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11.2 Valence Bond Method

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CHEM 110
Ariel Fenster

11.2 Introduction to Valence Bond Method  Orbitals overlap; forming a bond between the two atoms because of the high electron density probability found in the region between the atomic nuclei where the orbitals overlap  Overlap: the interpenetration of two orbitals  Localized electron: core electrons and lone-pair valence electrons retain the same orbital locations as in the separated atoms and charge density of the bonding electrons is concentrated in the region of orbital overlap Orbital overlap stabilization: accounts for 90% of the stabilization energy (this is how a bond is formed)  The more overlap = the stronger the bond  Maximum electron density around the bond axis  Building up density between the two nuclei Sigma bond: sigma bonds (σ bonds) are the strongest type of covalent chemical bond; they have the most electron density around the bond axis Covalent Bond:  The electrons are no longer confined to a single s orbital around one atom  Instead they can move over both orbitals for the entire molecule  Results in greater stability! Valence Bond Theory: the covalent bond results from the overlap of atomic orbitals containing one unpaired electron each (overlap of half-filled orbitals or a filled orbital of one atom with an empty orbital on the other) Valence Bond Theory Examples: Example #1: Hydrogen H for2s an s-overlap with unpaired electrons 11.2 Introduction to Valence Bond Method Example #2: F forms a p-overlap (head-to-head overlap). 2 Head to head overlaps are favored, as they result in the maximum overlap. Example #3: Helium (no unpaired electrons) therefore it has a filled orbital = no possible orbital; no place for the electrons to go. Overlap is possible but it will not be stabilizing. Non-polar Bonds:  Bond in homonuclear diatomic molecules (e.g. H , C2 , F2, 2tc.)  Bonding electrons equally shared and symmetrically distributed Example: Hydrogen Fluoride  s-p head to head overlap  Fluorine has a larger electron cloud; electrons spend more time around the fluorine; this is now a polar bond  Still a sigma bond  Fluorine has a greater EN than hydrogen, therefore it has a greater electron density 11.2 Introduction to Valence Bond Method Double Bonds:  First bond: head to head overlaps, therefore sigma bond  Second bond: side to side overlap (does not have as much overlap)  Second bond (overlap above and below the bond axis) this is a pi bond
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