Atomic Structure

blei2echem_ch02_p35 8/19/05 2:33 PM Page 35 CHAPTER 2 ATOMIC STRUCTURE Chemistry in Your Future When you were training as a sales representative for a pharmaceutical company, it surprised you to learn that there are many important medications whose mode of action is not at all well understood. A case in point is the drug lithium carbonate, which has long been used to control the psychiatric dysfunction known as bipolar disorder, or manic depression. Some scientists think its efficacy stems from lithium’s similarity to the elements sodium and potassium, which are known to play major roles in the transmission of nerve impulses. Chapter 2 presents the fundamental concepts that explain the chemical relatedness of these elements. (David J. Sams/Tony Stone.) For more information on this topic and others in this chapter, go to www.whfreeman.com/bleiodian2e Learning Objectives • Use Dalton’s atomic theory to explain the constant composition of matter and the conservation of mass in chemical reactions. • Define atomic mass. • Describe the structure of the atom in reference to its principal subatomic particles. • Identify main-group and transition elements, metals, nonmetals, and metalloids. • Correlate the arrangement of the periodic table with the electron configurations of the valence shells of the elements. • Describe the octet rule. 35 blei2echem_ch02_p35 8/19/05 2:33 PM Page 36 36 GENERAL CHEMISTRY or thousands of years, people have speculated about the fundamental properties of matter. The basic question to be answered was, Why are F there so many different substances in the world? Many explanations were proposed but the most enduring one—the one that has proved to be correct—was that all matter is composed of particles, or atoms. The atoms were assumed to combine in various ways to produce all the existing and future kinds of matter. This idea, first proposed by Greek philosophers in the fourth century B.C., remained more a philosophical argument than a scientific theory until the early nineteenth century. At that time, John Dalton, an English schoolmaster, proposed an atomic model that convincingly explained experimental observations on which chemists of the day had come to rely. In this chapter, we examine the nature of atomic structure and correlate it with the chemical reactivity of the elements. By the chapter’s end, we will know the answer to such questions as, Why does one atom of calcium react with one atom of oxygen but with two atoms of chlorine? 2.1 CHEMICAL BACKGROUND FOR THE EARLY ATOMIC THEORY By the early 1800s, two experimental facts had been firmly established: 1. Mass is neither created nor destroyed in a chemical reaction. When substances react chemically to create new substances, the total mass of the resulting products is the same as the total mass of the substances that have reacted. This is called the Law of Conservation of Mass. 2. The elements present in a compound are present in fixed and exact proportion by mass, regardless of the compound’s source or method of preparation. This is called the Law of Constant Composition. ❯❯ Vitamins receive special Consider vitamin C, or ascorbic acid, which can be obtained from citrus consideration in Chapters 19 fruits or synthesized in the chemical laboratory. No matter where vitamin C and 26. comes from, the relative amounts of carbon (symbol C), hydrogen (symbol H), and oxygen (symbol O) of which it is composed are the same. They are specified as the mass percent of each element, which taken together is called the percent composition of the compound. The mass percentages of C, H, and O in vitamin C are mass of carbon Mass percent of carbon in vitamin C ϭ ϫ 100% mass of vitamin C mass of hydrogen Mass percent of hydrogen in vitamin C ϭ ϫ 100% mass of vitamin C mass of oxygen Mass percent of oxygen in vitamin C ϭ ϫ 100% mass of vitamin C Multiplication of the mass ratios by 100% converts them into percentages. Example 2.1 Calculating the mass percent of a compound Analysis of 4.200 g of vitamin C (ascorbic acid) yields 1.720 g of carbon, 0.190 g of hydrogen, and 2.290 g of oxygen. Calculate the mass percentage of each element in the compound. Solution 1.720 g Mass percent of carbon in vitamin C ϭ ϫ 100% ϭ 40.95% 4.200 g 0.190 g Mass percent of hydrogen in vitamin C ϭ ϫ 100% ϭ 4.52% 4.200 g blei2echem_ch02_p35 8/19/05 2:33 PM Page 37 ATOMIC STRUCTURE 37 2.290 g Mass percent of oxygen in vitamin C ϭ ϫ 100% ϭ 54.52% 4.200 g Problem 2.1 Analysis of 4.800 g of niacin (nicotinic acid), one of the B-complex vitamins, yields 2.810 g of carbon, 0.1954 g of hydrogen, 0.5462 g of nitrogen (symbol N), and 1.249 g of oxygen. Calculate the mass percentage of each element in the compound. 