Curry School of Education, University of Virginia www.teacherlink.org/content/science/ Periodic Table Trends: Melting Point and Boiling Point This activity allows students to explore the relationship between an element's position in the periodic table, and its respective melting and boiling points. Students will create a graph from the Excel spreadsheet data provided. The graph provides a visual tool that helps students explore and identify the temperature range associated with gas, liquid, and solid phases of individual elements and observe periodic trends for these states of matter. The Chem Data set included with this lesson is a spreadsheet containing the following information for all elements in the periodic table: • • • • • • • • • • • • • Atomic symbols Group location A-group elements: groups 1, 2, 13-18 B-group elements: the transition elements in groups 3-12 F-block elements: the lanthanide and actinide series elements Group number Element name Atomic number Atomic mass Melting point and boiling point Electronegativity value First ionization energy Covalent atomic radius Grade level 9-12 Although there may be some advantage in terms of comprehension in students' building graphs by hand, this task quickly reaches a point of diminishing returns when students are making multiple graphs of similar data. Spreadsheets allow students to easily create multiple graphical representations of data and focus their classroom time and energies on the analysis of the graphical trends depicted, rather than on the tedious task of graphing by hand. Virginia Earth Science Standards of Learning addressed in this activity include: Chemistry.2 The student will investigate and understand that the placement of elements on the periodic table is a function of their atomic structure. The periodic table is a tool used for the investigations of: • • • • Mass/atomic number Families/groups Series/periods Trends/patterns Technology: • Computers with spreadsheet software. (The screenshots and instructions in this activity are from Microsoft Excel 2000, but other spreadsheets should work similarly.) • Chem Data files (choose from one of three available formats): Excel file: ChemData.xls Text file: ChemData.txt HTML: ChemData.htm (for better print version) Other Supplies: • Access to a periodic table. 2 NOTE TO TEACHERS: These procedures are written to show you how you might use these technologies to teach science concepts. Suggested questions, approaches, and expected answers are all provided. Therefore, these activity descriptions should be used as a guide for your instructional planning, rather than as a step-by-step activity guide for students. Getting Started Chemistry teachers use the study of periodic trends to help students recognize the repeating occurrence of chemical and physical properties of the elements when they are organized in family groups on the periodic table. This "periodicity" allows a chemist to predict properties of elements and make informed choices in designing chemical reactions for specific purposes. While chemistry textbooks frequently include discussions of periodicity for atomic radius, electronegativity and ionization energies, trends also exist for other chemical and physical properties such as melting and boiling points. Based on your knowledge of the physical and chemical properties of the elements, how would you expect melting point and boiling point to vary with increasing atomic number? To find out, you will graph the melting and boiling points of the elements using a spreadsheet. To begin, open the Excel ChemData file and select Save As from the File menu to save the file using your last name as the filename. [If you are using a different spreadsheet, you may need to use the text file version of the ChemData.] 3 Graphing Your Data First, convert melting and boiling point values to the Kelvin temperature scale, in order to eliminate negative temperatures that may be confusing. Copying the element symbol, atomic number, melting point and boiling point data from Sheet 1 on the spreadsheet to columns in Sheet 2 will simplify the table. Insert a new column between the Celsius melting point and boiling point data columns for calculating the melting point values in Kelvin. Label this new column "Melting Point, K," and label a last column "Boiling Point, K." Then use the formula bar to convert Celsius melting and boiling point data to Kelvin by adding 273 to each value in these columns (see a screenshot of Sheet 2 below). Now that you have created Sheet 2 with the appropriate melting point and boiling point data, you are ready to create a graph of your data. Select the atomic number, Kelvin 4 melting point, and Kelvin boiling point data from the chart. Tip: To select the data in all three columns at the same time, first click at the top of the atomic number column. Then holding down the control button (to select data in nonadjacent columns), click also at the top of the "Melting Point, K," data and then the "Boiling Point, K," data. Data in all three columns should now be highlighted. Select the Chart Wizard (graphing icon button) from the toolbar. Then, select an XY Scatter graph (we chose "scatter with data point connected by lines without markers"). 5 Follow the additional steps in the Wizard to label the graph, X-axis and Y-axis (see Procedure 3 for an example of a completed graph). 