B. Draw the best dot structure you can for HNNH, where the atoms are connected as written.

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1 Recitation, Week 15, Fall (Unit II) The proper assignments of oxidation numbers (ON) are necessary for recognizing and balancing redox reactions (e.g. questions IV and V on page 3), but they are also useful for understanding other aspects of chemical reactivity. For example, I mentioned in class that oxidation numbers allow one to deduce the relative strength of analogous acids. A. Assign the requested oxidation numbers for the following pairs of acids: 1. HNO 2 HNO 3 ON of N 2. H 2 SO 4 H 2 SO 3 ON of N See Solutions to Unit IV Homework for the answers. ON of S ON of S 3. HClO 4 HClO 2 ON of Cl ON of Cl B. Circle the stronger acid in each of the above pairs. C. What is the general relationship between oxidation number and acid strength? D. On October 12, 2008, an oleum spill at a chemical plant in western Pennsylvania forced about 2,500 residents to evaluate. Oleum also known as fuming sulfuric acid, H 2 SO 4 xso 3 is especially dangerous because the sulfur trioxide escapes as a gas and reacts with moisture in the air to form sulfuric acid according to the reaction SO 3 (g) + H 2 O(g) H 2 SO 4 (g). Is this a redox reaction? Explain your reasoning. 11. (Fall 2015) Draw dot structures as instructed below, and answer any associated questions. Be sure to show all non-zero formal charges. A. Draw the best dot structure you can for OF 2. B. Draw the best dot structure you can for HNNH, where the atoms are connected as written. C. Draw the best dot structure you can for the hypothetical molecule NCOF which contains all of the nonmetal elements from the second row of the periodic table, except neon. Which bond in NCOF do you expect to be the strongest? _ Why?

2 D. Draw two dot structures for SeO 2 : 1. best dot structure you can that obeys the octet rule, and 2. best dot structure you can that exceeds the octet rule to minimize formal charge. 16. (Fall 2015) Iodine is an important element. While its percent abundance on Earth is less than 0.05%, iodine is an essential element in biology, and iodine compounds have found numerous applications in industry and medicine. B. Resonance structure for the molecule ICN are given below. Assign nonzero formal charges for these dot structures. Which dot structure is the best resonance structure for ICN? 22. (Fall 2009) A dot structure for methyl amide is shown on the right. Draw a resonance structure for this dot structure. Which structure do you think is the better resonance structure: the one given above, or the one you drew? Why?

3 Steps for writing dot structures: 1. Add up the total number of valence electrons. For anions, add electrons to get the correct overall charge; for cations, subtract electrons to get the correct overall charge. 2. Connect the atoms using single bonds. 3. Starting with the terminal atoms, place remaining electrons around the atoms, giving no more than eight electrons to each atom. 4a. If any atom has less than an octet, then move pairs of electrons to form multiple bonds until every atom has an octet (BF 3 is a classic exception which can be understood in Step 5). There may be more than one way to satisfy Step 4a, giving different resonance structures. The relative importance of the different resonance structures can be evaluated by considering formal charge and electronegativity (see Step 5). 4b. You may exceed the octet for atoms with low-lying empty d-orbitals, but never expand the octet for B, C, N, O, or F. 5. Assign formal charges. The best dot structures will a. minimize formal charge b. separate like charges c. place opposite charges as close together as possible P As Sb S Se Te Cl Br I d. place negative charge on the most electronegative atoms, and positive charges on the least electronegative atoms One consequence of step 5 is that generally the least electronegative atom occupies the central position. We can also use this rule to predict which arrangement of atoms will be the most stable; e.g. OCN versus ONC. =====================================================================

4 Electron Accounting Systems # e atom has = # e atom should have # e atom actually has = # valence e {# lone pair e + # bonding e that belong to the atom} oxidation number, ON (assumes 100% ionic) = # valence e {# lone pair e + # bonding e } giving all to the most EN atom formal charge, FC (assumes 100% covalent) = # valence e {# lone pair e + # bonding e } split evenly between the atoms ===================================================================== To have zero formal charge (FC) on an atom: atom # bonds # lone pairs examples C 4 0 C C C N 3 1 N N N O 2 2 O O F 1 3 F Increasing # bonds and decreasing # lone pairs by one increases FC by one. Decreasing # bonds and increasing # lone pairs by one decreases FC by one. Common examples: N O

5 DOT STRUCTURE AND VSEPR PRACTICE SET Draw the best dot structure for the following molecules and polyatomic ions, and assign all nonzero formal charges. AsH 3 SF 6 BrF 5 AlCl 4 SF 3 SCl 2 ClO 3 HCN I 3 SOO IF 4 XeF 4 Now determine the molecular shape of each species (the shape formed by the atoms, as if the lone pairs were invisible ). Do not just give the name of the molecular shape (e.g. linear, bent, T-shaped, trigonal planar, trigonal pyramidal, tetrahedral, trigonal bipyramidal, distorted tetrahedron/ seesaw, square planar, square pyramidal, or octahedral), but also sketch the shape. As we discussed in class, use wedges to show bonds coming out of the plane of paper (towards you) and dashed lines for bonds going into the plane of the paper (away from you). It takes some practice to get the hang of this, and it helps if you use the same basic templates each time, such as shown below.

6 For example, using these templates, the VSEPR sketches for OPF 3, ClF 2, and PF 4 (ignoring distortions from ideal) would be Following the appropriate templates shown at the bottom of the first page, sketch the molecules/ions on that page and use arrows to show deviations from ideal geometries. Which of the neutral molecules would be polar?