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Important Concepts from Chapter 9 • DRAWING LEWIS ELECTRON DOT STRUCTURES • octet rule • resonance structures • bond polarity (ionic, polar covalent and covalent bonds) • ionic vs. covalent compounds • electronegativity • dipole vectors • calculating formal charges and partial charges • bond order • bond lengths • VSEPR and predicting shapes of molecules • polarity of molecules CHAPTER 10 – BONDING AND MOLECULAR STRUCTURE: ORBITAL HYBRIDIZATION AND MOLECULAR ORBITALS The electrons in an atom are located in the _____________ orbitals. Similarly when atoms combine to form molecules, molecular orbitals (MO) are used to describe the electrons in molecules. There are two theories available to explain the formation of chemical bonds based on orbitals: the valence bond (VB) theory and the molecular orbital (MO) theory What happens when two atoms come together to form a bond? An energy level diagram shows that a bond forms when two atomic orbitals overlap to make a stable molecule. The idea that _____________ ________________________ ________________________ is the basis for the VB theory. The covalent bond that arises from the overlap of two atomic orbitals, one each from the two atoms, is called a __________ _________. The electron density of a σ bond is greatest along the axis of the bond. In H2 molecule, a σ bond is formed by the overlap of ________ 1s orbitals of the hydrogen atoms. In HF molecule, a σ bond is formed by the overlap of the hydrogen ____ and the fluorine _______ orbitals. In F2 molecule, a σ bond is formed by the overlap of _______ _____ orbitals of the fluorine atoms. Hybridization of atomic orbitals The Lewis dot structure of boron trifluoride, BF3, shows _________ B⎯F covalent bonds. The VSEPR theory predicts a ____________ __________ configuration for BF3 with a ________ angle. How to account for three bonds with 120° angle using one spherical s orbital and three p orbitals (that are 90° to each other)? The boron atom in its ground state has only 1 unpaired electron, not the three needed to form three covalent bonds. Orbital hybridization theory explains bonding in molecules with more than two atoms. A new set of hybrid orbitals can be created by mixing the s, p, and d orbitals on an atom. 1. The number of hybrid orbitals is always the __________ as the number of atomic orbitals mixed to create them 2. The hybrid orbitals are more directed from the central atom toward the terminal atoms, than are the unhybridized atomic orbitals, leading to better overlap and a strong bond between the central and terminal atoms. _____ hybrid orbital is required for each σ bond or lone electron pair on the central atom. BF3 and sp2 hybridization In BF3, there are three σ bonds and hence we need 3 hybrid orbitals. One s orbital and two p orbitals can be mixed to form three _______ hybrid orbitals and they are separated by a 120° angle. The one p orbital which is empty does not take part in hybridization. 2s 2p An orbital (2p) from each F overlaps with one of the sp2 hybrid orbitals to form three σ bonds. The energy of a sp2 orbital is closer to the p energy because it has more ‘p character’ than ‘s character’ CF4 and sp3 hybridization CF4 is _________________ molecule with four C⎯F σ bonds and a 109.5° angle at the central atom. C has 4 valence electrons (________). To form 4 σ bonds, we mix ______ ___ and ________ ___ orbital to form _______ ____ hybrid orbitals and they are separated by a 109.5° angle. Each C⎯F bond is formed by the overlap of one sp3 orbital of C with a ______ of fluorine. Note that the energy of an sp3 orbital is very close to the energy of the p orbital. 2s 2p BeF2 and sp hybridization BeF2 is _______________ molecule with two Be⎯F σ 2s bonds and a 180° angle at the central atom. Be has 2 valence electrons in the 2s orbital. To form 2 σ bonds, we mix _______ and ________ orbital to form two sp hybrid orbitals and they are separated by a 180° angle. The 2 unoccupied p orbitals do not participate in hybridization. The energy of a sp orbital is midway between the energy of an s and a p orbital. Each Be⎯F bond is formed by the overlap of one sp orbital of Be with a 2p of fluorine. 2p The maximum numbers of hybrid orbitals that can be created from the s and p orbitals are _________. For atoms with coordination number higher than ________, s, p and d orbitals are involved in hybridization. Hybrid orbitals and VSEPR Electron pair geometry Linear Bond angle # bonds hybridization Example 180 2 Two electron pairs sp BeF2 120 3 Trigonal planar Three electron pairs sp2 BF3 Tetrahedral Four electron pairs 109.5 4 sp 3 CF4 Trigonal bipyramidal Five electron pairs axial – 90 equatorial –120 5 90 6 dsp3 or sp3d PF5 Octahedral Six electron pairs d2sp3 or sp3d2 SF6 Identify the hybridization of the central atom in the following compounds (a) BH4- (b) OSF4 (c) XeO64- Multiple bonds According to VB theory, two orbitals on adjacent atoms overlap to form a bond. Hence formation of double bond requires ________ sets of overlapping orbitals and for triple bond, overlap of _________ sets of atomic orbitals are required. Double bonds The Lewis structure of ethene, CH2CH2: Each carbon atom is bonded to 2 H atoms by a single bond and to the other carbon atom by a double bond. Hence each C atom has__________ ________ geometry with ______ bond angle. Carbon has ____ valence electrons; to form 3 bonds we mix one s and two p orbitals to form three sp2 orbitals of equal energy in the molecular plane and there is ____ electron present in the unhybridized p orbital, perpendicular to the molecular plane. Coplanar (flat) ethene molecule top view Three sp2 orbitals of the C overlap with orbitals of 2 H and 1 C to form three σ bonds. The two unhybridized p orbitals, one on each C, can overlap to form a second bond between the carbons. The overlap does not occur on the C⎯C axis, instead they overlap ______________ and the electron pair occupies an orbital with electron density above and below the molecular plane. The bond formed by sideways overlap of the p atomic orbitals is called a _____ bond. The π bond requires that all the six atoms of ethene lie in one plane (coplanar). π bond always forces the molecule to be coplanar. The C=C double bond in ethene consists of a σ and a π bond. Double bonds always consist of one σ and one π bond. Triple bond Consider ethyne, CHCH (C2H2) The molecular geometry is linear with a 180° bond angle. Each carbon atom is bonded to an H and to the other C by an σ bond. To form 2 bonds, we mix one s and one p orbitals, to get two sp hybrid orbitals. The two unhybridized p orbitals on each atom overlap to form 2 π bonds. These π bonds are perpendicular to the molecular axis and perpendicular to each other. Triple bonds always consist of one σ and two π bond. Cis-trans isomerism: A consequence of π bonding Free rotation (rotation of one end of a molecule relative to the other end) of molecules can occur around the axis of a single (σ) bond. For molecules containing π bonds like ethene, twisting one end of the molecule distorts the molecule away from planarity and the p orbitals, forming the π bond, would rotate out of alignment. Hence the p orbitals would no longer overlap and the π bond will be broken. It requires very large amount of energy to break π bonds, that rotation around C=C does not occur at room temperature. Due to restricted rotation, isomers occur for many compounds containing C=C bond. cis-1,2-dichloroethene trans-1,2-dichloroethene Isomers are compounds that have same _____________ but different ______________. The two isomers of 1,2-dichloroethene differs only in the orientation of the groups attached to the carbons of the double bond.