6 As you analyze your graph, try to answer the following questions: How would you describe the trend in melting point as the atomic number increases within a period? Melting points increase and then decrease as the atomic number increases within a period. How would you describe the trend in boiling point as the atomic number increases within a period? Boiling points increase and then decrease as the atomic number increases within a period. Boiling temperatures are generally higher than melting temperatures. The greatest differences between melting and boiling temperatures occur for elements in the middle of the period. What do the low points on the graph represent? The low points in the graphical representation mark the Noble gases, found at the ends of every period in the periodic table. Compare these trends to the properties of the elements in the third period (sodium, atomic number 11 to argon, atomic number 18). What are the similarities and differences? To make your graph even more informative, you can label the regions where different phases exist. Type the label, "Solid," in the equation bar at the top of the screen and click on the green check mark just to the left. The text will appear on the chart and can be moved to an appropriate location. Repeat these steps for the other phase labels, so that your graph looks like the one below. 7 Which elements are liquids at room temperature? This is easy to illustrate with your spreadsheet. Go back to Sheet 2 of the spreadsheet and create a column for "Room Temperature, K." Enter "298" in every cell of this column. Now create a new XY scatterplot graph with the Chart Wizard including the "Room Temperature" column. Try to identify the physical state at room temperature for each element in the periodic table. Locate the two elements that exist as liquids at room temperature from your graph. 8 (Check the atomic numbers of the elements you judge to be liquid at room temperature with a periodic table to see if your deductions are accurate. The two elements are bromine, 35, and mercury, 80.) Which elements enjoy the largest liquid ranges? Those elements with the greatest difference between their boiling point and melting point values have the largest liquid range. For example, molybdenum, (# 42) and its family member tungsten, atomic number 74, both have large temperature ranges over which they exist as liquids. Which have very small liquid ranges? Those elements with the smallest differences between their boiling and melting temperatures have very small liquid ranges. For example, the noble gases argon (#18) and krypton (#36). Now try to graph the melting and boiling point trend for elements only in Group 1, the alkali metals: (Note that by selecting the "Symbol" column instead of the "Atomic #" column, the symbols will appear on the X-axis. For chart type, select "line with markers displayed at each value.") Compare it to a graph of the melting and boiling point trends for elements in Group 17, the halogens: 9 What differences or similarities do you observe? Note that hydrogen clearly does not follow the pattern of the other elements. In fact, hydrogen (which is a gas at room temperature) is not included in the alkali metals, even though it is located in Group 1 on the periodic table. For the metals in this group, the melting and boiling temperatures decrease with increasing atomic number. These metals enjoy rather large liquid temperature ranges. For the halogens, just the opposite trend is observed. These elements show increasing melting and boiling point temperatures with increasing atomic number. The small differences between the melting and boiling temperatures for the halogens indicate that these elements have narrow temperature ranges for their liquid states. Extension What other properties of elements have recognizable periodic trends? Challenge your students to try graphing some of the other data provided in the Chem Data set to see if they can identify other periodic trends. 10 In addition to informal assessments of student's graphs, you could ask them to imagine living on Venus with an ambient temperature of 740 K. Challenge your students to determine which elements would have different physical states in this new environment from their physical states on Earth? How many liquid, solid, and gas phase elements would exist on the surface of Venus? Ask students to imagine how life might be different under these circumstances. How successful would you be using a lead tool on Venus? How about the outer planets of our solar system? Use the Internet to locate ambient temperatures on Pluto or Uranus. Which elements would exist in the gas or liquid phases on these planets far from the sun? An alternative assessment strategy would be to ask students to create graphs showing the melting point and boiling point data for the first two groups of elements and the first two periods of elements on the periodic table. Ask students to summarize the trends they observe for these elements. (See graphs below.) 11 Real World Connections: Mendeleev's Periodic Table, 1869 Check out Mendeleev's original periodic table at this site: http://chemlab.pc.maricopa.edu/periodic/foldedtable.html EnvironmentalChemistry.com: Periodic Table of the Elements. Comprehensive data on elements including scores of properties, element names in many languages and most known nuclides is provided by this periodic table. In addition environmental and chemistry articles round out this site. http://klbproductions.com/yogi/periodic/ WebElements Visit WebElements, a site that claims to be the first periodic table on the WWW. This web site features a printable version of the periodic table. http://www.webelements.com/webelements/scholar Contact: Randy L. Bell Asst Professor of Science Education Curry School of Education University of Virginia Charlottesville, VA 22904 email: [email protected] 12