2.2 DALTON’S ATOMIC THEORY In the years from 1803 to 1808, John Dalton showed how the atomic theory could be used to explain the quantitative aspects of chemistry considered in Section 2.1. Since that time, our knowledge of the nature of the atom has changed significantly; Dalton was not correct in every particular. Nevertheless, he succeeded in creating, on the basis of a small number of specific observations, the first workable general theory of the structure of matter. Dalton’s model consisted of the following proposals: • All matter is composed of infinitesimally small particles called atoms (believed in Dalton’s time to be indestructible but now known to be composed of even smaller parts). • The atoms of any one element are chemically identical. • Atoms of one element are distinguished from those of a different element by the fact that the atoms of the two elements have different masses. • Compounds are combinations of atoms of different elements and possess properties different from those of their component elements. • In chemical reactions, atoms are exchanged between starting compounds to form new compounds. Atoms can be neither created nor destroyed. Later in this chapter, we will examine some of the ways in which these proposals have undergone modification, but the key ideas have survived to this day. Dalton’s idea of chemical reactions is illustrated in Figure 2.1. Atoms (called elementary particles by Dalton) that are combined in compounds (called compound particles by Dalton) rearrange to form new compounds. (You can see where the modern names element and compound, Section 1.1, come from.) This model shows that, in a chemical process, mass is conserved. The new compounds possess the mass of their constituent atoms. No mass has been gained or lost. The atoms have simply become rearranged. Dalton’s model explained why the elements in a compound are present in fixed and exact proportion by mass. If the atoms of each element have a defi- nite and unique mass and if compounds consist of a fixed ratio of given atoms—say, one atom of sulfur to two atoms of hydrogen—then the ratio of their masses also must be constant. ++ AB CD CA BD Figure 2.1 A chemical reaction as John Dalton might have described it. Atoms of elements are represented by different shapes and colors and labeled A, B, C, and D. The elements have combined to form two compounds, AB and CD, which react to produce two new compounds CA and BD. Note that no mass has been gained or lost and that the composition of all compounds is constant. blei2echem_ch02_p35 8/19/05 2:33 PM Page 38 38 GENERAL CHEMISTRY 2.3 ATOMIC MASSES In Dalton’s definition, the principal difference between different elements is the different masses of their atoms. Because he could not weigh individual atoms to measure their mass, he proposed a relative mass scale instead. He identified the lightest element then known—hydrogen—and proposed that it be assigned a relative mass of 1.0. He then assigned masses to other elements by comparing them with hydrogen. Dalton also examined compounds in which hydrogen combined with other elements in what Dalton believed were 1:1 ratios of the different atoms. By determining the percent compositions of these compounds, he could com- pare the masses of other elements with the mass of hydrogen in each compound and thus establish their relative atomic masses. For example, the mass percentages of hydrogen and chlorine in the compound hydrogen chloride are 2.76% and 97.24%, respectively. If it is true that there is a 1:1 ratio of hydrogen and chlorine atoms in this compound, their relative masses in 100 grams of hydrogen chloride are Mass of a chlorine atom ϭ 97.24 ϭ 35.2 Mass of a hydrogen atom 2.76 1 or mass of a chlorine atom ϭ 35.2 ϫ mass of a hydrogen atom. Today the relative masses of atoms are determined by a technique called mass spectrometry. A mass spectrometer can be seen in Figure 2.2. The relative masses of atoms are called the atomic masses of the elements, although chemists often call them atomic weights, because mass is determined by weighing. In this book, we will use the term atomic mass. Atomic masses were first assigned by assuming hydrogen’s mass to be 1. Atomic masses are now defined relative to the mass of the most common isotope of the element carbon, whose mass is specified as exactly 12 atomic mass units, which now makes hydrogen’s mass 1.008.

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