Key Objectives 6.3.1 DESCRIBE trends among elements for 6.3 Periodic Trends atomic size. 6.3.2 EXPLAIN how ions form. 6.3.3 DESCRIBE trends for first ionization energy, ionic size, and electronegativity. CHEMISTRY & YOUY Additional Resources Q: How are trends in the weather similar to trends in the properties of elements? Although the weather changes from day to day. The weather Reading and Study Workbook, Lesson 6.3 you experience is related to your location on the globe. For example, LESSON 6.3 Available Online or on Digital Media: Florida has an average temperature that is higher than Minnesota’s. Similarly, a rain forest receives more rain than a desert. These differ- • Teaching Resources, Lesson 6.3 Review ences are attributable to trends in the weather. In this lesson, you will • Small-Scale Chemistry Laboratory Manual, Lab 9 learn how a property such as atomic size is related to the location of an element in the periodic table. Key Questions Trends in Atomic Size What are the trends among the What are the trends among the elements for atomic size? ? elements for atomic size One way to think about atomic size is to look at the units that form How do ions form? when atoms of the same element are joined to one another. These What are the trends among the units are called molecules. Figure 6.14 shows models of molecules Engage elements for first ionization energy, (molecular models) for seven nonmetals. Because the atoms in each ionic size, and electronegativity? molecule are identical, the distance between the nuclei of these atoms CHEMISTRY YOUYOY U Have students read the can be used to estimate the size of the atoms. This size is expressed & Vocabulary opening paragraph. As a class, discuss the seasonal as an atomic radius. The atomic radius is one half of the distance tBUPNJDSBEJVT between the nuclei of two atoms of the same element when the atoms trends in weather typical for your region of the tJPO are joined. country. Ask How is knowing the trends in weather tDBUJPO The distances between atoms in a molecule are extremely small. for a specific region helpful? (Sample answer: It can tBOJPO So the atomic radius is often measured in picometers (pm). Recall that tJPOJ[BUJPOFOFSHZ 12 help in determining the type of plants that will grow there are one trillion, or 10 , picometers in a meter. The molecular tFMFDUSPOFHBUJWJUZ model of iodine in Figure 6.14 is the largest. The distance between the well in your yard.) Have students consider how nuclei in an iodine molecule is 280 pm. Because the atomic radius is knowing trends in elemental properties might be one half the distance between the nuclei, a value of 140 pm (280/2) is helpful to scientists. Figure 6.14 Atomic Radii assigned as the radius of the iodine atom. In general, atomic size This diagram compares the atomic radii increases from top to bottom within a group and decreases from Access Prior Knowledge of seven nonmetals. left to right across a period. Distance between nuclei Ask student volunteers to summarize what they have Nucleus learned about the organization of the periodic table. Hydrogen (H2) Oxygen (O2) Nitrogen (N2) Have students compare and contrast properties and 30 pm 66 pm 70 pm characteristics of periods with those of groups. Atomic radius Fluorine (F2) Chlorine (Cl2) Bromine (Br2) Iodine (I2) 62 pm 102 pm 120 pm 140 pm National Science Education Standards 174 $IBQUFSt-FTTPO A-1, A-2, B-1, B-2, B-3 Focus on ELL 1 CONTENT AND LANGUAGE Have students write the lesson title Periodic Trends in their vocabulary notebook. Have students determine the common definitions of each word and write them in their notebook. Have students predict the lesson content based on these definitions and what they have learned previously about the periodic table. 2 FRONTLOAD THE LESSON Provide four sets of photos of fashion trends from the 1950s, the 1970s, the 1990s, and the current decade. Ask students to identify the fashion trends for each decade. Explain that a trend is a pattern over time. Then ask students if they notice any similarities between clothes in the pictures. Explain that in fashion, and in element properties, trends repeat periodically. 3 COMPREHENSIBLE INPUT Play “The Elements” song written by Tom Lehler. Use this as a tool for engaging students’ interest in the content. Point out that, even though the element names are never repeated, sections of the music are repeated. 174 Chapter 6 • Lesson 3 Similarly, trends in the periodic table are repeated in each period. LESSON 6.3 Interpret(SBQIT Foundations for Reading Atomic Radius vs. Atomic Number Figure 6.15 This graph plots 300 atomic radius versus atomic BUILD VOCABULARY Have students‘ attention make Period 4 Period 5 Period 3 Cs number for elements with a compare/contrast table for cation and anion, and Period 2 Rb atomic numbers from 1 to 55. 