CHEMISTRY THE CENTRAL SCIENCE

22 CHEMISTRY OF THE NONMETALS

AN OCEAN SUNSET SEEN THROUGH a glass window.

WHAT'S AHEAD

22.1 PERIODIC TRENDS AND CHEMICAL REACTIONS

We begin with a review of periodic trends and types of chemical reactions, which will help us focus on general patterns of behavior as we examine each family in the periodic table.

22.2 HYDROGEN

The first nonmetal we consider, hydrogen, forms compounds with most other nonmetals and with many metals.

22.3 GROUP 8A: THE NOBLE GASES

Next, we consider the noble gases, the elements of group 8A, which exhibit very limited chemical reactivity.

22.4 GROUP 7A: THE HALOGENS

We then explore the most electronegative elements: the halogens, group 7A.

22.5 OXYGEN

We next consider oxygen, the most abundant element by mass in both Earth's crust and the human body, and the oxide and peroxide compounds it forms.

22.6 THE OTHER GROUP 6A ELEMENTS: S, Se, Te, AND Po

We study the other members of group 6A (S, Se, Te, and Po), of which sulfur is the most important.

22.7 NITROGEN

We next consider nitrogen, a key component of our atmosphere. It forms compounds in which its oxidation number ranges from –3 to +5, including such important compounds as NH3 and HNO3.

22.8 THE OTHER GROUP 5A ELEMENTS: P, As, Sb, AND Bi

Of the other members of group 5A (P, As, Sb, and Bi), we take a closer look at phosphorus—the most commercially important one and the only one that plays an important and beneficial role in biological systems.

22.9 CARBON

We next focus on the inorganic compounds of carbon.

22.10 THE OTHER GROUP 4A ELEMENTS: Si, Ge, Sn, AND Pb

We then consider silicon, the element most abundant and significant of the heavier members of group 4A.

22.11 BORON

Finally, we examine boron—the sole nonmetallic element of group 3A.

THE CHAPTER-OPENING PHOTOGRAPH shows the sun setting over the ocean, viewed through a window. Everything we see there comes from nonmetallic elements. The heat and light of the Sun result from the nuclear reactions of hydrogen. We know that water is H2O; glass, too, is nonmetallic, being based on silicon dioxide, SiO2.

In this chapter we take a panoramic view of the descriptive chemistry of the nonmetallic elements, starting with hydrogen and progressing group by group across the periodic table. We will consider how the elements occur in nature, how they are isolated from their sources, and how they are used. We will emphasize hydrogen, oxygen, nitrogen, and carbon because these four nonmetals form many commercially important compounds and account for 99% of the atoms required by living cells.

As you study this descriptive chemistry, it is important to look for trends rather than trying to memorize all the facts presented. The periodic table is your most valuable tool in this task.

22.1 PERIODIC TRENDS AND CHEMICAL REACTIONS

Recall that we can classify elements as metals, metalloids, and non-metals.  (Section 7.6) Except for hydrogen, which is a special case, the nonmetals occupy the upper right portion of the periodic table. This division of elements relates nicely to trends in the properties of the elements as summarized in FIGURE 22.1. Electronegativity, for example, increases as we move left to right across a period and decreases as we move down a group. The nonmetals thus have higher electronegativities than the metals. This difference leads to the formation of ionic solids in reactions between metals and nonmetals.  (Sections 7.6, 8.2, 8.4) In contrast, compounds formed between two or more nonmetals are usually molecular substances.  (Sections 7.8 and 8.4)

FIGURE 22.1 Trends in elemental properties.

The chemistry exhibited by the first member of a nonmetal group can differ from that of subsequent members. For example, nonmetals in period 3 and below can accommodate a larger number of bonded neighbors.  (Section 8.7) Another important difference is that the first element in any group can more readily form π bonds. This trend is due, in part, to size because small atoms are able to approach each other more closely. As a result, the overlap of p orbitals, which results in the formation of π bonds, is more effective for the first element in each group (FIGURE 22.2). More effective overlap means stronger π bonds, reflected in bond enthalpies.  (Section 8.8) For example, the difference between the enthalpies of the C—C bond and the C═C bond is about 270 kJ/mol;  (Table 8.4) this value reflects the “strength” of a carbon-carbon π bond, and the difference between Si—Si and Si═Si bonds is only about 100 kJ/mol, significantly lower than that for carbon.

FIGURE 22.2 π bonds in period 2 and period 3 elements.

As we shall see, π bonds are particularly important in the chemistry of carbon, nitrogen, and oxygen, each the first member in its group. The heavier elements in these groups have a tendency to form only single bonds.

SAMPLE EXERCISE 22.1 Identifying Elemental Properties

Of the elements Li, K, N, P, and Ne, which (a) is the most electronegative, (b) has the greatest metallic character, (c) can bond to more than four atoms in a molecule, (d) forms π bonds most readily?

SOLUTION

Analyze We are given a list of elements and asked to predict several properties that can be related to periodic trends.

