1 Organizing the Elements

Name Class DateCHAPTER 51 Organizing the ElementsSECTIONThe Periodic TableKEY IDEASAs you read this section, keep these questions in mind:• How did Dmitri Mendeleev organize his periodic table?• How are the elements arranged in a modern periodictable?How Are the Elements Organized?People organize things so that a particular item iseasier to find. For example, a person may organize herCDs by the names of the bands. Another person mayorganize his books by subject.In the 1860s, scientists knew of about 60 elements.A Russian schoolteacher named Dmitri Mendeleev wastrying to organize these elements. Mendeleev listed theproperties of each element on a separate piece of paper.He then tried to arrange the pieces of paper into rowsand columns so that they formed a pattern.MENDELEEV’S PERIODIC TABLEIn 1869, Mendeleev published the first periodic tableof the elements. In his periodic table, Mendeleevarranged elements in rows by increasing atomic mass.Within a row, elements with lower atomic masses wereon the left. Mendeleev started a new row every time thechemical properties of the elements repeated. Thus, allthe elements in a column had similar properties.READING TOOLBOXOrganize Create a SpiderMap that has two legs andseveral lines on each leg.Use the map to compareMendeleev’s periodic tablewith the modern periodictable.READING CHECK1. Identify What propertydid Mendeleev use toorganize his periodic table?Each row of Mendeleev’s periodic table represented a repeating pattern.Because the pattern of chemical properties repeated by rows, all elementslined up in a column had similar properties.Copyright © by Holt, Rinehart and Winston. All rights reserved.Interactive Reader 87 The Periodic Table

Name Class DateSECTION 1Organizing the Elements continuedREADING CHECK2. Explain Why didMendeleev leave gaps in theperiodic table?PREDICTING ELEMENTSWhen Mendeleev arranged the elements in a table, heleft gaps, or spaces, in the table. Look again at Mendeleev’stable on the last page. Notice that Mendeleev includedquestion marks in his table. The question marks representedelements with certain properties that scientists hadnot discovered yet. Because he saw patterns of chemicalproperties, Mendeleev predicted that scientists wouldeventually find the elements that filled those gaps.Mendeleev was not the only person to develop aperiodic table. However, he was the first to use the tableto make predictions. For example, Mendeleev left a spacein his table for an element after silicon. He predicted thatthis element would be a gray metal that had a highmelting point. In 1886, the element germanium wasdiscovered.Properties of GermaniumMendeleev’spredictionActualPropertyAtomic mass 70 72.6Density 5.5 g/cm 3 5.3 g/cm 3Appearance Dark gray metal Gray metalloidMelting point High 937 °CGermanium has the properties similar to those that Mendeleev predicted.READING CHECK3. Explain Why didMendeleev have problemsarranging the elements Teand I?PROBLEMS WITH MENDELEEV’S TABLEMendeleev found that some elements did not fit thepattern in his table. For example, Mendeleev had to placetellurium (Te) before iodine (I) in his table so that theyfit the pattern of chemical properties. However, whenhe switched Te and I, they were no longer in order ofincreasing atomic mass.Mendeleev thought that the values for the atomicmasses of Te and I might be incorrect. He thought thatcareful measurements would show that the atomic massof Te was actually less than that of I. However, measurementsby other scientists showed that the atomic massesof the two elements were correct. This problem wasfinally solved about 40 years later by an English chemistnamed Henry Moseley.Copyright © by Holt, Rinehart and Winston. All rights reserved.Interactive Reader 88 The Periodic Table

Name Class DateSECTION 1Organizing the Elements continuedHow Is the Periodic Table Organized Today?Unlike Mendeleev, Mosely did not organize elementsby increasing atomic masses. Instead, he organized theelements into a periodic table by atomic number. Recallthat an element’s atomic number is the number ofprotons in an atom of the element.The new way of organizing did not change thelocations of most elements in the periodic table.However, a few elements, including Te and I, did move.Although Te has a higher atomic mass than I, it hasa lower atomic number. Thus, Mosely could place Tebefore I in the periodic table without disturbing thepattern of chemical properties.The modern periodic table has more than 100 elements,and organizes the elements by atomic number. Becauseelements are arranged by atomic number, elementswith similar properties are located in the same column.Therefore, the properties of the elements on the periodictable repeat at regular intervals. This principle is known asthe periodic law.PERIODS AND GROUPSA horizontal row on the periodic table is called aperiod. There are seven periods on the periodic table. Theproperties of the elements in a period are different. Avertical column on the periodic table is called a group. Allthe elements in a group have similar chemical properties. READING CHECK4. Identify What propertydid Mosely use to organizethe periodic table?5. Compare Whichinformation helps youpredict the properties of anelement—the period it is in,or the group it is in? Explainyour answer. Copyright © by Holt, Rinehart and Winston. All rights reserved.Interactive Reader 89 The Periodic Table

Name Class DateSection 1 ReviewSECTION VOCABULARYgroup a vertical column of elements in theperiodic table; elements in a group sharechemical propertiesperiod in chemistry, a horizontal row ofelements in the periodic tableperiodic law the law that states that therepeating chemical and physical properties ofelements change periodically with the atomicnumbers of the elements1. Compare Find oxygen, sulfur, and fluorine in the periodic table in the back of thebook. Are the chemical properties of oxygen more similar to those of sulfur or tothose of fluorine? Explain your answer.2. Identify Complete the table below to describe several elements. Use the periodictable in the back of the book to help you. Round off atomic masses to the nearestwhole number. For example, change 15.9994 to 16.Element Symbol Atomic number Atomic mass Period GroupGold474 2Fe3. Predict If scientists found element 117, into which period and group wouldthey place it? Identify one element that would have properties similar to those ofelement 117.4. Infer Before 1937, scientists had not found element 43. Chemists predicted theproperties of element 43. How was it possible for chemists to predict theseproperties?Copyright © by Holt, Rinehart and Winston. All rights reserved.Interactive Reader 90 The Periodic Table