Background Periodic trends are the patterns observed in elemental properties across a row or down a column on the Periodic Table. Some of these trends were observed when the Periodic Table was first being constructed. It helped scientists, like Dmitri Mendeleev, group certain elements together and observe the periodicity of the elements. Later discoveries found that the characteristic properties of elemental families can be attributed to the valence electrons in the outermost electron shell of each element. Having the same number of valence electrons means having similar chemical properties as the atoms of each element gain or lose electrons to achieve a stable octet of electrons. The exception to this trend include hydrogen and helium, which will achieve a duet. All periodic trends can be explained by three simple concepts: attraction of the nucleus for the valence electrons (opposite charges attract), the repulsion of the electrons from each other (like charges repel), and the concept of energy levels within the atom (that the valence electrons become further from the nucleus as energy levels are added.) When comparing different periodic trends, you will find that some exhibit a positive correlation when one value increases, the other value increases. An can be seen in Graphs One and Two below. Different periodic trends may also exhibit a negative correlation when one value increases, the other value decreases. An example can be seen in Graphs Two and Three. Regardless of the type of correlation, periodic trends are related to each other and can be explained by using the three concepts listed above. Complete the Background questions in your Student Journal. 1
Putting It All Together In this activity, you will be given 20 specific elements to investigate to create three graphs to help you analyze patterns. First, you will graph the atomic radii, ionic radii, and 1 st ionization energies in relation to the elements atomic numbers. Then, you will analyze your graphs and the information given on the Student Reference Sheets to determine periodic trends. Procedure: 1. For your first two graphs, the x-axis should contain the atomic numbers of the 20 elements that you will be investigating. Make sure that the numbers are in numerical order. The y-axis should represent the radii, in picometers (pm), of the atoms and ions that you will be investigating. Check the largest radius and plan your axis accordingly. 2. On your first sheet of graph paper, plot the atomic radii in pencil and label each data point with the correct element symbol. When all elements have been plotted, connect the data points with a ruler in order of increasing atomic number. Make sure to include a title for your graph, and label the x-axis and the y-axis. Color each data point with a blue pencil or blue highlighter. 3. On a second sheet of graph paper, create a similar graph for ionic radii in pencil and label each data point with the correct element symbol. When all elements have been plotted, connect the data points with a ruler in order of increasing atomic number. Make sure to include a title for your graph, and label the x-axis and the y-axis. Color each data point with a red pencil or red highlighter. 4. On a third sheet of graph paper, create a graph for the ionization energy of the same 20 elements. The x-axis will include the atomic number of the element, and the y-axis will include the ionization energy values. Plan your axes accordingly. Label each data point with the correct element symbol and connect the data points with a ruler in order of increasing atomic number. Make sure to include a title for your graph and label the x-axis and the y-axis. 5. Each graph contains evidence of characteristic trends found within the elements. Use a pencil to connect the values for the Alkali metals, then color over that line with the same color you used when color-coding the periodic table of elements. Next connect the values for the Alkaline earth metals in the same color used on the color-coded periodic table of elements. Connect the data points for each of the other three families: Aluminum group, Oxygen group, and the Halogens. 6. Create a legend on the each graph to KEY which colors represent each element group. 2
Putting it All Together Use the Student Reference Sheet and your graphs to help you answer the following questions. Do you recognize any trends from this information alone? 1. Which element below has the smallest atomic radius? Beryllium (Be) Oxygen (O) Sodium (Na) Fluorine (F) 2. Which element below has the smallest ionic radius? Lithium (Li) Sodium (Na) Rubidium (Rb) Potassium (K) 3. Which element below has the lowest electronegativity? Calcium (Ca) Gallium (Ga) Selenium (Se) Bromine (Br) 4. Which element below has the highest ionization energy? Strontium (Sr) Beryllium (Be) Magnesium (Mg) Calcium (Ca) 5. Which element has the highest electronegativity on the Periodic Table? 6. Which representative element has the largest atomic radius? 7. Place the elements below in order of decreasing ionization energy. Aluminum (Al) Chlorine (Cl) Magnesium (Mg) Sulfur (S) [largest] [smallest] 8. Place the elements in order of increasing atomic radius. Gallium (Ga) Nitrogen (N) Indium (In) Tellurium (Te) [smallest] [largest] 9. Choose which of the two is larger, the atom or its ion. A. Magnesium atom (Mg) OR Magnesium ion (Mg 2+ ) B. Oxygen atom (O) OR Oxygen ion (O 2- ) 3
Putting It All Together, continued Interpret this graph of electronegativities and answer the questions found below. 10. What do you notice about the order of the elements on this graph? How do the rows on this graph relate to the Periodic Table? 11. What element shown on this graph has the highest electronegativity? 12. Which element shown on this graph has the lowest electronegativity? 13. Explain how the information shown on this graph can help you identify the periodic trend of electronegativity on the Periodic Table. 4
Putting It All Together, continued 14. Now that you have completed your graphs, analyze them to draw vertical arrows on the right and horizontal arrows on the top of the Periodic Table below to show the direction of the trends. Follow these instructions: Draw blue arrows for the increasing atomic radii trend. Draw red arrows for the increasing ionic radii trend. Draw black arrows for the increasing 1 st ionization energy trend. Draw brown arrows for the increasing electronegativity trend. 5
Reflection and Conclusions 1. Across a period, the ionic radii of the anions is larger than that of the cations. Use what you know about ions and electric charges to help explain this trend. 2. Fluorine is the most electronegative element on the Periodic Table. Use what you know about the halogens to help explain why fluorine is such a highly electronegative element. (Hint: It has to do with electrons.) 3. The ionization energy of an element relates to the amount of energy that is required to remove an electron from a neutral atom. The first ionization energy is the amount of energy it takes to remove the first electron. Use the trends you have learned to explain why the first ionization energy for lithium is 520 kj/mol, while the first ionization energy for oxygen is 1314 kj/mol. What characteristics of these two neutral atoms could account for the large difference in energy? 6