Pitt CHEM 0110 - Chapter 10- Molecular Geometry

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Chapter 10- Molecular Geometry and Chemical Bonding Theory In a cis compound, both H atoms are on the same side of the carbon bond; in the tras compound, they are on opposite sides Molecular Geometry - general shape of a molecule as determined by the relative positionsof the atomic nuclei10.1 The Valence-Shell Electron-Pair Repulsion (VSEPR) Model The VSEPR Model - predicts the shape of molecules and ions by assuming that the valence shell electron pairs are arranged about each atom so that the electron pairs are kept as far away from one another as possible thus minimizing electron pair repulsions  When you determine the geometry of a molecule experimentally, you locate the positionsof the atoms, not the electron pairs  The direction in space of the bonding pairs gives you the molecular geometry  Steps in the prediction of Geometry by the VSEPR Model o Step 1: Write the electron dot formula from the molecular formulao Determine from the electron-dot formula the number of electron pairs around the central atom, including bonding and nonbonding pairs. (Count a multiple bond as one pair. If resonance occurs, use the resonance formula to determine this number) o Determine the arrangement of these electron pairs about the central atom o Obtain the molecular geometry from the directions of the bonding pairs for this arrangement Bond Angles and the effect of Lone Pairs  A lone pair tends to require more space than a corresponding bonding pair  Multiple bonds require more space than single bonds because of the greater number of electrons Central Atoms with Five or Six Valence-Shell Electron Pairs Axial Directions- form through the central atom; 180 degrees apart Equatorial Directions- point through the corners of the equilateral triangle that lies on a plane through the central atom, perpendicular to the axial directions; are 120 degrees apart10.2 Dipole Moment and Molecular Geometry  Dipole Moment - A quantitative measure of the degree of charge separation in a molecule and is therefore an indicator of the polarity of the molecule  Q= the charge  D= the distance  Dipole Moments are usually measured in units of debyes (D)  A polar substance is place between two metal plates. The polar substance attracts to the sides of the metal plates. Why?o Because the polar molecules in the substance align themselves so that their negative end is pointing towards the positive plate and their positive end is pointing towards the negative plate The dipole moment arrow points from the positive partial charge to towards the negative partial charge Each bond dipole is like a vector quantity, it has both magnitude and direction o Two bond dipoles of equal magnitude but opposite direction cancel each other  Moles like linear, trigonal planar, and tetrahedral etc. give molecules of zero dipole moment. That is the molecules are nonpolar Effect of Polarity on Molecular Properties A cis compound boils at a higher temperature because it’s polar and therefore has higher intermolecular forces which increases the energy required for the liquid to boil which increases the boiling point10.3 Valence Bond Theory Valence Bond Theory - an approximate theory to explain the electron pair or covalent bond by quantum mechanics  A bond forms between two atoms when the follow conditions are met o 1. An orbital on one atom comes to occupy a protion of the same region of space as an orbital on the other atom. The orbitals are said to overlapo 2. The total number of electrons in both orbitals is no more than two The strength of the bonding depends on orbital overlap Orbitals bond in the directions in which they protrude or point, to obtain maximum overlapHybrid Orbitals  Hybrid Orbitals- orbitals used to describe bonding that are obtained by taking combinations of atomic orbitals of the isolated atomso Ex. CH4 The number of hybrid orbitals formed always equals the number of atomic orbitals used Steps to obtain the bonding description about any atom in a moleculeo 1. Write the lewis electron dot formula of the moleculeo 2. Form the lewis formula, use the VSEPR model to obtain the arrangement of electron pairs about this atom o 3. From the geometric arrangement of the electron pairs, deduce the type of hybrid orbitals on this atom required for the bonding description o 4. Assign valence electrons to the hybrid orbitals of this atom one at a time, pairing them only when necessaryo 5. Form bonds to this atom by overlapping singly occupied orbitals of other atoms with the singly occupied hybrid orbitals of this atom10.4 Description of Multiple Bonding One hybrid orbital is needed for each bond (whether a single or a multiple bond) and for each lone pair A sigma bond has a cylindrical shape about the bond axiso Formed when two s orbitals overlap or when an orbital with directional character,such as a p orbital or a hybrid orbital, overlaps another orbital along their axis  A pi bond has an electron distribution above and below the bond axis o Formed by the sideways overlap of two parallel p orbitals  A single bond = 1 sigma bond  A double bond = 1 sigma & 1 pi bond  A triple bond = 1 sigma and 2 pi bonds Isomers- compounds of the same molecular formula but with different arrangements of the atomsMolecular Orbital Theory Molecular Orbital Theory- a theory of the electronic structure of molecules in terms of molecular orbitals, which may spread over several atoms or the entire molecule  Any molecule with an even number of electrons should be diamagnetic, because the electrons are paired  Any molecule with an uneven number of electrons are paramagnetic, like liquid oxygen o This is why liquid oxygen sticks to the poles of a strong magnet 10.5 Principles of Molecular Orbital TheoryBonding and Antibonding Orbitals Bonding Orbitals - molecular orbitals that are concentrated in regions between nuclei  Antibonding Orbitals- Molecular orbitals having zero values in the region between two nuclei and therefore concentrated in other regions The asterisk, star, tells us that the molecular orbital is antibonding Bond Order  Bond order refers to the number of bonds that exist between two atomso Bond Order = .5 (nb-na)o Nb= the difference between the number of electrons in bonding orbitals o Na= the difference between the number of electrons in antibonding orbitals Factors that determine Orbital Interaction 


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Pitt CHEM 0110 - Chapter 10- Molecular Geometry

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