Page 1 of 7 KEY CONCEPT Elements make up the periodic table. BEFORE, you learned NOW, you will learn • Atoms have a structure • How the periodic table is • Every element is made from organized a different type of atom • How properties of elements are shown by the periodic table VOCABULARY EXPLORE Similarities and Differences of Objects atomic mass p. 17 How can different objects be organized? periodic table p. 18 group p. 22 PROCEDURE MATERIALS period p. 22 buttons 1 With several classmates, organize the buttons into three or more groups. 2 Compare your team’s organization of the buttons with another team’s organization. WHAT DO YOU THINK? • What characteristics did you use to organize the buttons? • In what other ways could you have organized the buttons? Elements can be organized by similarities. One way of organizing elements is by the masses of their atoms. Finding the masses of atoms was a difficult task for the chemists of the past. They could not place an atom on a pan balance. All they could do was find the mass of a very large number of atoms of a certain element and then infer the mass of a single one of them. Remember that not all the atoms of an element have the same atomic mass number. Elements have isotopes. When chemists attempt to measure the mass of an atom, therefore, they are actually finding the average mass of all its isotopes. The atomic mass of the atoms of an element is the average mass of all the element’s isotopes. Even before chemists knew how the atoms of different elements could be different, they knew atoms had different atomic masses. Chapter 1: Atomic Structure and the Periodic Table 17 DB Page 2 of 7 Mendeleev’s Periodic Table In the early 1800s several scientists proposed systems to organize the elements based on their properties. None of these suggested methods worked very well until a Russian chemist named Dmitri Mendeleev (MENH -duh-LAY-uhf) decided to work on the problem. In the 1860s, Mendeleev began thinking about how he could organ- ize the elements based on their physical and chemical properties. He made a set of element cards. Each card contained the atomic mass of an atom of an element as well as any information about the element’s properties. Mendeleev spent hours arranging the cards in various ways, looking for a relationship between properties and atomic mass. The exercise led Mendeleev to think of listing the elements in a chart. In the rows of the chart, he placed those elements showing similar chemical properties. He arranged the rows so the atomic masses increased as one moved down each vertical column. It took Mendeleev quite a bit of thinking and rethinking to get all the relation- ships correct, but in 1869 he produced the first periodic table of the elements. We call it the periodic table because it shows a periodic, or repeating, pattern of properties of the elements. In the reproduction of Mendeleev’s first table shown below, notice how he placed carbon (C) and silicon (Si), two elements known for their similarities, in the same row. check your reading What organizing method did Mendeleev use? Dmitri Mendeleev (1834–1907) first published a periodic table of the elements in 1869. DB 18 Unit: Chemical Interactions Page 3 of 7 Predicting New Elements When Mendeleev constructed his table, he left some empty spaces where no known elements fit the pattern. He predicted that new ele- ments that would complete the chart would eventually be discovered. He even described some of the properties of these unknown elements. At the start, many chemists found it hard to accept Mendeleev’s predictions of unknown elements. Only six years after he published the table, however, the first of these elements—represented by the question mark after aluminum (Al) on his table—was discovered. This element was given the name gallium, after the country France (Gaul) where it was discovered. In the next 20 years, two other elements Mendeleev predicted would be discovered. The periodic table organizes the atoms of the MAIN IDEA WEB Make a main idea web elements by properties and atomic number. to summarize the infor- The modern periodic table on pages 20 and 21 differs from Mendeleev’s mation you can learn from the periodic table. table in several ways. For one thing, elements with similar properties are found in columns, not rows. More important, the elements are not arranged by atomic mass but by atomic number. Reading the Periodic Table Each square of the periodic table gives particular information about the atoms of an element. 