SECTION 5.1 History of the Periodic Table By 60, more than 60 elements had been discovered. Chemists had to learn the properties of these elements as well as those of the many compounds that the elements formed. The elements were not organized into any patterns and the factors that determined the properties of the elements were unknown. In addition, there was no method for determining an element s atomic mass or the number of atoms of an element in a compound. Different chemists used different atomic masses for the same elements, resulting in different compositions being listed for the same compounds. This made communication of results between chemists difficult. periodic law periodic table Key Terms lanthanides actinides 1. Infer How could organizing the elements according to trends or patterns help chemists study the properties of the elements? In 60, Italian chemist Stanislao Cannizzaro presented a method for accurately measuring the relative masses of atoms. This allowed chemists to agree on standard values of atomic mass. It also allowed them to search for relationships between atomic mass and other properties of the elements. Mendeleev s periodic table grouped elements by their properties. A Russian chemist, Dmitri Mendeleev, decided to write a chemistry textbook and include the standard values of atomic masses. For the book, Mendeleev hoped to organize the elements according to their properties. Mendeleev went about organizing elements much as you might organize information to study for a test or write a research paper. He placed the name of each known element on a card, together with its atomic mass and a list of its observed physical and chemical properties. He then arranged the cards according to various properties and looked for trends or patterns. These note cards could be used to determine patterns in the properties of the elements. 125
Mendeleev s First Periodic Table Ti = 50 Zr = 90? = 0 V = 51 Nb = 94 Ta = 2 Cr = 52 Mo = 96 W = 6 Mn = 55 Rh = 104.4 Pt = 197.4 Fe = 56 Ru = 104.4 Ir = 19 Ni = Co = 59 Pl = 106.6 Os = 199 H = 1 Cu = 63.4 Ag = 10 Hg = 200 Be = 9.4 Mg = 24 Zn = 65.2 Cd = 112 B = 11 Al = 27.4? = 6 Ur = 116 Au = 197? C = 12 Si = 2? = 70 Su = 1 N = 14 P = 31 As = 75 Sb = 112 Bi = 210 O = 16 S = 32 Se = 79.4 Te = 12? F = 19 Cl = 35.5 Br = 0 I = 127 Li = 7 Na = 23 K = 39 Rb = 5.4 Cs = 133 Tl = 204 Ca = 40 Sr = 7.6 Ba = 137 Pb = 207? = 45 Ce = 92?Er = 56 La = 94?Yt = 60 Di = 95?In = 75.6 Th = 1? Mendeleev noticed that when the elements were arranged in order of increasing atomic mass, certain similarities in their properties appeared at regular intervals. For example, there are seven elements between F and Cl atoms, which have similar properties. There are also seven elements between two other atoms that have similar properties: Na and K. This repeating pattern is referred to as periodic. Something is periodic if it repeats again and again. For example, the second hand on a clock is periodic because it passes over the same spot every 60 seconds. Mendeleev s arrangement of the elements is shown in the table above. He switched two elements, iodine, I, and tellurium, Te, to maintain the patterns. He also hypothesized undiscovered elements to fill in the gaps in his pattern. By 6, three of these elements had been discovered. READING CHECK 2. Name three things in everyday life that are periodic. 126 CHAPTER 5
Moseley arranged elements by their atomic numbers. The periodic law states that the physical and chemical properties of the elements are periodic functions of their atomic numbers. In other words, when the elements are arranged in order of increasing atomic number, elements with similar properties appear at regular intervals. Mendeleev s periodic table ordered the elements by atomic mass, not atomic number. In 1911, English scientist Henry Moseley found that the elements fit into patterns better when they were ordered by nuclear charge, or the amount of positive charge in the nucleus. Moseley s work led to the modern definition of the atomic number and also to the reorganization of the elements by atomic number instead of atomic mass. Modern periodic tables arrange the elements by both atomic number and properties. The periodic table has undergone extensive changes since Mendeleev s time. Chemists have discovered more than 40 