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DRAFT 01/05 PERIODICITY Teacher Resource Graphing Trends Part I: Summary of Periodic Trends In your groups, share the graphs of physical properties that you created. Based on the trends you see in these graphs, complete the following table together. SUMMARY OF PERIODIC TRENDS Trend From Left to Property Definition Trend Down a Group Right Across a Period The distance between nuclei of two bonded Decreases Increases Atomic Radius atoms. The energy required to remove a mole of Increases Decreases First Ionization Energy electrons from gaseous atoms. The number of electrons in the outermost Number of Valence Increases Constant energy level of an Electrons atom. The temperature at which a solid turns Decreases Increases Melting Point into a liquid. The ability of a Decreases Increases Thermal Conductivity substance to conduct heat. Part II: Explaining Periodic Trends Use the trends discovered in the graphs to answer the following questions. The easier it is to lose an electron the smaller the amount of energy needed to remove it (smaller ionization energy). 2. What happens to the amount of positive charge in the nucleus as you go from left to right across a period? The amount of positive charge increases as the number of protons increases from left to right across a period. a) How would this account for the trend you discovered in atomic radius? Electrons are held more tightly and are pulled in closer by the nucleus as the number of protons increases. CHEMISTRY • PERIODICITY Y • © 2005 MCPS SCIENCE 55 DRAFT 01/05 PERIODICITY b) How would this account for the trend you discovered in first ionization energy? Electrons are held more tightly by the nucleus, making it harder to lose them. 3. What is different about the number of energy levels of electrons as you go down a group? The number of energy levels increases as you go down a group. a) How would this account for the trend you discovered in atomic radius? More energy levels of electrons take up more space, and the valence electrons cannot be held as tightly by the nucleus since they are farther away. b) How would this account for the trend you discovered in first ionization energy? The electrons are not held as tightly by the nucleus, and can therefore be lost more easily. It takes less energy to remove them. 4. What is the relationship between the group and the number of valence electrons? The group number is equal to the valence electrons. 5. Which properties from the Summary of Periodic Trends table describe metallic character? Melting point and thermal conductivity a) In general, what happens to the metallic character that elements have as you go from left to right across a period? Metallic character decreases. b) In general, what happens to the metallic character that elements have as you go down a group? Metallic character increases. 56 CHEMISTRY • PERIODICITY Y • © 2005 MCPS SCIENCE