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Chem 1201
Unit 4: Chemical Bonding II - Advanced Bonding Theories

Unit 4: Chemical Bonding II - Advanced Bonding Theories

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Equipped with a basic model to describe bonding in chemical species, we are now prepared to explore more advanced models. This unit will include a variety of novel representations for molecular and polyatomic ionic species.

We apply two distinct approaches for describing covalent bonds:

a localized model to describe bonding in molecules with two or more atoms attached to a central atom and
a delocalized model to explain and predict which diatomic species exist and which do not exist.

We conclude by describing more complex molecules and ions with multiple bonds. The tools you acquire in this chapter will enable you to explain why Ca₂ is too unstable to exist in nature and why the unpaired electrons on O₂ are crucial to the existence of life as we know it. You will also discover why carbon, the basic component of all organic compounds, forms four bonds despite having only two unpaired electrons in its valence electron configuration and how the structure of retinal, the key light-sensing component in our eyes, allows us to detect visible light.

The learning objectives of this unit are:

Unit	Topic	Learning Objectives
4	Need for a theory beyond Lewis	Justify the need for a bonding theory beyond Lewis Theory Describe the five basic shapes of molecules as predicted by VSEPR Theory Assign the VSEPR notation for the central atom in a molecule or ion
4	VSEPR	Use VSEPR Theory to predict the geometry of a molecule or polyatomic ion from its Lewis structure Approximate the bond angles in a molecule or ion with and without lone pairs on the central atom
4	VSEPR	Apply VSEPR Theory to molecules or ions with more than one central atom Sketch the Structural Formula (in 3D) for a molecule or ion
4	Bond Dipoles Molecular Polarity	Define bond dipole Predict the polarity of bonds Predict the polarity of molecules based on molecular shape and bond dipoles Represent the polarity of a bond and a molecule using the dipole moment vector notation and using the δ⁺ δ^- notation. Predict the molecular geometry based on bond dipoles and the experimental molecular dipole moment
4	Valence Bond Theory	Justify the need for a bonding theory beyond VSEPR Theory Describe the conceptual basis for Valence Bond Theory
4	Orbital Hybridization	Explain the need for hybridization of atomic orbitals Use molecular geometries to predict the orbitals involved in hybridization Sketch the energy level diagram for the hybridization of s and p atomic orbitals to create sp³ and sp² and sp orbitals
4	Sigma and Pi Bonding	Define sigma and pi bonds and differentiate between them in a multiply-bonded system
4	Orbital Hybridization	Sketch the energy level diagram for the hybridization of s and p and d atomic orbitals to create sp³d and sp³d² orbitals Predict the type of hybridization required for each atom in a molecular based on the molecular geometry
4	Valence Bond Representation	Sketch the Valence Bond Representation (overlapping orbitals) for a molecule
4	Valence Bond Representation	Label each of the bonds in a molecule in terms of their symmetry type and overlapping orbitals
4	Carbon Skeletons (Line Drawings)	Interpret skeletal formulae (line drawings) in terms of number of carbon and hydrogen atoms, orbital hybridization, and σ or π bonding
4	The need for a theory beyond VB	Justify the need for a bonding theory beyond Valence Bond Theory Differentiate between valence bond theory and molecular orbital theory
4	Molecular Orbital Theory	Describe and sketch bonding and antibonding molecular orbitals Sketch Molecular Orbital Energy-Level Diagrams for homonuclear diatomic molecules of the first period, labelling the diagram appropriately
4	Molecular Orbital Theory: Bond Order	Calculate bond order according to Molecular Orbital Theory Predict molecular stability based on bond order Express the electron configuration of a molecule based on molecular orbital theory
4	Molecular Orbital Theory	Sketch Molecular Orbital Energy-Level Diagrams for homonuclear diatomics involving Li and Be, labelling the diagram appropriately Sketch the shapes of molecular orbitals created from 1s, 2s, and 2p atomic orbitals
4	Molecular Orbital Theory	Sketch Molecular Orbital Energy-Level Diagrams for all second period homonuclear diatomics, labelling the diagram appropriately Predict the magnetic properties of a diatomic species based its MO energy-level diagram Define photolysis Describe the effect of electron excitation according to MO Theory, and represent it on an energy-level diagram
4	Molecular Orbital Theory	Sketch molecular orbital energy-level diagrams for heteronuclear diatomic species and use them to predict their magnetic properties and bond orders Give examples of molecules with delocalized molecular orbitals and compare this description with the concept of resonance structures