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Chapter 11

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Judith C Poe

Chapter 11: Theories Of Covalent Bonding 11.1: Valence Bond (VB) Theory & Orbital Hybridization A. The Central Themes of VB Theory  the basic principle is that a covalent bond forms when orbitals of two atoms overlap and a pair of electrons occupy the overlap region. The overlap of two orbitals means their wave functions are in phase (constructive interference), so the amplitude b/w the nuclei increases, this gives rise to the central themes: 1. Opposing spins of the electron pair: A set of overlapping orbitals has a maximum of two electrons that must have opposite spins. 2. Maximum overlap of bonding orbitals: The greater the orbital overlap, the stronger (more stable) the bond as bond strength depends on the attraction b/w the nuclei and shared electrons 3. Hybridization of atomic orbitals: During bonding the valence atomic orbitals in the isolated atoms become different when they are in the molecule. If we mathematically mix certain combinations of orbitals, we form new ones whose spatial orientations do math the observed molecular shapes, the process of orbital mixing is called hybridization and the new atomic orbitals are called hybrid orbitals. 4. Features of hybrid orbitals: here a. the number of hybrid orbitals formed equals the number of atomic orbitals mixed. b. the type of hybrid molecules formed varies with types of atomic orbitals mixed c. the shape and orientation of a hybrid orbital maximized overlap with the orbital of the other atom in the bond B. Types of Hybrid Orbitals  sp Hybridization: when two electron groups surround the central atom, we observe a linear shape which means the bonding orbitals must have a linear orientation: 1. Orbitals mixed and orbitals formed: VB theory proposes two non-equivalent orbitals of a central atom, one s and one p, mix and form two equivalent sp hybrid orbitals that are oriented 180 degrees apart. The shape differs from the atomic orbitals with one large and one small lobe, the orbital orientation increase electron density in bonding direction. 2. Overlap of Orbitals from central and surrounding atoms:  sp Hybridization: 2 1. Orbitals mixed and orbitals formed: mixing one s and 2p orbitals gives 3 sp hybrid orbitals that point to the corner of an equilateral triangle, their axis 120 degrees apart. (In hybrid orbitals, the superscript numbers refer to the number of atomic orbitals of the given type) 2. Overlap of orbitals from central and surrounding atoms: BF 2 3. Placement of Lone Pairs: to account for other molecular shapes within a given electron-group arrangement, one or more hybrid orbitals contains a lone pair.  sp Hybridization 1. Orbitals mixed and orbital formed: mixing one s orbital with three p orbitals gives 4 sp hybrid orbitals that point to the corners of a tetrahedron 2. Overlap of orbitals from central and surrounding atoms: 3. Placement of Lone pairs: the trigonal pyramidal shape of NH ar3ses when a lone pair fills any one of the four sp orbitals of N. 3 
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