1 2 1 atomic chemical The number at the top of the square is the atomic number, number symbol which is the number of protons in the nucleus of an atom of that element. 2 The chemical symbol is an abbreviation for the element’s 1 name. It contains one or two letters. Some elements that have not yet been named are designated by temporary three-letter symbols. H 3 The name of the element is written below the symbol. Hydrogen 4 The number below the name indicates the average 1.008 atomic mass of all the isotopes of the element. 3 name 4 atomic The color of the element’s symbol indicates the physical mass state of the element at room temperature. White letters—such as the H for hydrogen in the box to the right—indicate a gas. Blue letters indicate a liquid, and black letters indicate a solid. The background colors of the squares indicate whether the element is a metal, nonmetal, or metalloid. These terms will be explained in the next section. Chapter 1: Atomic Structure and the Periodic Table 19 DB Page 4 of 7 The Periodic Table of the Elements 1 1 1 H Hydrogen 1.008 2 Period 3 4 2 Li Be Each row of the periodic table is called Lithium Beryllium a period. As read from left to right, 6.941 9.012 one proton and one electron are added from one element to the next. 11 12 3 Na Mg Sodium Magnesium 22.990 24.305 3 4 5 6 7 8 9 19 20 21 22 23 24 25 26 27 4 K Ca Sc Ti V Cr Mn Fe Co Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt 39.098 40.078 44.956 47.87 50.942 51.996 54.938 55.845 58.933 37 38 39 40 41 42 43 44 45 5 Rb Sr Y Zr Nb Mo Tc Ru Rh Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium 85.468 87.62 88.906 91.224 92.906 95.94 (98) 101.07 102.906 55 56 57 72 73 74 75 76 77 6 Cs Ba La Hf Ta W Re Os Ir Cesium Barium Lanthanum Hafnium Tantalum Tungsten Rhenium Osmium Iridium 132.905 137.327 138.906 178.49 180.95 183.84 186.207 190.23 192.217 87 88 89 104 105 106 107 108 109 7 Fr Ra Ac Rf Db Sg Bh Hs Mt Francium Radium Actinium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium (223) (226) (227) (261) (262) (266) (264) (269) (268) 58 59 60 61 62 Group Ce Pr Nd Pm Sm Cerium Praseodymium Neodymium Promethium Samarium Each column of the table is called a 140.116 140.908 144.24 (145) 150.36 group. Elements in a group share similar properties. Groups are read 90 91 92 93 94 from top to bottom. Th Pa U Np Pu Thorium Protactinium Uranium Neptunium Plutonium 232.038 231.036 238.029 (237) (244) Metal Metalloid NonmetalFe SolidHg LiquidO Gas DB 20 Unit: Chemical Interactions Page 5 of 7 18 2 Metals and Nonmetals He This zigzag line separates Helium metals from nonmetals. 13 14 15 16 17 4.003 5 6 7 8 9 10 B C N O F Ne Boron Carbon Nitrogen Oxygen Fluorine Neon 10.811 12.011 14.007 15.999 18.998 20.180 13 14 15 16 17 18 Al Si P S Cl Ar Aluminum Silicon Phosphorus Sulfur Chlorine Argon 10 11 12 26.982 28.086 30.974 32.066 35.453 39.948 28 29 30 31 32 33 34 35 36 Ni Cu Zn Ga Ge As Se Br Kr Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton 58.69 63.546 65.39 69.723 72.61 74.922 78.96 79.904 83.80 46 47 48 49 50 51 52 53 54 Pd Ag Cd In Sn Sb Te I Xe Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon 106.42 107.868 112.4 114.818 118.710 121.760 127.60 126.904 131.29 78 79 80 81 82 83 84 85 86 Pt Au Hg Tl Pb Bi Po At Rn Platinum Gold Mercury Thallium Lead Bismuth Polonium Astatine Radon 195.078 196.967 200.59 204.383 207.2 208.980 (209) (210) (222) 110 111 112 Ds Uuu Uub Lanthanides & Actinides Darmstadtium Unununium Ununbium (269) (272) (277) The lanthanide series (elements 58–71) and actinide series (elements 90–103) are usually set apart from the rest of the periodic table. 63 64 65 66 67 68 69 70 71 Eu Gd Tb Dy Ho Er Tm Yb Lu Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium 151.964 157.25 158.925 162.50 164.930 167.26 168.934 173.04 174.967 95 96 97 98 99 100 101 102 103 Am Cm Bk Cf Es Fm Md No Lr Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium (243) (247) (247) (251) (252) (257) (258) (259) (262) Atomic Number Symbol number of protons 1 Each element has a symbol.