250 use the table to decide which type of ion an element K a. Read Graphs Which alkali metal has an atomic is likely to form. Na radius of 238 pm? 200 READING STRATEGY Tell students that they will Period 1 b. Draw Conclusions Sc Li Cd Based on the data for alkali be learning about trends related to the location of 150 Zn metals and noble gases, how elements in the periodic table. Guide students to does atomic size change within Xe a group? read the visuals throughout the lesson closely, as the 100 Kr Atomic radius (pm) Ar c. Predict Is an atom of visuals summarize the trends described in the text. barium, atomic number 56, Ne 50 He smaller or larger than an atom of cesium (Cs)? Explain 0 10 20 30 40 50 60 Atomic number Trends in Atomic Size USE VISUALS Guide students’ attention to Figure Group Trends in Atomic Size Look at the data for the alkali metals and noble gases in Figure 6.15. The atomic radius within these groups increases as 6.14. Ask What is the main reason why a scientist the atomic number increases. This increase is an example of a trend. cannot measure the diameter of a single atom? As the atomic number increases within a group, the charge on the (because an atom does not have a sharply defined nucleus increases and the number of occupied energy levels increases. These border) Discuss how measuring the distance variables affect atomic size in opposite ways. The increase in positive charge draws electrons closer to the nucleus. The increase in the number of occupied between nuclei solves this problem. (NOTE: In orbitals shields electrons in the highest occupied energy level from the attrac- Chapter 8 there are formal definitions of molecule tion of protons in the nucleus. The shielding effect is greater than the effect of and diatomic molecule. The operational definition the increase in nuclear charge, so the atomic size increases. of a molecule should be sufficient for a discussion Period Trends in Atomic Size Look again at Figure 6.15. With increas- of atomic radii.) ing atomic number, each element has one more proton and one more elec- CRITICAL THINKING Emphasize the key roles tron than the preceding element. Across a period, the electrons are added to the same principal energy level. The shielding effect is constant for all the electrical attraction and repulsion play within atoms elements in a period. The increasing nuclear charge pulls the electrons in and ions. Review the effects of increasing nuclear the highest occupied energy level closer to the nucleus, and the atomic size charge and changes in the shielding effect of decreases. Figure 6.16 summarizes the group and period trends in atomic size. electrons on the size of an atom: nuclear charge increases within groups and across periods; the Size generally decreases Figure 6.16 Trends in Atomic Size shielding effect increases within groups, but it is The size of atoms tends to decrease from left to right across a period and constant across periods. Have students use these increase from top to bottom within effects to describe the trends for atomic size within a group. Predict If a halogen and an alkali a period and within groups. metal are in the same period, which USE AN ANALOGY As an analogy to positions and one will have the larger radius? trends in properties of elements in the periodic INETIC K See periodic trends table, use seating charts and pricing data from Size generally increases animated online. A R T local theaters or sports venues to discover trends. Instruct students to determine patterns that relate 5IF1FSJPEJD5BCMF 175 the position of a seat to its price. Students should discover that variables such as distance from the stage or field, location relative to the center of the action, and whether the view will be obstructed, all affect price. Elements and the Big Bang At the time of the Big Bang, the temperature was many billions of degrees. Neutrons, protons, and electrons may have formed within 10–4 second after the Big Bang, and the lightest nuclei formed within 3 minutes. Matter was in the form of plasma, a sea of positive nuclei and negative electrons. It took an estimated 500,000 years for electrons and nuclei to cool enough to form atoms. According to the Big Answers Bang theory, Earth, with its wealth of chemical elements, formed from the debris of FIGURE 6.15 supernova explosions. a. potassium b. It increases with increasing atomic number. c. smaller FIGURE 6.16 the alkali metal The Periodic Table 175 Figure 6.17 Cation Formation Lose one electron When a sodium atom loses an ź1eź electron, it becomes a positively Explore charged ion.