Created by Lori A. Watson, Earlham College ( watsolo@earlham.edu) and posted on VIPEr on March 28, 2010. Copyright Lori A. Watson 2010. This work is licensed under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.
Athletic Periodic Trends: Instructor Notes and Solutions
O, O2-, S, F, F-, Ar, K+, K, Li, Na, Rb, Cs, C, N, O, Cl, Si, Li+, N3-
For general chemistry, these are the groups and trends we do:
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Neutral Main Group Atoms:
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Size: Group 1 (smallest to largest: Li, Na, K, Rb, Cs)
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Size: 1st row (largest to smallest: Li, C, N, O, F)
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Size: P block (smallest to largest: F, O, C, Cl, S, Si)
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Ionization Energies: Group 1 (largest to smallest: Li, Na, K, Rb, Cs)
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Ionization Energies: 1st row (smallest to largest: Li, C, O, N, F)
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Electronegativities: Group 1 (largest to smallest: Li, Na, K, Rb, Cs)
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Electronegativities: 1st row (smallest to largest: Li, C, N, O, F)
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Charged Main Group Ions:
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Size: 1st row (smallest to largest: Li+, F-, O2-, N3-)
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Size: 1st row pairs (smallest to largest): Li+, Li; N, N3-; O, O2-; F, F-
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Challenge: Put Cl-, Ar, and K+ in order (smallest to largest): K+, Ar, Cl-
Common Misconceptions:
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On size: Students generally do well on ordering by size, but sometimes have incomplete or incorrect reasoning. For example, Cs is bigger than Li because it has a higher number, is “down further”, or has a bigger mass. They often need help articulating their reason in terms of additional shells of electrons added and effective nuclear charge changes. For ions, some students read the (+) as it must be larger (or that you’ve added an electron); they need to be reminded of the fact that a positive charge means fewer electrons (thus, held more tightly). Only about ½ of the students will get an isoelectronic series (i.e., Cl-, Ar, and K+) right on the first try; encouragement to think in terms of effective nuclear charge often results in most of the students seeing it on the 2nd try without further prompting.
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On Ionization Energies: Students never (except for the few who closely read their assigned book sections!) get the C-O-N order, but it gets a lot of good discussion going about “trends” and exceptions in them, and why THIS particular exception happens. We don’t do them “athletically” but we talk about the changes from sp (i.e. Be and B), as well as the half filled d subshells too (we don’t do hardly any transition metals in gen chem.) The most common misconception is that a low ionization energy means that it is hard to remove that electron.
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On Ionization Energies: By this point, most students have “got” the trends. Most are able to rationalize their arguments using the magical words “Effective Nuclear Charge.” There is sometimes some confusion about whether big or small electronegativities mean the atom attracts electrons to itself better.
Athletic Periodic Trends: Instructor Notes and Solutions
O, O2-, S, S2-,F, F-, Cl-, Ar, K+, Ti, Ti4+,Fe, Fe2+,Fe3+,Ru2+, Li
At Earlham College, Inorganic Chemistry is a one-semester, Junior/Senior level course. Many have had the first semester of P-Chem (but not all) and some are enrolled in Quantum Chemistry. I usually have a small enrollment 12-16 students, and so have many fewer atom “choices” of atoms and ions than in our General Chemistry course. In my course, this is a second or third day “review” activity right after we have reminded ourselves about effective nuclear charge and talked through its implications for the transition metals. For inorganic chemistry, these are the groups and trends we do:
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Neutral Atoms:
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Size: (largest to smallest: Ti, Fe, Li, S, O, F)
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Electronegativities: (smallest to largest: Li, Ti, Fe, S, O, F)
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Ionization Energies: (smallest to largest: Li, Ti, Fe, S, O, F)
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Polarizability: (greatest to least: Ti, Fe, Li, S, O, F)
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All Atoms and Ions:
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Size: pairs and triads (smallest to largest): O, O2-; F, F-; S, S2-; Ti4+, Ti; Fe3+, Fe2+, Fe
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Put Cl-, Ar, and K+ in order (smallest to largest): K+, Ar, Cl-
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Size of atoms and ions together (challenging!): (largest to smallest, radii in angstroms: S2- (1.84), Li (1.57), Ti (1.47), K+ (1.38), O2-(1.35), F-(1.28), Fe (1.26), S (1.04), Ru2+ (0.82), Fe2+ (0.78—high spin), O (0.66), F (0.64), Ti4+ (0.64), Fe 4+ (0.58))1
Common Misconceptions:
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Students sometimes demonstrate misconceptions similar to those displayed by general chemistry students (see page 1). Students sometimes question why transition metals are quite a bit more electronegative, etc. than the alkali metals. Though we don’t have enough students to show it using our atom cards, we also talk about (based on a figure of the variation of ionization energy with atomic number I display) why there are “blips” at half filled p and d, and why the ionization energy gets smaller going from the s to p shell and then d to p (Be to B, and Zn to Ga, for example).
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The combined size exercise (all the atoms and ions in together) is difficult! There are always several tries, and much arguing! I take this time to talk a bit about different ways of measuring atomic and ionic radii and the real difficulty in trying to compare the two and how they might vary with coordination number and high spin/low spin states. Once the students get “close”, or are able to justify their arrangement with a good argument based on effective nuclear charge, I stop the exercise and show the answers. By this point, we are usually right at the 15 minutes I allotted for the exercise.
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