Lecture 11. Chemical bonding: Lewis structures

Thursday 22 February 2024

Further development of skill in drawing valid Lewis structures from chemical formulas. Alternative skeletal structure and structural isomers. Introduction to formal charge and resonance structures. Examples of Lewis structures and assignment of formal charges to individual atoms. Patterns of formal charge and normal valence of an atom type. Using formal charge in choosing between structural isomers (alternate skeletak structures). Resonance forms and the resonance hybrid. Equivalent and non-equivalent resonance forms.

Reading: Tro NJ. Chemistry: Structure and Properties (3rd ed.) - Ch.5, §5.3-5.4 (pp.220-228t).

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Summary

We work further with Lewis structures, writing out valid structures for molecules and polyatomic ions, analyzing both cases, paying particular attention to formal charge assignments for every atom, and drawing all valid resonance forms. Further, in cosidering examples, we see how to distinguish between equivalent and nonequivalent resonance forms. We discuss how assigning of formal charges and the electronegativities of the atoms involved allow us to assess relative contributions of nonequivalent resonance forms to the resonance hybrid. (Equivalent resonance forms would make equal contributions to the hybrid.)