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BYUI CHEM 105 - Molecular Structure and Covalent Bonding Theories

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Molecular Structure and Covalent Bonding TheoriesThe Valence Shell ElectronsModels to Describe Covalent BondingVSEPR TheorySlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Molecular Geometry and PolaritySlide 14Slide 15Valence Bond(VB) TheorySlide 17Slide 18Slide 19Molecular Shapes and BondingSlide 21AB2 Molecules - No Lone Pairs on A - Linear MoleculesSlide 23Slide 24Slide 25Slide 26AB3 Molecules - No Lone Pairs on A - Trigonal Planar MoleculesSlide 28Slide 29Slide 30Slide 31AB4 Molecules - No Lone Pairs on A - Tetrahedral MoleculesSlide 33Slide 34AB4 Molecules - No Lone Pairs on A - Tetrahedral MoleculesSlide 36Alkanes CnH2n+2AB3U Molecules - One Lone Pair - Pyramidal MoleculesSlide 39Slide 40Slide 41Slide 42Slide 43AB2U2 - Two Lone Pairs - V-Shaped MoleculesSlide 45Slide 46Slide 47Trigonal Bipyramidal Electronic GeometryTrigonal Bipyramidal Electronic Geometry, AB5Slide 50Slide 51Trigonal Bipyramidal Electronic Geometry with Lone PairsSlide 53Slide 54Octahedral Electronic GeometryOctahedral Electronic Geometry: AB6Slide 57Variations of Octahedral ShapeLarger MoleculesMolecular Structure and Covalent Bonding TheoriesChapter 8The Valence Shell Electrons•Valence shell electrons–These electrons are largely responsible for____–Electrons not present in the preceding ___ ___•Ignore filled sets of d and f orbitals–Used to determine the Lewis structure of a compound containing covalent bonds•Works well for molecules containing atoms from the ____ ____ elementsModels to Describe Covalent Bonding•Valence shell electron pair repulsion (VSEPR) model – predicts the _____ ______ of atoms in a molecule–This will be related to a physical property called ______•Valence bond theory – predicts how bonding will take place by ______ of atomic orbitalsVSEPR Theory•Valence shell electron are present as either ___ ___ or ____ ____. –Regions of high electron density are created.–These regions arrange themselves to be as far away as possible form on another. As a result specific geometries are created around atoms in the molecule•Single, double, and triple bonds are counted as one region of electron density•Unshared pairs of valence electrons are also counted as one region of electron densityDrawing the Lewis structure accurately will reveal the number of electron density regions around the center atomsVSEPR Theory•Draw the Lewis structures for CO2, H2CO, and CH4–How will these regions of electron density arrange themselves to be as far away as possible from one another?•There are five basic shapes based on the number of electron density regions around a center atom(s)–Illustration of models with next few slidesVSEPR TheoryTwo regions of high electron densityVSEPR TheoryThree regions of high electron densityVSEPR TheoryFour regions of high electron densityVSEPR TheoryFive regions of high electron densityVSEPR TheorySix regions of high electron densityVSEPR Theory•Encountered geometries–Electronic geometry – determined by the location of “___” the regions of electron density around the center atom(s)–Molecular geometry – determined by the arrangement of _____ only around the center atom(s)•The does not include lone electron pairs. The molecular shape differs from the electron shape if lone pairs are presentAn example is H2OVSEPR Theory•Lone pairs of electrons occupy more space than bonding pairs. As a consequence, there is an order of the magnitude of repulsions –lp/lp > lp/bp > bp/bpAs a result, the bond angles around a center atoms can be distorted (reduced) from the predicted valuesCH4 and H2O What are the H-C-H and H-O-H bond angles. If a change is observed, why?Molecular Geometry and Polarity•The polarity can be determine once the geometry is known•A polar bond is created if the atoms sharing the electron pair have different electronegativities–HCl and the associated dipole moment. This molecule is polar. For diatomics, determination of polarity is easy. What if the molecule has two or more atoms? All the dipole have to be summed. If the sum equals zero, the molecule has no dipole.Molecular Geometry and Polarity•A dipole moment (bond dipole) has _____ and _____. Both must be considered when determines if a molecule is polar.–CO2 and H2O. Do these molecules have net dipoles?•Conditions for polarity–There must be at least one polar bond or lone pair on a central atom–The bond dipoles must not cancel or if there are two or more lone pairs on the central atom, they must not be arranged so that their polarities cancelCO2, H2O, and O3Molecular Geometry and PolarityValence Bond(VB) Theory•VB theory describes how bonding occurs•Describes how the atomic orbitals overlap to produce the bonding geometry predicted by VSEPR–Go back and review atomic orbitals if necessary•Electrons are arranged in atomic orbitals according to energy. The set of atomic orbitals, however, may not be of lowest possible energy upon bonding covalently to neighboring atoms.Valence Bond(VB) Theory•The valence shell orbitals (atomic orbitals) commonly combine to change their character in order to obtain a lower energy ‘mixed’ orbital set for bonding in a particular geometry–Which atomic orbitals would participate in bonding in H, O, and C? These atomic orbitals can form a new set of hybrid orbitals upon bonding.•Hybrization – process by which ____ ____ combine to form a set of ‘mixed’ orbitals of lower energy when bonding covalently–The ‘mixed’ orbitals are called hybrid orbitalsValence Bond(VB) Theory•Hybrid orbitals on a center atom align themselves with the bonding orbitals on the neighboring atoms–A ‘good overlap’ is necessary for sharing electrons in a bond.•Table 8-2 (refer to it)–The label given to a set of hybridized orbitals reflects the number and type of atomic orbitals used to produce the set. •Indicates the electronic geometry in agreement with VSEPRValence Bond(VB) TheoryMolecular Shapes and Bonding•Simples structures will be analyzed based on geometry type.•Experimentally determined findings will be discussed in light of these models.•Terminology–A – central atom–B – atoms bonded to A–U – lone pairs of electrons around AAB3U represents three atoms bonded to a central atom with one lone pair. An example would be NH3Molecular Shapes and BondingDiscussion sequence•Experimental facts and Lewis formula•VSEPR–Electronic geometry–Molecular

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