?
(b) Write the balanced chemical equation for the reaction of scandium oxide with nitric acid.
Analyze and Plan We are asked about one physical property of scandium oxide—its state at room temperature—and one chemical property—how it reacts with nitric acid.
(a) Because scandium oxide is the oxide of a metal, we expect it to be an ionic solid. Indeed it is, with the very high melting point of 2485 °C.
(b) In compounds, scandium has a 3+ charge, Sc3+, and the oxide ion is O2−. Consequently, the formula of scandium oxide is Sc2O3. Metal oxides tend to be basic and, therefore, to react with acids to form a salt plus water. In this case the salt is scandium nitrate, Sc(NO3)3:
Write the balanced chemical equation for the reaction between copper(II) oxide and sulfuric acid.
Nonmetals can be solid, liquid, or gas. They are not lustrous and generally are poor conductors of heat and electricity. Their melting points are generally lower than those of metals (although diamond, a form of carbon, is an exception and melts at 3570 °C). Under ordinary conditions, seven nonmetals exist as diatomic molecules. Five of these are gases (H2, N2, O2, F2, and Cl2), one is a liquid (Br2), and one is a volatile solid (I2). Excluding the noble gases, the remaining nonmetals are solids that can be either hard, such as diamond, or soft, such as sulfur (FIGURE 7.16).
Because of their relatively large, negative electron affinities, nonmetals tend to gain electrons when they react with metals. For example, the reaction of aluminum with bromine produces the ionic compound aluminum bromide:
A nonmetal typically will gain enough electrons to fill its outermost occupied p subshell, giving a noble-gas electron configuration. For example, the bromine atom gains one electron to fill its 4p subshell:
Compounds composed entirely of nonmetals are typically molecular substances that tend to be gases, liquids, or low-melting solids at room temperature. Examples include the common hydrocarbons we use for fuel (methane, CH4; propane, C3H8; octane, C8H18) and the gases HCl, NH3, and H2S. Many drugs are molecules composed of C, H, N, O, and other nonmetals. For example, the molecular formula for the drug Celebrex is C17H14F3N3O2S. Most nonmetal oxides are acidic, which means that those that dissolve in water form acids:
The reaction of carbon dioxide with water (FIGURE 7.17) accounts for the acidity of carbonated water and, to some extent, rainwater. Because sulfur is present in oil and coal, combustion of these common fuels produces sulfur dioxide and sulfur trioxide. These substances dissolve in water to produce acid rain, a major pollutant in many parts of the world. Like acids, most nonmetal oxides dissolve in basic solutions to form a salt plus water:
FIGURE 7.16 Sulfur, known to the medieval world as “brimstone,” is a nonmetal.
FIGURE 7.17 The reaction of CO2 with water containing a bromthymol blue indicator. Initially, the blue color tells us the water is slightly basic. When a piece of solid carbon dioxide (“dry ice”) is added, the color changes to yellow, indicating an acidic solution. The mist is water droplets condensed from the air by the cold CO2 gas.
GIVE IT SOME THOUGHT
A compound ACl3 (A is an element) has a melting point of −112 °C. Would you expect the compound to be molecular or ionic? If you were told that A is either scandium or phosphorus, which do you think is the more likely choice?
SAMPLE EXERCISE 7.9 Nonmetal Oxides
Write the balanced chemical equation for the reaction of solid selenium dioxide, SeO2(s), with (a) water, (b) aqueous sodium hydroxide.
Analyze and Plan We note that selenium is a nonmetal. We therefore need to write chemical equations for the reaction of a nonmetal oxide with water and with a base, NaOH. Nonmetal oxides are acidic, reacting with water to form an acid and with bases to form a salt and water.
(a) The reaction between selenium dioxide and water is like that between carbon dioxide and water (Equation 7.13):
(It does not matter that SeO2 is a solid and CO2 is a gas under ambient conditions; the point is that both are water-soluble nonmetal oxides.)
(b) The reaction with sodium hydroxide is like the reaction in Equation 7.15:
Write the balanced chemical equation for the reaction of solid tetraphosphorus hexoxide with water.
Metalloids have properties intermediate between those of metals and those of nonmetals. They may have some characteristic metallic properties but lack others. For example, the metalloid silicon looks like a metal (FIGURE 7.18), but it is brittle rather than malleable and does not conduct heat or electricity nearly as well as metals do. Compounds of metalloids can have characteristics of the compounds of metals or nonmetals.
Several metalloids, most notably silicon, are electrical semiconductors and are the principal elements used in integrated circuits and computer chips. One of the reasons metalloids can be used for integrated circuits is that their electrical conductivity is intermediate between that of metals and that of nonmetals. Very pure silicon is an electrical insulator, but its conductivity can be dramatically increased with the addition of specific impurities called dopants. This modification provides a mechanism for controlling the electrical conductivity by controlling the chemical composition. We will return to this point in Chapter 12.