Lecture 7: Chemical bonding Lecture 7 Topics Brown, chapter 8 Basic bonding background information 8.1 Types of chemical bonds Valence electrons & Lewis structures The ‘Octet rule’ Ionic bonding 8.2 Lattice energy Charge & size affect lattice energy Transition metal ions Covalent bonding 8.3 Lewis structures & unequal sharing Unequal sharing & electronegativity 8.4 Bond polarity Guidelines for Lewis Structures 8.5 Dipole moment & overall molecular polarity Multiple bonds & formal charge Resonance structures 8.6 Exceptions to the octet rule 8.7

Resonance: alternate arrangement of electrons Molecules may have multiple resonance structures. Resonance hybrids are more realistic. The general process of advancing scientific knowledge by making experimental observations and by formulating hypotheses, theories, and laws. It’s a systematic problems solving process AND it’s hands-on….. Experiments must be done, data generated, conclusions made. This method is “iterative”; it requires looping back and starting over if needed. [Why do you think they call it REsearch?] Often years, decades or more of experiments are required to prove a theory. While it’s possible to prove a hypothesis wrong, it’s actually NOT possible to absolutely prove a hypothesis correct as the outcome may have had a cause that the scientist hasn’t considered.

Resonance structures Take a look at the geometry of this molecule. What suggests that the two “outer” oxygen atoms are behaving the same way? Now try to draw the Lewis dot structure of this molecule, O3, or ozone. p.309-10

Resonance structures… … are Lewis structures that are equivalent except for e- placement. .. .. .. ozone (O3) 1.54Å O 1.34Å O O .. .. .. .. .. .. : O O O O : O : .. .. .. .. O .. .. 1.40Å resonance structures resonance hybrid The actual structure is a blend of the two alternative structures shown above, & is best represented by the resonance hybrid. Note that the actual structure does not oscillate between 2 resonance structures. Both structures exist - as both blue & yellow are present in green. p.309-10

Example: resonance structure Which has shorter S - O bonds, SO3 or SO3-2 ? SO3 = 6 + (6x3) = 24 ve- For these 3 equivalent resonance structures all bonds also have equal length. But it’s 1/3 shorter than a single bond. .. .. : O : : O : : O : S S S : : O : : : : .. O : .. O : : O : O : : O .. .. SO3-2 = 6 + (6x3) + 2 = 26 ve- .. .. .. :O S O: | :O: .. .. For this structure all bonds have equal length – that of a single bond (long). .. p.311

Example: resonance structures & hybrid HCO2-1 has resonance structures. How many resonance structures? Draw each resonance structure. Draw the resonance hybrid. .. : O : : O : : O : C C C H : O : H : O : : O : .. H p.311