new elements and, in more recent years, manufactured new ones in the laboratory. Each of these elements, however, can still be placed in a group of other elements using the same type of pattern originally described by Mendeleev. The modern periodic table is an arrangement of the elements in order of their atomic numbers so that elements with similar properties fall in the same column, or group. The Noble Gases An important addition to the periodic table was made near the end of the nineteenth century. In 94, English physicist Lord Rayleigh and Scottish chemist William Ramsay discovered an unreactive gas called argon, with atomic mass 40. However, no spot was available for argon on the periodic table. In 95, Ramsay isolated another such gas, helium, which previously had been observed in the Sun, using the Sun s emission spectrum. William Ramsay won the Nobel Prize for his discoveries in 1904. Over the next several years, four more unreactive gases were discovered: neon, krypton, xenon, and radon. Scientists realized that a new column was needed to account for these elements, which they called noble gases. READING CHECK 3. In what way is the periodic table periodic? 7 N 15 P 33 As 51 Sb 3 Bi O 16 S 34 Se 52 Te 4 Po 9 F 17 Cl 35 Br 2 He 10 Ne Ar 36 Kr 53 I 54 Xe 5 At The noble gases, or Group elements, are all unreactive. 6 Rn 127
The Lanthanides The next step in the development of the periodic table was completed in the early 1900s. The lanthanides are the 14 elements with atomic numbers from 5 (cerium, Ce) to 71 (lutetium, Lu). These elements have very similar properties. The process of separating and identifying these elements required a large effort by many chemists. However, finally it was understood that the elements were best located in the sixth period, between Group 3 and Group 4. The Actinides Another major step in the development of the periodic table was the discovery of the actinides. The actinides are the 14 elements with atomic numbers from 90 (thorium, Th) to 103 (lawrencium, Lr). The actinides belong between Group 3 and Group 4 directly below the lanthanides. They are located in the seventh period of the table. To save space, the lanthanides and the actinides are often broken off and displayed below the rest of the periodic table. The periodic table in Section 2 is one example of this form of display. Periodicity The periodicity of the atomic numbers is one of the most important features of the periodic table. Consider the noble gases in Group. The first noble gas is helium, He. It has an atomic number of 2. The next element that has similar properties to helium is neon, Ne, with atomic number 10. The rest of the noble gases are argon (Ar, atomic number ), krypton (Kr, atomic number 36), xenon (Xe, atomic number 54), and radon (Rn, atomic number 6). The progression of the differences between the atomic numbers of these elements is shown at the right. The elements in Group 1 follow the same pattern. As the figure at the right shows, the atomic number of each successive element in the group is,,,, and 32 higher than the previous element. Group Group 1 Element and atomic number He 2 Ne Ar Kr Xe Rn Li Na K Rb Cs Fr 10 36 54 6 3 11 19 37 55 7 Difference in atomic numbers 32 32 In Groups 1 and, the differences between the atomic numbers of successive elements are,,,, and 32, respectively. 4. Calculate What is the pattern of differences between the atomic numbers of Groups 2 and 13 17 of the periodic table? 12 CHAPTER 5
SECTION 5.1 REVIEW VOCABULARY 1. State the periodic law. REVIEW 2. a. Who is credited with developing a method that led to the determination of standard atomic masses? b. Who discovered the periodic law? c. Who established atomic numbers as the basis for organizing the periodic table? 3. Name three sets of elements that have been added to the periodic table after Mendeleev s time. 4. How do the atomic numbers of the elements within each of Groups 1, 2, and 13 of the periodic table vary? 5. RELATING IDEAS Why are the atomic masses of the elements not strictly in increasing order in the periodic table, even though the properties of the elements are similar? For example, by atomic mass, tellurium, Te, should be in Group 17 and iodine, I, should be in Group 16, but grouping by properties has Te in Group 16 and I in Group 17. 129