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Chem 1201
Unit 3: Chemical Bonding I - Lewis Theory

Unit 3: Chemical Bonding I - Lewis Theory

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In this unit, we begin with a general method for predicting the structures of simple covalent molecules and polyatomic ions; we will follow up in the next unit by discussing the actual distribution of electrons in covalent bonds. Even though we will find Lewis Theory has problems with predicting experimental results, it is still used by chemists to establish atomic connectivity and represent chemical reaction mechanisms. Lewis Theory will often serve as your intellectual foundation when predicting molecular properties, upon which you will add using the theories in the next unit.

A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electrostatic force of attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction. The strength of chemical bonds varies considerably.

The learning objectives of this unit are:

Unit	Topic	Learning Objectives
3	Ionic Bonding	Draw Lewis Dot structures for atoms and ions of the representative elements Define ionic bonding
3	Lattice Energy	Define lattice energy and relate it to Coulomb’s Law Order ionic compounds in terms of their melting point based on Coulomb’s Law
3	Bond Energies	Define bond energy Use bond energies to calculate a reaction enthalpy
3	Covalent Bonding	Differentiate between ionic and covalent bonding Describe bonding in terms of the balance of attractive and repulsive forces, and sketch a potential energy well Relate bond energy to bond strength and length Define electronegativity and use it to predict the degree of iconicity in a bond
3	Lewis Structures and the Octet Rule	Identify Lewis Structures Represent the polarity of a bond in a Lewis Structure using the bond dipole moment vector notation and using the δ⁺ δ^- notation. Define the octet rule
3	Lewis Structures	Draw Lewis structures of covalent compounds
3	Formal Charge	Draw Lewis structures of complex ions Assign formal charges to all the atoms in a Lewis structure Use formal charges to identify a best Lewis Structure
3	Resonance	Define resonance Draw all the resonance structures for a molecule or ion Use "curvy" arrows to show the redistribution of electron density to form resonance structures Use the concept of bond order and bond energy to explain molecular properties
3	Exceptions to the Octet Rule	Describe the situations when the octet rule does not apply Give examples and draw Lewis structures of compounds exhibiting incomplete octets Give examples and draw Lewis structures of compounds exhibiting expanded octets Give examples and draw Lewis structures of radicals Explain why radicals are highly reactive