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 YOYYOUU 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 t
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FMFDUSPOFHBUJWJUZ 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 $IBQUFS

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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 5IF
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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.

Nucleus á Ions 10 eź 11 p Nucleus 11 e ź 12 n0 11 pá Class Activity 12 n0 Sodium atom (Na) Sodium ion (Naá) PURPOSE To give students practice identifying positive and negative ions PROCEDURE Give students a list of elements. Ions

LESSON 6.3 Ask them to locate each element in the periodic How do ions form? table, and decide whether its atoms are likely to Some compounds are composed of particles called ions. An ion is an atom form positive or negative ions. Have students make or group of atoms that has a positive or negative charge. An atom is elec- a list of elements that are likely to form positive ions trically neutral because it has equal numbers of protons and electrons. For and another list of elements that are likely to form example, an atom of sodium (Na) has 11 positively charged protons and negative ions. 11 negatively charged electrons. The net charge on a sodium atom is zero [(à11) à (Ź11) â 0]. Positive and negative ions form when electrons are transferred Misconception Alert between atoms. Atoms of metals, such as sodium, tend to form ions by losing Many students will associate the words “losing” and one or more electrons from their highest occupied energy levels. Figure 6.17 compares the atomic structure of a sodium atom and a sodium ion. In the “gaining” with subtraction and addition, respectively. sodium ion, the number of electrons (10) is not equal to the number of pro- Make sure they understand that when an atom loses tons (11). Because there are more positively charged protons than negatively an electron, its charge becomes more positive, rather charged electrons, the sodium ion has a net positive charge. An ion with a than more negative. Similarly, when an element gains positive charge is called a cation. The charge for a cation is written as a num- ber followed by a plus sign. If the charge is 1, the number in 1à is usually an electron, it becomes more negative rather than omitted from the symbol for the ion. For example, Na1à is written as Naà. more positive. It may be helpful to remind students Atoms of nonmetals, such as chlorine, tend to form ions by gaining one or that they are adding or subtracting the total charge more electrons. Figure 6.18 compares the atomic structure of a chlorine atom of the electrons gained or lost, rather than the total and a chloride ion. In a chloride ion, the number of electrons (18) is not equal to the number of protons (17). Because there are more negatively charged number of electrons. For example, the elemental electrons than positively charged protons, the chloride ion has a net negative form has a charge of 0, and it loses a single electron, Figure 6.18 Anion Formation charge. An ion with a negative charge is called an anion. The charge for an which has a charge of −1. The charge can be When a chlorine atom gains an anion is written as a number followed by a minus sign. calculated as 0 – (−1) = 0 + 1 = +1. For chlorine, electron, it becomes a negatively charged ion. which gains an electron to become negative, the Interpret Diagrams What Gain one electron calculation would be 0 + (−1) = −1. happens to the protons and á1eź neutrons during this change?

Nucleus á 17 p 17 eź 0 18 n 18 eź Nucleus 17 pá 18 n0 Chlorine atom (Cl) Chloride ion (Clź)

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Check for Understanding How do ions form? Assess students’ knowledge about the formation of ions by asking them the following questions: a. What occurs when an atom in Group 2 becomes an ion? (It loses two electrons.) b. Is the ion that forms from a Group 2 atom called an anion or a cation? How do you know? (It is a cation because it has a positive charge, +2.)

ADJUST INSTRUCTION If students are confused, have them use a copy of the periodic table as a reference as they read the Ions lesson or as they review. Review the information provided in the Misconception Alert with students and show them how to use the group numbers shown in red in Figure 6.9 to help them determine the number of electrons lost or gained in groups 1A–7A. Note that determining the charge on transition metal ions will not be covered until Chapter 20. 176 Chapter 6 • Lesson 3 LESSON 6.3 Trends in Ionization Energy What are the trends among the elements for first ionization energy? Explain Recall that electrons can move to higher energy levels when atoms absorb energy. Sometimes the electron has enough energy to overcome the attrac- tion of the protons in the nucleus. The energy required to remove an electron Trends in Ionization Energy from an atom is called ionization energy. This energy is measured when an element is in its gaseous state. The energy required to remove the first elec- MAKE A CONNECTION Explain that ionization energy tron from an atom is called the first ionization energy. The cation produced is a measure of the difficulty in removing an electron has a 1à charge. First ionization energy tends to decrease from top to from the highest occupied energy level. Ask Why bottom within a group and increase from left to right across a period. is the first ionization energy of a nonmetal much Ionization energies can help you predict what ions an element will form. Look at the data in Table 6.1 for lithium (Li), sodium (Na), and potassium (K). higher than that of an alkali metal? (Because the The increase in energy between the first and second ionization energies is nuclear charge increases from left to right across a large. It is relatively easy to remove one electron from a Group 1A metal atom, period and the shielding effect stays the same, it is but it is difficult to remove a second electron. This difference indicates that more difficult to remove an electron.) Group 1A metals tend to form ions with a 1à charge. Misconception Alert Some students may be tempted to place a negative sign on the ionization energy value. Students Interpret Data may confuse the phrase “losing an electron” Table 6.1 The table with a negative energy value. Explain that the Ionization Energies of First 20 Elements (kJ/mol*) compares ionization energies ionization energy is the energy change associated for elements with atomic with removing one electron from a neutral atom. Symbol First Second Third numbers 1 through 20. a. Read Tables What are Explain that this energy value is always a positive H1312 the values for the first, second, number since energy must be added to the atom He (noble gas) 2372 5247 and third ionization energies Li 520 7297 11,810 for sodium and aluminum? system to remove an electron. (NOTE: The unit for Be 899 1757 14,840 b. Compare Is it easier to ionization energy is kJ/mol. The footnote in Table 6.1 remove an electron from a supplies an operational definition of mole, which is B80124303659sodium (Na) or aluminum (Al) C108623524619atom? From Naá or Alá? From introduced in Lesson 10.1.) N140228574577Na2á or Al2á? O 1314 3391 5301 c. Draw Conclusions Which ion is more common— Explore F168133756045Na3á or Al3á? Ne (noble gas) 2080 3963 6276 Na 496 4565 6912 Teacher Demo Mg 738 1450 7732 Note: The second Al 578 1816 2744 ionization energy is the PURPOSE To help students understand the concepts Si 786 1577 3229 energy needed to remove of effective nuclear charge and electron shielding P101218962910an electron from an ion with a 1à charge. This produces PURPOSE Choose four students to be “protons” and S 999 2260 3380 an ion with a 2à charge. The four students to be “electrons.” Construct a lithium Cl 1256 2297 3850 third ionization energy is the Ar (noble gas) 1520 2665 3947 energy needed to remove an “nucleus” by having three protons stand together at K41930694600electron from an ion with a the front of the room. Note that for purposes of this à Ca 590 1146 4941 2 charge. This produces an demo, you are ignoring the neutrons. ion with a 3à charge. *An amount of matter equal to the atomic mass in grams • Place two electrons together at a short distance from the nucleus to represent the 1s electrons. • Place the third electron a bit farther away to The Periodic Table 177 represent the 2s electron. You should be able to draw a line from the nucleus through the 1s electrons to the 2s electron. • Point out that there are no other electrons Differentiated Instruction between the 1s electrons and the nucleus.

L1 LESS PROFICIENT READERS Have students refer back to their KWL charts to Explain that these electrons experience the full review their notes on atomic structure. Have them use this information to identify impact of the 3+ charge because the third electron’s three of the factors that affect ionization energy: nuclear charge, number of energy “view” of the nucleus is partially blocked. Convey levels, and shielding. that this means the nucleus it is shielded somewhat from the full force of the 3+ charge. ELL ENGLISH LANGUAGE LEARNERS Use student volunteers to set up a tug-of-war game that to demonstrate why the amount of energy needed to remove successive Answers electrons increases. Assign one team to be “protons” and the other team to be FIGURE 6.18 nothing “electrons.” Show students how the amount of force exerted by the “protons” on the “electrons” increases as each “electron” is removed from the game, and that TABLE 6.1 a. sodium: 496, 4565, 6912 kJ/mol; aluminum: the “electrons” then have to pull harder against the “protons.” 578, 1816, 2744 kJ/mol L3 ADVANCED STUDENTS Have students create a three-dimensional tactile or b. Na; Al+; Al2+ technological model that depicts the energy needed to remove an atom from an c. Al3+ electron. The Periodic Table 177 Interpret
(SBQIT

Explain First Ionization Energy vs. Atomic Number Figure 6.19 This graph 2500 reveals group and period He trends for ionization energy. Trends in Ionization Energy Ne a. Read Graphs Which 2000 element in Period 2 has the USE VISUALS Direct students to Table 6.1, lowest first ionization energy? Figure 6.19, and their copy of the periodic table. Ar In Period 3? Show students how to use Table 6.1 and the 1500 N Kr b. Describe What are the group trends for first periodic table to create ordered pairs that are then Xe H P ionization energy for noble Be As plotted in the graph in Figure 6.19. Call out various 1000 Zn Cd gases and alkali metals? Mg atomic numbers and have students estimate the c. Predict If you drew a graph for second ionization

LESSON 6.3 ionization energy, and vice versa. When students 500 energy, which element would

First ionization energy (kJ/mol) Li are comfortable reading the graph, direct them to Na you have to omit? Explain. K Rb answer the questions. Cs CRITICAL THINKING Challenge students to explain 0 10 20 30 40 50 60 why the portion of the graph for Periods 4 and 5 is Atomic number different from the portion of the graph for Periods 2 and 3. (Periods 4 and 5 include transition metals, Group Trends in Ionization Energy Figure 6.19 is a graph of first ioniza- tion energy versus atomic number. Look at the data for the noble gases and whose atoms have electrons in d orbitals.) the alkali metals. In general, first ionization energy decreases from top to bottom within a group. Recall that the atomic size increases as the atomic number increases within a group. As the size of the atom increases, nuclear Explore charge has a smaller effect on the electrons in the highest occupied energy level. Less energy is required to remove an electron from this energy level, and the first ionization energy is lower. Teacher Demo Period Trends in Ionization Energy In general, the first ionization energy of representative elements tends to increase from left to right across a period. PURPOSE Students observe the relative reactivities This trend can be explained by the nuclear charge and the shielding effect. of magnesium and calcium and predict relative The nuclear charge increases across the period, but the shielding effect remains constant. As a result, there is an increase in the attraction of the reactivities for other pairs of elements. nucleus for an electron. Thus, it takes more energy to remove an electron MATERIALS 20 mL 1M HCl, two 50-mL beakers, from an atom. Figure 6.20 summarizes the group and period trends for first overhead projector, 20 cm magnesium ribbon, ionization energy. 1 g calcium

SAFETY Wear goggles for this demo. Figure 6.20 Energy generally increases Trends in First Ionization Energy PROCEDURE Pour 20 mL HCl into each beaker. First ionization energy tends to Set the beakers on an overhead projector. Coil the increase from left to right across a magnesium ribbon and drop it into one beaker. period and decrease from top to bottom within a group. Drop 1 g calcium into the other beaker. Compare Predict Which element would have the reaction rates in the two beakers. Point out the larger first ionization energy— an alkali metal in Period 2 or an the positions of the two elements in the periodic alkali metal in Period 4? table, and relate the difference in reactivity to their first and second ionization energies. Ask students Energy generally decreases to predict the relative reactivities of other pairs of

elements in Groups 1A and 2A. 178 $IBQUFS

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 EXPECTED OUTCOME The calcium fizzes in the HCl. The magnesium reacts more slowly with the HCl.

Check for Understanding What are the trends among the elements for first ionization energy and ionic size and electronegativity? Assess students’ knowledge about the trends among the elements for first ionization energy by having students use arm gestures to answer the following questions. Ask In which direction on the periodic table does the first ionization energy generally increase in value? (Accept arm gestures from left to right, and upward.) Ask In which direction on the periodic table does the first ionization energy generally decrease in value? (Accept arm gestures downward and from right to left.)

ADJUST INSTRUCTION If students are having trouble answering, have them write some of the values in Table 6.1 on the element squares in a copy of the periodic table and examine their table for ionization patterns.

178 Chapter 6 • Lesson 3 LESSON 6.3

Figure 6.21 Trends in Ionic Size Comparing Atomic and Ionic Sizes What are the trends among the elements for ionic size? This diagram compares the relative sizes of atoms and ions for selected alkali metals Explain During reactions between metals and nonmetals, metal atoms (Group 1A) and halogens (Group 7A). The tend to lose electrons and nonmetal atoms tend to gain electrons. numbers are measurements of the radii This transfer of electron has a predictable effect on the size of the given in picometers (pm). Trends in Ionic Size ions that form. Cations are always smaller than the atoms from which they form. Anions are always larger than the atoms from MAKING CONNECTIONS Relate the periodic trends which they form. Ionic size tends to increase from top to Group 1A in ionic size to those discussed earlier for atomic bottom within a group. Generally, the size of cations and anions eź Liá size. Explain that the effective nuclear charge decrease from left to right across a period. Li experienced by an electron in the highest occupied 60 Group Trends in Ionic Size Figure 6.21 compares the relative 156 orbital of an atom or ion is equal to the total eź sizes of the atoms and ions for three metals in Group 1A—lithium á nuclear charge (the number of protons) minus the (Li), sodium (Na), and potassium (K). For each of these elements, Na Na shielding effect due to electrons in lower energy the ion is much smaller than the atom. For example, the radius of 95 levels. Point out that the effective nuclear charge a sodium ion (95 pm) is about half the radius of a sodium atom 191 (191 pm). When a sodium atom loses an electron, the attraction eź determines the atomic and ionic radii. Explain á between the remaining electrons and the nucleus is increased. As K that as you proceed from left to right in any given a result, the electrons are drawn closer to the nucleus. Metals that K period, the principal quantum number, n, of the are representative elements tend to lose all their outermost elec- 133 trons during ionization. Therefore, the ion has one fewer occupied 238 highest occupied energy level remains constant, but energy level. the effective nuclear charge increases. Therefore, The trend is the opposite for nonmetals, like the halogens in atomic and ionic radii decrease as you move to the Group 7A Group 7A. Look at Figure 6.21, and compare the relative sizes of right in a period. Convey that in contrast, within the atoms and ions for fluorine (F), chlorine (Cl), and bromine eź F any group, as you proceed from top to bottom, the (Br). For each of these elements, the ion is much larger than the Fź 62 effective nuclear charge remains nearly constant, atom. For example, the radius of a fluoride ion (133 pm) is more 133 than twice the radius of a fluorine atom (62 pm). As the number ź but the principal quantum number, n, increases. of electrons increases, the attraction of the nucleus for any one e Cl ź Consequently, point out that atomic and ionic radii electron decreases. Cl 102 increase from top to bottom within a group. 181 Period Trends in Ionic Size Look ahead at Figure 6.23. From left to right across a period, two trends are visible—a gradual decrease eź Br in the size of the positive ions (cations), followed by a gradual Brź decrease in the size of the negative ions (anions). Figure 6.22 sum- Explore 120 marizes the group and period trends in ionic size. 196 Teacher Demo

PURPOSE Students observe an analogy for the effect Size of cations decreases Size of anions decreases Figure 6.22 Trends in Ionic Size of adding or removing electrons from an atom. The ionic radii for cations and MATERIALS washers or other small circular items, anions decrease from left to right across periods and increase from smaller item (such as a button) to represent the top to bottom within groups. nucleus, overhead projector PROCEDURE On the overhead projector, make a circle of washers to represent an electron cloud in a neutral atom. The washers should be touching. Place the “nucleus” in the center of the circle. Add

Size generally increases or subtract washers to mimic ion formation. With each change, adjust the circle so that the washers 5IF
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179 are still touching. Explain that the change in the diameter of the circle is analogous to the change in the effective attraction of the nuclear charge for electrons. Differentiated Instruction

L1 LESS PROFICIENT READERS Have students examine Table 6.2. Act out the meaning of electronegativity. Then have students predict if there are any elements other than noble gases that do not have a value for electronegativity. Guide students in researching their prediction. (Predictions will vary; students will most likely find electronegativity tables lacking values for manmade elements with atomic numbers greater than 102.) Note these elements rarely, if ever, have been demonstrated to Answers form compounds because their atoms exist only momentarily due to their instability. FIGURE 6.19 a. lithium; sodium ELL SPECIAL NEEDS STUDENTS Provide tactile spherical models of various atoms. b. First ionization energy decreases as atomic Have students arrange the models on a copy of the periodic table to visually convey number increases. the trend in ionic size. c. Hydrogen; it has only one electron. L3 ADVANCED STUDENTS Have students research and describe the phenomenon FIGURE 6.20 an alkali metal in Period 2 of the lanthanide contraction. Ask them to discuss how the lanthanide contraction accounts for the fact that zirconium and hafnium have virtually the same atomic The Periodic Table 179 radius even though hafnium is below zirconium in Group 4B of the periodic table. Quick Lab

Explore Purpose To use a graph to iden- Periodic Trends in Ionic Radii tify period and group trends Trends in Ionic Size Materials Procedure Use the data presented in Figure 6.23 to r
graph paper plot ionic radius versus atomic number. Quick Lab r
pencil

OBJECTIVE AfterAf completing this activity, students Analyze and Conclude will be able to identify periodic trends in ionic size. Ionic Radius vs. Atomic Number 1. Compare How does the size change when an atom forms a cation SKILLS FOCUS Using tables and graphs, predicting, 250 and when an atom forms an anion? 2. Describe How do the ionic radii vary within a group of metals? drawing conclusions 200 How do they vary within a group of nonmetals? LESSON 6.3 PREP TIME none 150 3. Describe What is the shape of a portion of the graph that corre- sponds to one period? CLASS TIME 40 minutes 100 4. Compare and Contrast Is the trend across a period similar or TEACHING TIPS 50 If time is too limited for students Ionic radius (pm) different for Periods 2, 3, 4, and 5?

to make the graph, use Figure 6.23 to answer 0 5. Explain Propose explanations for the trends you have described for Questions 1, 2, 4, and 5. You may want to reference 0 1020 3040 50 60 ionic radii within groups and across periods. the radii diagrams in the Elements Handbook on R3, Atomic number R7, R11, R15, R21, R25, and R29. EXPECTED OUTCOME Ionic radii increase from top Figure 6.23 Atomic and Ionic Radii to bottom within a group. The radii of cations and Atomic and ionic radii are an 156 Atomic radius Metal atom indication of the relative size of atoms Cation anions decrease from left to right across a period. and ions. The data listed are reported Li Metalloid atom 60 Ionic radius Anion ANALYZE AND CONCLUDE in picometers (pm). Nonmetal atom 1A 1. Cations are smaller than their atoms; anions are 8A 30 50 larger than their atoms. H 2. Ionic radii increase from top to bottom within He a group of metals or within a group of 2A 3A 4A 5A 6A 7A 156 113 83 77 70 66 62 70 nonmetals. Li Be BCNO FNe 1á 2á 3á 4á 3ź 2ź 1ź 3. Two portions of the curve slope down from left 60 44 23 15 146 140 133

to right. 191 160 143 109 109 105 102 94 4. The trend is similar for the periods. Na Mg Al Si P S Cl Ar 1á 2á 3á 4á 3ź 2ź 1ź 5. The radii increase within a group because the 95 66 51 41 212 184 181 number of occupied energy levels increases. 238 197 141 122 122 120 120 111 The radii of cations decrease across a period K Ca Ga Ge As Se Br Kr 1á 2á 3á 4á 3ź 2ź 1ź because the nuclear charge increases, the 133 99 62 53 222 198 196 shielding effect is constant, and the number 255 215 166 139 137 139 140 130 of electrons decreases. (The effect is smaller Rb Sr In Sn Sb Te I Xe 1á 2á 3á 4á 5á 2ź 1ź with anions because the number of electrons 148 112 81 71 62 221 220 increases.) 273 224 172 175 170 168 140 140 FOR ENRICHMENT Have students use the graph on Cs Ba Tl Pb Bi Po At Rn 1á 2á 3á 4á 5á page R37 to describe the periodic trend in atomic 169 134 95 84 74 size for transition metals. Have students examine how the trend for transition metals compares to the 180 $IBQUFS

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Focus on ELL

4 LANGUAGE PRODUCTION Have students work in groups or pairs to complete the lab. Review plotting data on a coordinate grid with the class. Pair students with higher proficiency in English with students of lower proficiency.

BEGINNING: LOW/HIGH Help students create a set of ordered pairs to plot on the graph. Use gestures to visually show how to plot an ordered pair on a coordinate grid.

INTERMEDIATE: LOW/HIGH Paraphrase the questions in the Analyze and Conclude section. Guide students to answer one question at a time.

ADVANCED: LOW/HIGH Direct students to read the graph aloud, pointing out trends and making predictions prior to answering the questions.

180 Chapter 6 • Lesson 3 LESSON 6.3 Trends in Electronegativity What are the trends among the elements for electronegativity? Explore In Chapters 7 and 8, you will study two types of bonds that can exist in compounds. Electrons are involved in both types of bonds. There is a prop- erty that can be used to predict the type of bond that will form during a Trends in Electronegativity reaction. This property is called electronegativity. Electronegativity is the ability of an atom of an element to attract electrons when the atom is in a START A CONVERSATION Lead a class discussion compound. Scientists use factors such as ionization energy to calculate values on periodic and group trends in electronegativities. for electronegativity. Point out that electronegativity values help chemists Table 6.2 lists electronegativity values for representative elements in predict the type of bonding that exists between Groups 1A through 7A. The elements are arranged in the same order as in the periodic table. The noble gases are omitted because they do not form atoms in compounds. Ask Why are the noble gases many compounds. The data in Table 6.2 is expressed in Pauling units. Linus not included in a discussion on electronegativity? Pauling won a Nobel Prize in Chemistry for his work on chemical bonds. He (because they form very few compounds) Ask was the first to define electronegativity. Which element represented in Table 6.2 is the In general, electronegativity values decrease from top to bottom within a group. For representative elements, the values tend to increase most electronegative and which is the least from left to right across a period. Metals at the far left of the periodic table electronegative? (fluorine; cesium) Stress that have low values. By contrast, nonmetals at the far right (excluding noble electronegativity is a calculated value rather than a gases) have high values. The electronegativity values among the transition measured quantity. metals are not as regular. The least electronegative element in the table is cesium, with an electro- APPLY CONCEPTS Explain that the values for negativity value of 0.7. It has the least tendency to attract electrons. When it electronegativity are often based on values for reacts, it tends to lose electrons and form cations. The most electronegative ionization energy and electron affinity. Explain that element is fluorine, with a value of 4.0. Because fluorine has such a strong tendency to attract electrons, when it is bonded to any other element it either ionization energy is a measure of an atom’s ability attracts the shared electrons or forms an anion. to lose electrons and electron affinity is a measure Figure 9.24, on the next page, summarizes several trends that exist of an atom’s ability to gain electrons. among the elements. Refer to this figure as you study the periodic trends presented in this chapter. Misconception Alert Students often confuse the meanings of Table 6.2 electronegativity and ionization energy. As a Electronegativity Values for Selected Elements class, brainstorm ideas for how to remember H the meanings of each term. Then have students 2.1 pictorially illustrate both concepts. Li Be B C N O F 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Na Mg Al Si P S Cl 0.9 1.2 1.5 1.8 2.1 2.5 3.0 K Ca Ga Ge As Se Br 0.8 1.0 1.6 1.8 2.0 2.4 2.8 Rb Sr In Sn Sb Te I 0.8 1.0 1.7 1.8 1.9 2.1 2.5 Cs Ba Tl Pb Bi 0.7 0.9 1.8 1.9 1.9

The Periodic Table 181

Focus on ELL

4 ENABLE LANGUAGE PRODUCTION Have students work in small groups to complete the Small-Scale Lab on page 184. Make sure each group has ELLs of varied language proficiencies, so that more proficient students can help less proficient ones. Have students work according to their proficiency level. BEGINNING LOW Model the procedure and have students mimic you. Show students how to read measures from measuring tools. HIGH Rephrase steps 1 and 3 in the procedure as several single-direction steps. INTERMEDIATE: LOW/HIGH Restate the Analyze and Conclude questions in simpler terms. Allow students to orally respond to the Analyze and Conclude questions. ADVANCED: LOW/HIGH Have students paraphrase the questions in Analyze and Conclude and You’re the Chemist and read them aloud to students with lower English proficiencies. The Periodic Table 181 Atomic size decreases Ionization energy increases Explain Electronegativity increases Nuclear charge increases USE VISUALS Direct students’ attention to Figure Shielding is constant 6.24. Point out that this diagram incorporates 1A 8A information from several diagrams earlier in the chapter. Lead a discussion as to whether students 2A 3A 4A 5A 6A 7A Ionic size increases find this or earlier diagrams more helpful, and why. Atomic size increases Ionization energy decreases Electronegativity decreases Nuclear charge increases Shielding increases

CHEMISTRY & YOYYOUU Trends in atomic size, ionization energy, ionic size, and electronegativity

LESSON 6.3 can be idendified with the help of the periodic table.

Evaluate

Size of cations decreases Size of anions decreases Informal Assessment Assign each student two elements in the same group CHEMISTRY & YYOU and have the student compare the elements in terms Q: You are familiar with using of atomic radius, ionic radius, ionization energy, and a weather map to identify trends Figure 6.24 Summary of Periodic Trends in the weather. For example, cer- Trends for atomic size, ionization energy, ionic size, and electronegativity vary electronegativity. For successful students, repeat the tain areas are typically warmer within groups and across periods. The trends that exist among these properties exercise with a metal and nonmetal from the same than other areas. What trends can be explained by variations in atomic structure. The increase in nuclear charge in the properties of elements can within groups and across periods explains many trends. Within groups, an increase period. Have students write general statements to in the number of occupied energy levels and an increase in shielding both have a you identify with the help of the significant effect on each trend. summarize the trends revealed by these comparisons. periodic table? Then, have students complete the 6.3 Lesson Check. Interpret Diagrams Which properties tend to decrease across a period? Which properties tend to decrease down a group? Reteach Review the terms used in Figure 6.24. Then, use the NLINE periodic table and the terms to play a version of “I’m O

P thinking of . . . .” For example, choose fluorine and R S O B L E M LessonCheck say you are thinking of an element that has a very 6.3 small atomic size and a very high electronegativity. Let 18. Review How does atomic size change 23. Explain In general, how can the periodic trends students guess, and then discuss the correct answer. within groups and across periods? displayed by elements be explained? Have students continue the game in small groups. 19. Explain When do ions form? 24. Sequence Arrange these elements in order of decreasing atomic size: sulfur, chlorine, alu- 20. Summarize How do first ionization ener- minum, and sodium. Does your arrangement gies vary within groups and across periods? demonstrate a periodic trend or a group trend? 21. Describe Compare the size of ions to the 25. Identify Which element in each pair has the size of the atoms from which they form. larger first ionization energy? 22. Review How do electronegativity values a. sodium, potassium vary within groups and across periods? b. magnesium, phosphorus

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Lesson Check Answers 18. Atomic size generally increases 22. Electronegativity values generally within a group and decreases from decrease from top to bottom within left to right across a period. a group and increase from left to 19. Ions form when electrons are right across a period. transferred between atoms. 23. The trends can be explained by 20. First ionization energy generally variations in atomic structure. decreases within a group and 24. sodium, aluminum, sulfur, chlorine; increases from left to right across a periodic trend period. 25. a. sodium b. phosphorus 21. Anions are larger and cations are smaller than the atoms from which they form.

182 Chapter 6 • Lesson 3 CHEMISTRY & YOU CHEMISTRYY & YOU:YOU:O EVEVERYDAYVERYDAY MATTERMATTER CHEMISTRY & YOYYOUU Have students look at Elements of Life the photos and read about the Elements of Life. Engage students in a conversation of ways the four Like everything else in the universe, your body is made up of elements. main elements of life—hydrogen, oxygen, carbon, Your body uses these elements for different functions. Roughly 97 percent of the human body consists of just four elements: oxygen, and nitrogen—are a part of their everyday lives. carbon, hydrogen, and nitrogen. The remaining 3 percent contains Students should realize that everything they do, about 20 other elements that are essential to life. every moment of the day, involves these elements. Pose the following question to students: How many CIRCULATORY SYSTEMM IrIronon andand different encounters have you had with these four oxygen are critical to the ccirculatoryirculatory elements today? You may need to assist students in system—the system thathat carriescarries blbloodood the following ways: throughout the body. Iron, whichwhich is contained in red bloodod ccells,eells, hhelpselps • activities involving eating, drinking, transport oxygen fromm the lungs to or performing personal hygiene tasks other cells in your body.dy. TwTwoo other • activities involved in getting to school elements—copper andd cocobalt—arebalt—are • activities the body carries out to live necessary for the formationmation of red blood cells. • objects encountered in nature

NERVOUS SYSTEM Sodium Explain and potassium are essential to the nervous system, in START A CONVERSATION Explain to students that particular the nerve cells. These about 97% of the atoms in the body are either elements allow your brain to hydrogen, oxygen, carbon, or nitrogen. Encourage communicate with other tissues in your body. Other elements them to think about what properties might make that are important for proper these elements so special. Explain that in later nervous system function include chapters they will learn about how atoms combine calcium, chlorine, zinc, and by forming chemical bonds. Atoms of these four magnesium. elements are small and light. Also, the arrangement of electrons in the atoms allows them to form SKELETAL SYSTEM Your bones bonds in such a way that the atoms can combine and teeth—two components into the large, stable molecules necessary to carry of the skeletal system—are out life functions. largely comprised of calcium TakeTake ItIt FurtherFurther and phosphorus, which give bones and teeth their 11.. DescribeDescribe UUsese thethe informationinformation pprovidedrovided on strength. Fluorine, boron, ppageage R1 to estimate tthehe composition ooff tthehe hhumanuman Extend magnesium, and silicon bbodyody in terms ofof metals,metals, nonmetanonmetals,ls, anandd metametalloids.lloids. are also important 2. Predict ThThee elements elements sodium, sodium, ma magnesium,gnesium, for bone growth and ppotassium,otassium, and calcium are the most abundant Connect to PHYSIOLOGY for maintaining bone metametalsls in tthehe hhumanuman bbodyody anandd are prepresentsent as ions. WhWhatat is tthehe cchargeharge ooff eaceachh ooff tthesehese ions? strength. Point out to students that even though hydrogen, 3. SeqSequenceuence Use FigureFigure 6.23 to list the ions in QueQuestionstion 2 from smallest to largest.largest. oxygen, carbon, and nitrogen are by far the most common atoms in organisms, they are not the only atoms that are necessary for life. Have students Chemistry & You 183 research the relative percentages of all the different elements present in the human body and to present their findings to the class. Differentiated Instruction

L1 STRUGGLING STUDENTS Help students make a circle graph of the distribution of elements in living organisms as a percentage of body weight. Write these numbers on the board for them to use: oxygen, 61%; carbon, 23%; nitrogen, 2.6%; hydrogen, 10%, and other elements, 3.4%. You may wish to have students use a Answers graphing program to make the graphs, or have them make the graph on the board as a class project. FIGURE 6.24 sizes of atoms and ions; ionization energy and electronegativity LPR LESS PROFICIENT READERS Have students read the feature together with a TAKE IT FURTHER partner. First, have students read a paragraph independently, and then have them discuss what they have read with their partners. Afterwards, discuss the feature 1. metals: 2%; nonmetals: 98%; metalloids: 0% together as a class. 2. sodium: 1+; magnesium: 2+; potassium: 1+; calcium: 2+ L3 ADVANCED STUDENTS Ask students to research and write a report, including 3. magnesium, sodium, calcium, potassium tables, that distinguishes between the percentages by mass and percentages by number of atoms of each of the four main elements in the body. Chemistry & You 183 Small-Scale Lab Explore Periodicity in Three Dimensions

Small-Scale Lab Purpose To build three-dimensional models for OBJECTIVE AfterAft completingl this activity, students periodic trends should be able to build concrete models to reinforce periodic trends. They should also be able to apply a procedure to a new variable and design a model on Materials their own. r
96-well spot plate r
metric ruler r
straws r
permanent fine-line PREP TIME 10 minutes r
scissors marker CLASS TIME 40 minutes MATERIALS 96-well spot plates, straws, scissors, metric rulers, permanent fine-line markers Procedure ADVANCE PREPARATION Straws with a 1/4-inch 1. Measure the depth of a well in the spot plate by inserting a straw into a well and holding the straw diameter fit snugly in the wells. upright as shown in the photograph. Make a mark TEACHING TIPS Students can use colored straws to on the straw at the point where the straw meets the color code groups or periods. If you do not have surface of the plate. Measure the distance from the

SMALL-SCALE LAB end of the straw to the mark in centimeters (cm). spot plates, press a lump of clay the size of a golf Record this distance as well depth. 3. Interpret Diagrams Relate the trend in electro- ball flat on a table with a block of wood. Students 2. Cut the straw to a length that is 4.0 cm plus well negativity across a period to the location of metals can mark out a 1-cm square grid and insert the depth. The straw will extend exactly 4.0 cm above and nonmetals in the periodic table. straws in the clay. the surface of the plate. 4. Use Models Based on your model, what is the 3. Fluorine has an electronegativity value of 4.0. On general trend in electronegativity within a group? EXPECTED OUTCOME Students produce 3-D models a scale of 1.0 cm equals 1.0 unit of electronegativ- Are there any notable exceptions? for periodic trends. ity, the portion of the straw that extends above the 5. Explain Why do you think that the electronega- surface of the plate represents the electronegativity ANALYZE AND CONCLUDE tivity value for hydrogen is so high given its location value for fluorine. Using the same scale, cut straws in the periodic table? 1. fluorine to represent the electronegativity values for all the 2. Electronegativity generally increases from left to elements listed in Table 6.2. Remember to add the You’re the Chemist right along a period. well depth to the electronegativity value before cut- 1. Design an Experiment Construct a similar ting a straw. As you cut the straws, mark each straw three-dimensional model for first ionization ener- 3. Metals, which are on the left side of the with the chemical symbol of the element that the gies. Use the data in Table 6.1 to construct the table, have lower electronegativity values than straw represents. model. Use a scale of 1.0 cm equals 300 kJ/mol. nonmetals, which are on the right. 4. Arrange the straws in the spot plate in rows and 2. Design an Experiment Design and construct columns to match the locations of the elements in 4. Electronegativity generally increases from a three-dimensional model that shows trends in the periodic table. atomic and ionic radii for the elements in Groups 1A bottom to top within a group. Except for boron, and 7A. Devise a way to display both ionic and the rest of Group 3A shows a reverse in this Analyze and Conclude atomic radii in the same model. 1. Use Models Which element represented in your trend. 3. Analyze Data Xenon has an electronegativ- model is the most electronegative? 5. Hydrogen is placed in Group 1A based on its ity value of 2.6. Cut and place a straw in your first 2. Use Models Based on your model, what is the model to represent xenon. Does xenon support the electron configuration, but is classified as a general trend in electronegativity from left to right nonmetal. trend for electronegativity across a period? Is xenon across a period? likely to form compounds? Explain your answers. FOR ENRICHMENT Have students use the data on page R37 to make a 3-D model of trends in atomic size for transition metals. 184 $IBQUFS

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-BC YOU’RE THE CHEMIST 1. Students divide the values of first ionization energies by 300 and measure the appropriate length of straws. 2. Students must determine their own scale before they begin. Students often use two wells to represent both ionic and atomic radii. Other students cut a straw to a length that represents the larger radius of an atom and mark the straw to show the smaller radius of the corresponding cation. 3. The value for xenon is similar to iodine, which is consistent with the general trend. Based on this value, xenon appears to have the ability to attract electrons and form compounds.

184 Chapter 6 • Small-Scale Lab