Periodic Trends - Atomic Radius Name Hr How and why is atomic radius a periodic trend? Model 1 The diagram to the right shows electron-electron interactions and nucleus-electron interactions in atoms of Helium and Lithium. The diagrams are not to scale. The size of the nucleus is actually much, much smaller than shown. 1. Write the electron configurations of a. Helium b. Lithium 2. Why is the third electron of Lithium drawn further away from the nucleus? 3. Is the electron-electron interaction attractive or repulsive? Why? 4. Is the electron-nucleus interaction attractive or repulsive? Why? Model 2 (Reference: Modern Chemistry pg. 150-152) FIGURE 1: Bohr Diagrams of boron, carbon and nitrogen nucleus charge = +5 nucleus charge = +6 nucleus charge = +7 Boron Carbon Nitrogen Because the nucleus is positively charged, it exerts an attractive force on the electrons. However, the three electrons in boron s outer energy level do not feel the full +5 attraction from the 5 protons in boron s nucleus. Before the +5 attraction gets to the outer energy level it gets shielded (partially cancelled) by the two electrons in the first energy level. Therefore the outer energy level only feels a +3 charge rather than a +5 charge from the nucleus. Information above for questions 1-4 Copyright 2010 by David M. Hanson and for questions 5-25 Copyright 2002-2004 by Jason Neil. All rights reserved. To make copies permission must be obtained from www.chemistryinquiry.com. Page 1 POGIL: Periodic Trends Atomic Radius
Consider the diagram of carbon. An electron in the outer energy level only feels a charge of +4 coming from the nucleus because the two electrons in the first energy level shield two of the positive charges. 5. How large is the charge that the second energy level of nitrogen feels from the nucleus? 6. Why does the first energy level in each of the three above diagrams only contain two electrons? 7. How many electrons can fit in the second energy level of any atom? 8. How many electrons can fit in the third energy level? 9. How many energy levels does aluminum have? How many electrons should be in each energy level? 10. Draw a Bohr diagram for aluminum similar to those above. 11. Explain why the second energy level of aluminum only feels a +11 attraction instead of a +13 attraction from aluminum s nucleus. 12. How large is the charge that the third energy level of an aluminum atom feels from the nucleus? Why? Model 3 (Reference: Modern Chemistry pg. 150-152) As you know, opposite charges attract. Examine the following diagrams of charged metal spheres. Diagram A Diagram B Diagram C +3-3 +4-4 +5-5 The attraction between the two charged metal spheres in each diagram is represented by an arrow. The metal spheres are pulled closer together in diagram C because of the +5 to -5 attraction is stronger than the +4 to -4 attraction in Diagram B and the +3 to -3 attraction in Diagram A. Electrons behave the same way as the metal spheres are depicted in Figure 2. Consider the Bohr diagrams of boron, carbon and nitrogen in Figure 1. Recall that Boron s outer electrons feel a +3 attraction from the Page 2
nucleus. Carbon s outer electrons feel a +4 attraction. In question 5, you found out that nitrogen s outer electrons feel a +5 attraction. This attraction between the nucleus and outer energy level determines the size of the atom. If the attraction is strong the atom is small because the electrons are pulled close to the nucleus. If the attraction is weak, the electrons spread out further from the nucleus and the atom is larger. 13. Which atom is larger: nitrogen or carbon? Why? 14. In a silicon atom, the force of attraction from the nucleus to the outer energy level is +4. Using a Bohr diagram of a silicon atom as an illustration, explain in why this is true. 15. Draw Bohr diagrams for sulfur and chlorine. 16. a) Find the size of the attraction between the nucleus and outer energy level for sulfur and for chlorine. b) Which atom do you predict to be larger: sulfur or chlorine? c) Explain, in detail, your reasoning to part b. 17. Notice and compare the locations of boron, carbon and nitrogen on the periodic table. Now compare their sizes in Figure 1. Do the same with sulfur and chlorine and your diagrams in question 15. There is a general trend in size as you proceed from left to right across the periodic table. What is this trend? In other words, how do atoms in the same row of the periodic table compare to each other in size? Page 3
Model 4 (Reference: Modern Chemistry pg. 150-152) Examine the following Bohr Diagrams of three atoms from the periodic table. Atom A Atom B Atom C 18. Give the name and atomic number of each atom from Figure 3. Atom A Atom B Atom C Name of the element Atomic Number 19. a) Concerning atoms A, B and C, what is similar about their location in the periodic table? b) Compare the force of attraction from the nucleus to the outer energy level in atoms A, B, and C. 20. Draw Bohr diagrams for neon and argon. 21. a) What is similar about the location of neon and argon in the periodic table? b) Compare the force of attraction between the outer energy level and the nucleus for neon and argon. c) Using your answers to 19b and 21b, what can be said about elements in the same column and the force of attraction between their outer energy level and nucleus? Page 4
22. Atom A has more electrons than atom B. Atom A s atomic radius is larger than atom B s. The attraction between the nucleus and the electrons in the outer energy level is equal in atoms A and B, so what other reason could there be for Atom A s larger size? Propose an explanation based on what you learned about the structure of atoms in the last unit. 23. Which is larger neon or argon? Why is this atom the larger? 24. In general, there is a trend in the sizes of atoms as you move down a column of the periodic table. a) What is this trend? b) Why does this trend exist? (Explain the basis for the trend.) 25. Order the following lists of elements in order from smallest atomic radius to largest atomic radius. a) K, As, Br b) P, Sb, N c) S, Ca, Mg, Cl # Symbol Atomic Radius, pm # Symbol Atomic Radius, pm 1 H 79 18 Ar 88 2 He 49 19 K 277 3 Li 205 20 Ca 223 4 Be 140 31 Ga 181 5 B 117 32 Ge 152 6 C 91 33 As 133 7 N 75 34 Se 122 8 O 65 35 Br 112 9 F 57 36 Kr 103 10 Ne 51 37 Rb 298 11 Na 223 38 Sr 245 12 Mg 172 49 In 200 13 Al 162 50 Sn 172 14 Si 146 51 Sb 153 15 P 123 52 Te 142 16 S 109 53 I 132 17 Cl 97 54 Xe 124 Model 5: Graphing Atomic Radius To the right is a table of the atomic radii for selected elements. On the graph, plot the atomic radius vs. atomic number. Graph the Atomic Radius for each element in Period 2 in one color. On the same graph, graph the atomic radius for each element in Period 3 in a second color, Period 4 in a third color and period 5 in a fourth color. Be sure to complete the key. Use the graph to answer the that follow. Page 5
Atomic Radius (pm) 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 3 4 5 6 7 8 9 10 11 19 12 20 13 31 14 32 15 33 16 34 17 35 18 36 37 38 49 50 51 52 53 54 Atomic Number Period 2 Period 3 Period 4 Period 5 26. How does the organization of the atomic numbers on the graph relate to the arrangement of the periodic table? Key: 27. Why does the graph skip from atomic number 20 to 31 and atomic number 38 to 49? 28. a) As you go across a period what happens to the amount of positive charge in the nucleus of an atom? b) In general, as you go across a period, what happens to the atomic radius? c) Use your answer to a) to explain your answer to b). 29. a) As you go down a group what happens to the energy level that electrons are filling? b) In general as you go down a group, what happens to the atomic radius? c) Use your answer to a) to explain your answer to b). Page 6