Periodic Trends. Alscher graphing periodic trends Page 1 of 6 11/15/16

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1 Periodic Trends The study of the property of the various elements of the periodic table and their classification into a usable system reflects two centuries of understanding and exploration of the elements. Dmitri Mendeleev is known to be the father of the periodic table, but he shares that honor with Lothar Meyer. This work was based on the experiments and discoveries of scientists such as Dobereiner, Chancourtois, and Newlands. Mendeleev and Meyer noticed that when the elements were arranged by increasing atomic mass, there was a repetition or periodic pattern. This early table was accepted because successfully predicted the existence and properties of undiscovered elements. Henry Moseley advanced the concept of atomic numbers and rearranged the table so the elements increased based on their atomic number, not their atomic mass. Moseley s arrangement of elements by atomic number resulted in a clear periodic pattern of properties. ( mass.htm is an excellent table with gobs of information). This ordering strongly enforced periodic law. Periodic law recognizes the periodic repetition of chemical and physical properties of the elements when arranged by increasing atomic number. The repeating patterns that develop in the rows of the periodic table show trends that reflect similar chemical and physical behavior in a column or family, and a gradual change in these properties across a row. In addition to repeating patterns there are clear trends. A trend is the tendency to head in a certain direction. In this lab, we hope to answer the question, Are certain properties of elements periodic functions of their atomic number? We will look at trends of the main group elements because they have the strongest presentation of periodic law and trends. You will make observations about the data and explain the trends based on Z eff, quantum effects (n), shielding, and Slater calculations. Remember that shielding and Z eff do not explain all trends. Side note: There is an inconsistency with the data from text book to text book and reference book to reference book. I chose the data that is listed in your text. Bear in mind that if you go to other sources, this data could change. HOWEVER, the concepts do not! Even if the numbers are different (ionic radius of the lithium atom 60 pm by one source and 90 pm by your text book while the size of the beryllium ion is 31 pm for the same source and 59 pm by your text book), the size of the lithium ion IS larger than the beryllium ion. This is an important part of explaining trends. The numbers help give you context, but do not explain the differences in sizes of ions and parent atoms; describing and analyzing the underlying reasons for the size variation does. Alscher graphing periodic trends Page 1 of 6 11/15/16

2 Data and Graphs: Figure 1 Atomic Radius of the first 36 elements Atomic Radius of the first 36 elements 3 ATOMIC RADIUS (ANGSTROMS) ATOMIC NUMBER Figure 2 Comparison of atomic and ionic radii &thid=OIP.M47c5cbc810528aa7ed ec60H0&ajaxhist=0 Alscher graphing periodic trends Page 2 of 6 11/15/16

3 Figure 3 Atomic radius trends for selected Representative elements 3.25 Atomic Radius (Angstroms) Atomic Radius Trend Atomic Number "Alkali Metals" (IA) Halogens (VIIA) Alkaline Earth Metals (IIA) Aluminum Family (IIIA) Carbon Family (IVA) Part 1 Investigating atomic radius: Atomic radius is the distance from the nucleus of an atom to the outer most energy level (the 90% probability). The atomic radius for the first 36 elements and the atomic radius for the main group elements are given above. We can assume that patterns that appear for the graphs are repeated with some regularity for other elements in the periodic table, due to periodic law and the organization of the periodic table. 1. Looking at FIGURE 3, what is the general trend of the atomic radius as the atomic number increases down a family? 2. Looking at FIGURE 1, what is the general trend of the atomic radius as the atomic number increases across a period? Alscher graphing periodic trends Page 3 of 6 11/15/16

4 3. Notice in FIGURE 1, that the line dips at the atomic numbers of 10, 18, and 36. What is an explanation for the dip. 4. Give an explanation for the trends you are observing in the graphed data. Part 2: Investigating Ionic Radius: Table 1 Trend data for selected elements IA IIA IIIA IVA VA VIA VIIA VIIIA Li Be B C N O F Ne g/mL 1.85/mL 2.34g/mL 2.25g/mL 1.25g/L 1.42g/L 1.70g/L 0.900g/L Na Mg Al Si P S Cl Ar g/mL 1.74g/mL 2.70g/mL 2.33g/mL 1.82g/mL 2.07g/mL 3.21g/L 1.78g/L K Ca g/mL 1.55g/mL Atomic number Rb Sr O Atomic symbol Atomic radius in pm Ionization energy 1.53g/mL 2.54g/mL 1.251g/L Density Cs Ba g/mL 3.5g/mL You will make a graph with data for the alkali and alkaline earth metals. The atomic number will go on the x- axis, while other data will go on the y- axis. These graphs are done in your lab book. The data for the graph is found in Figure 2. The values are in angstroms. 1. Make a graph of ionic radius as a function of atomic number for the alkali metals. On the same graph, use a different color to do the same for the alkaline earth metals. 2. Make a graph of ionic radius as a function of atomic number for the 2 nd period elements. Draw a smooth line connecting the cations and a smooth line connecting the anions. 3. What trends did you observe in your graphs? Alscher graphing periodic trends Page 4 of 6 11/15/16

5 4. Looking at FIGURE 2, what is the general trend of the atomic radius as the atomic number increases down a family? Is this trend the same for ionic radii? Explain. 5. Looking at FIGURE 2, what is the general trend of the atomic radius as the atomic number increases across a period? Is this this trend the same for ionic radii? Explain. 6. Give an explanation for the trends you are observing in the graphed data. Part 3: Investigating ionization energy. Table 2 Ionization energies for the 2nd period elements Atomic number Chemical symbol 1 st I.E. 2 nd I.E. 3 rd I.E. 1 H He Li , Be ,800 5 B C N O F Ne Alscher graphing periodic trends Page 5 of 6 11/15/16

6 Figure 4 Ionization energies for periods 2 & 3 Looking at FIGURE 4 and TABLE 2, what happens to the ionization energy as the atomic number increases down a family Why doesn t a Li 2+ ion form in nature? The formation of the Li 2+ ion comes at a high energy cost, yet Be 2+ does form as you can see in TABLE 2. Explain the energy jump. The ionization energies in FIGURE 4 and TABLE 2 show anomalies for oxygen in the 1 st ionization energies. Draw the box diagram of ground state nitrogen and oxygen. Draw the box diagram of the N + and O +. What happens to the orbital stability for the nitrogen and oxygen atoms vs the orbital stability the respected ions? You see a similar trend for Be and B. Alscher graphing periodic trends Page 6 of 6 11/15/16