1. CHEMICAL BONDING Set 4 Cocaine SAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNcheck "Background Printing")!

2. Credits Thank you to Mr. Neil Rapp who provided the bulk of this powerpoint on his website www.chemistrygeek.co m www.chemistrygeek.co m Other information comes from Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

3. Violations of the Octet Rule Usually occurs with B and elements of higher periods. Common exceptions are: Be, B, P, S, and Xe. BF 3 SF 4 Be: 4 B: 6 P: 8 OR 10 S: 8, 10, OR 12 Xe: 8, 10, OR 12

4. Lewis Structures: Comments about the Octet Rule The second-row elements C, N, O and F should always be assumed to obey the octet rule. The second-row elements B and Be often have fewer than eight electrons around them in their compounds. These electron-deficient compounds are very reactive. The second-row elements never exceed the octet rule, since their valence orbitals (2s and 2p) can accommodate only eight electrons. Third-row and heavier elements often satisfy the octet rule but can exceed the octet rule by using their empty valence d orbitals. When writing the Lewis structure for a molecule, satisfy the octet rule for the atoms first. If electrons remain after the octet rule has been satisfied, then place them on the elements having available d orbitals (elements in Period 3 or beyond).

5. Resonance Resonance occurs when more than one valid Lewis structure can be written for a particular molecule. Example: Nitrate Ion

6. Resonance Explained If any one of these structures accurately represented the bonding in the nitrate ion, there should be two types of N-O bonds observed in the molecule: one shorter bond (the double bond) and two identical longer ones (the two single bonds). Experiments clearly show that the nitrate ion exhibits only ONE bond length and bond strength between those expected for a double and single bond. Resonance structures are the depictions of all possible structures, but in reality, the molecule’s bonding is the average of all three.

7. Structural Isomers http://xsiborganic1011.wikispaces.com/TangYaoFlo Examine the three models of the three alkanes above to determine how they are similar and how they are different. All three have 5 carbon atoms and 12 hydrogen atoms, so they all have the molecular formula C 5 H 12. BUT, you can see that the arrangements of the atoms results in three different compounds: pentane, 2-methyl butane, and 2,2 –dimethyl propane

8. Isomer Definitions Isomers are two or more compounds that have the same molecular formula, but their atoms are bonded in different arrangements. Structural isomers have different chemical and physical properties despite having the same chemical formula. The b.p. of each of the three isomers shown in the previous slide are as follows (in order as shown): 36°C, 28°C, 9°C – This observation supports a main principle of chemistry: structure determines properties. – How does the trend in boiling points of C 5 H 12 isomers relate to their structures?

9. Number of Possible Isomers As the number of carbons in a hydrocarbon increases, the number of possible structural isomers increases. – There are nine alkanes with the molecular formula C 7 H 16 – There are more than 300,000 structural isomers with the formula C 20 H 42

10. Odd-Electron Molecules Relatively few molecules formed from nonmetals contain odd numbers of electrons. Two common examples are NO and NO 2 (try drawing them) Since the Lewis structure model is based on electron pairs, it does not handle odd-electron cases in a natural way. We usually just tack on the odd electron onto the central atom.

11. Homework Questions 1) Which has the greater bond lengths: NO 2 - or NO 3 - ? Explain. (HINT: You’ll have to draw the Lewis structures first) 2) The following ions are best described with resonance structures. Draw the resonance structures:a) NCO - b) CNO - 3) Explain the terms resonance and its connection to delocalized electrons. Use additional resources if necessary. 4) Explain the difference between a resonance structure and an isomer.