Plan We can use Figures 22.1 and 22.2 to guide us to the answers.

Solve

(a) Electronegativity increases as we proceed toward the upper right portion of the periodic table, excluding the noble gases. Thus, N is the most electronegative element of our choices.

(b) Metallic character correlates inversely with electronegativity—the less electronegative an element, the greater its metallic character. The element with the greatest metallic character is therefore K, which is closest to the lower left corner of the periodic table.

(c) Nonmetals tend to form molecular compounds, so we can narrow our choice to the three nonmetals on the list: N, P, and Ne. To form more than four bonds, an element must be able to expand its valence shell to allow more than an octet of electrons around it. Valence-shell expansion occurs for period 3 elements and below; N and Ne are both in period 2 and do not undergo valence-shell expansion. Thus, the answer is P.

(d) Period 2 nonmetals form π bonds more readily than elements in period 3 and below. There are no compounds known that contain covalent bonds to Ne. Thus, N is the element from the list that forms π bonds most readily.

PRACTICE EXERCISE

Of the elements Be, C, Cl, Sb, and Cs, which (a) has the lowest electronegativity, (b) has the greatest nonmetallic character, (c) is most likely to participate in extensive π bonding, (d) is most likely to be a metalloid?

Answers: (a) Cs, (b) Cl, (c) C, (d) Sb

The ready ability of period 2 elements to form π bonds is an important factor in determining the structures of these elements and their compounds. Compare, for example, the elemental forms of carbon and silicon. Carbon has five major crystalline allotropes: diamond, graphite, buckminsterfullerene, graphene, and carbon nanotubes.  (Sections 12.7, 12.9) Diamond is a covalent-network solid that has C—C σ bonds but no π bonds. Graphite, buckminsterfullerene, graphene, and carbon nanotubes have π bonds that result from the sideways overlap of p orbitals. Elemental silicon, however, exists only as a diamondlike covalent-network solid with σ bonds; it has no forms analogous to graphite, buckminsterfullerene, graphene, or carbon nanotubes, apparently because Si—Si π bonds are weak.

GIVE IT SOME THOUGHT

Can silicon-silicon double bonds form in elemental silicon?

We likewise see significant differences in the dioxides of carbon and silicon (FIGURE 22.3). CO2 is a molecular substance containing C═O double bonds, whereas SiO2 is a covalent-network solid in which four oxygen atoms are bonded to each silicon atom by single bonds, forming an extended structure that has the empirical formula SiO2.

FIGURE 22.3 Comparison of the bonds in SiO2 and CO2.

GIVE IT SOME THOUGHT

Nitrogen is found in nature as N2(g). Would you expect phosphorus to be found in nature as P2(g)? Explain.

Chemical Reactions

Because O2 and H2O are abundant in our environment, it is particularly important to consider how these substances react with other compounds. About one-third of the reactions discussed in this chapter involve either O2 (oxidation or combustion reactions) or H2O (especially proton-transfer reactions).

In combustion reactions,  (Section 3.2) hydrogen-containing compounds produce H2O. Carbon-containing ones produce CO2 (unless the amount of O2 is insufficient, in which case CO or even C can form). Nitrogen-containing compounds tend to form N2, although NO can form in special cases or in small amounts. A reaction illustrating these points is:

The formation of H2O, CO2, and N2 reflects the high thermodynamic stability of these substances, indicated by the large bond energies for the O— H, C═O, and N≡N bonds (463, 799, and 941 kJ/mol, respectively).  (Section 8.8)

When dealing with proton-transfer reactions, remember that the weaker a Brønsted–Lowry acid, the stronger its conjugate base.  (Section 16.2) For example, H2, OH, NH3, and CH4 are exceedingly weak proton donors that have no tendency to act as acids in water. Thus, the species formed by removing one or more protons from them are extremely strong bases. All react readily with water, removing protons from H2O to form OH. Two representative reactions are:

SAMPLE EXERCISE 22.2 Predicting the Products of Chemical Reactions

Predict the products formed in each of the following reactions, and write a balanced equation:

SOLUTION

Analyze: We are given the reactants for two chemical equations and asked to predict the products and then balance the equations.

Plan We need to examine the reactants to see if we might recognize a reaction type. In (a) the carbon compound is reacting with O2, which suggests a combustion reaction. In (b) water reacts with an ionic compound. The anion, P3–, is a strong base and H2O is able to act as an acid, so the reactants suggest an acid–base (proton-transfer) reaction.

Solve

(a) Based on the elemental composition of the carbon compound, this combustion reaction should produce CO2, H2O, and N2:

(b) Mg3P2 is ionic, consisting of Mg2+ and P3– ions. The P3– ion, like N3–, has a strong affinity for protons and reacts with H2O to form OH and PH3 (PH2–, PH2, and PH3 are all exceedingly weak proton donors).

Mg(OH)2 has low solubility in water and will precipitate.

PRACTICE EXERCISE

Write a balanced equation for the reaction of solid sodium hydride with water.

Answer: