chem_TE_ch06.fm Page 170 Monday, April 17, 2006 8:32 AM 6.3 6.3 Periodic Trends 1 FOCUS An atom doesn’t have a sharply Objectives Guide for Reading defined boundary. So the radius of an atom cannot be measured directly. 6.3.1 Describe trends among the There are ways to estimate the sizes of atoms. In one method, a solid is Key Concepts bombarded with X rays, and the paths of the X rays • elements for atomic size. What are the trends among the are recorded on film. Sodium chloride (table elements for atomic size? 6.3.2 Explain how ions form. • How do ions form? salt) produced the geometric pattern in the 6.3.3 Describe periodic trends for • What are the trends among the photograph. Such a pattern can be used to elements for first ionization calculate the position of nuclei in a solid. first ionization energy, ionic energy, ionic size, and size, and electronegativity. electronegativity? The distances between nuclei in a solid • What is the underlying cause are an indication of the size of the parti- of periodic trends? cles in the solid. In this section, you will Guide for Reading Vocabulary learn how properties such as atomic size atomic radius are related to the location of elements in the Build Vocabulary L2 ion periodic table. cation Graphic Organizer Have students anion make a compare/contrast table for ionization energy cation and anion, and use the table to electronegativity Trends in Atomic Size decide which type of ion an element is Reading Strategy Another way to think about atomic size is to look at the units that form Building Vocabulary After you when atoms of the same element are joined to one another. These units likely to form. read this section, explain the are called molecules. Figure 6.13 shows models of molecules (molecular difference between a cation and L2 models) for seven nonmetals. Because the atoms in each molecule are Reading Strategy an anion. identical, the distance between the nuclei of these atoms can be used to Relate Text and Visuals Tell students estimate the size of the atoms. This size is expressed as an atomic radius. that they will learn about trends The atomic radius is one half of the distance between the nuclei of two related to the location of elements in atoms of the same element when the atoms are joined. The distances between atoms in a molecule are extremely small. So the the periodic table. They should look for atomic radius is often measured in picometers. Recall that there are one the visuals throughout the section that Figure 6.13 This diagram trillion, or 1012, picometers in a meter. The molecular model of iodine in lists the atomic radii of summarize the trends. Figure 6.13 is the largest. The distance between the nuclei in an iodine mol- seven nonmetals. An atomic radius is half the distance ecule is 280 pm. Because the atomic radius is one half the distance between 2 between the nuclei of two the nuclei, a value of 140 pm (280/2) is assigned as the radius of the iodine INSTRUCT atoms of the same element atom. In general, atomic size increases from top to bottom within a when the atoms are joined. group and decreases from left to right across a period. Distance between nuclei Have students read the opening para- Nucleus graph. Ask, What does the geometric Hydrogen (H2) Oxygen (O2) Nitrogen (N2) pattern indicate about the arrange- 30 pm 66 pm 70 pm ment of particles in salt? (It is orderly.) Trends in Atomic Size Atomic radius Fluorine (F2) Chlorine (Cl2) Bromine (Br2) Iodine (I2) Use Visuals L1 62 pm 102 pm 120 pm 140 pm Figure 6.13 Ask, Why can't a scien- 170 Chapter 6 tist measure the diameter of a single atom? (because an atom does not have a sharply defined border) Discuss how measuring the distance between Section Resources nuclei solves this problem. Print Technology FYI •Guided Reading and Study Workbook, •Interactive Textbook with ChemASAP, In Chapter 8 there are formal defini- Section 6.3 Animation 7, Assessment 6.3 tions of molecule and diatomic mol- •Core Teaching Resources, •Go Online, Section 6.3 ecule. The operational definition of a Section 6.3 Review molecule should be sufficient for a •Transparencies, T70–T74 discussion of atomic radii. •Small-Scale Chemistry Laboratory Manual, Lab 9 •Laboratory Practicals, 6-1, 6-2 170 Chapter 6 chem_TE_ch06_IPL.fm Page 171 Friday, August 6, 2004 12:27 AM Figure 6.14 This graph plots L2 Atomic Radius Versus Atomic Number Interpreting Graphs atomic radius versus atomic number for 55 elements. a. potassium 300 Period 4 b. It increases. Period 5 Period 3 Cs c. smaller Period 2 Rb INTERPRETING GRAPHS 250 K a. Analyzing Data Which Because of the amount of data in this alkali metal has an atomic graph, you may need to help students Na 200 radius of 238 pm? get oriented before they begin to inter- Period 1 Sc b. Drawing Conclusions Li Cd Based on the data for alkali pret the graph. 150 Zn metals and noble gases, how Enrichment Question L3 Xe does atomic size change within a group? 100 Kr Emphasize the key roles electrical Atomic radius (pm) Ar c. Predicting Is an atom of barium, atomic number 56, attraction and repulsion play within Ne 50 He smaller or larger than an atoms and ions. Review the effects of atom of cesium (Cs)? increasing nuclear charge and changes in the shielding effect of electrons on 0 10 20 30 40 50 60 the size of an atom. (Nuclear charge Atomic number increases within groups and across periods; the shielding effect increases Group Trends in Atomic Size In the Figure 6.14 graph, atomic radius is within groups, but is constant across plotted versus atomic number. Look at the data for the alkali metals periods.) Have students use these and noble gases. The atomic radius within these groups increases as the effects to describe the trends for atomic number increases. This increase is an example of a trend. atomic size within a period and within As the atomic number increases within a group, the charge on the nucleus increases and the number of occupied energy levels increases. groups. These variables affect atomic size in opposite ways. The increase in positive Discuss L2 charge draws electrons closer to the nucleus. The increase in the number of occupied orbitals shields electrons in the highest occupied energy level As an analogy to positions and trends in from the attraction of protons in the nucleus. The shielding effect is greater properties of elements in the periodic than the effect of the increase in nuclear charge. So the atomic size increases. table, use seating charts and pricing Periodic Trends in Atomic Size Look again at Figure 6.14. In general, data from local theaters or sports atomic size decreases across a period from left to right. Each element has venues to discover trends. Ask students one more proton and one more electron than the preceding element. to determine patterns that relate the Across a period, the electrons are added to the same principal energy level. position of a seat to its price. Students The shielding effect is constant for all the elements in a period. The increas- ing nuclear charge pulls the electrons in the highest occupied energy level should discover that variables such as closer to the nucleus and the atomic size decreases. Figure 6.15 summa- distance from the stage or field, location rizes the group and period trends in atomic size. relative to the center of the action, and whether the view will be obstructed, all Trends in Atomic Size affect price. Size generally decreases Figure 6.15 The size of atoms tends to decrease from left to right across a period and increase from top to bottom within a group. Predicting If a halogen and an alkali metal are in the same period, which one will Size generally increases have the larger radius? Section 6.3 Periodic Trends 171 Facts and Figures Elements and the Big Bang At the time of the Big Bang, the temperature nuclei and negative electrons. It took an esti- was many billions of degrees. Neutrons, pro- mated 500,000 years for electrons and nuclei tons, and electrons may have formed within to cool enough to form atoms. According to 10–4 second after the Big Bang, and the light- the Big Bang theory, Earth, with its wealth of est nuclei formed within 3 minutes. Matter chemical elements, formed from the debris was in the form of plasma, a sea of positive of supernova explosions. Answers to... Figure 6.15 the alkali metal The Periodic Table 171 chem_TE_ch06.fm Page 172 Monday, April 17, 2006 8:35 AM Section 6.3 (continued) Lose one electron 1e؊ Ions Nucleus 11 p Nucleus 12 n0 11 p CLASS Activity 12 n0 10 e e؊ 11 Listing Elements L2 (Give students a list of elements. Ask Sodium atom (Na) Sodium ion (Na؉ them to locate each element in the periodic table, and decide whether its Gain one electron atoms are likely to form positive or ؉1e Figure 6.16 When a sodium negative ions. Have students make a atom loses an electron, it Nucleus list of elements that are likely to form becomes a positively charged 17 p 18 n0 positive ions and another list of ele- ion. When a chlorine atom gains an electron, it becomes 17 e؊ Nucleus ments that are likely to form negative a negatively charged ion.