Chem 120 (sec. 1-16) Study Guide for Exam 4Coverage: chapters 9 and 24 Chapter 9:Molecular geometry – It uses the VSEPR model to predict shape of molecules.- It is called the valence-shell electron-pair repulsion model.- Electron pairs (bonding or nonbonding) around central atoms are referred as electron domains.- Each pair (single, double, or triple) counts as one electron domain.- Electron-domain geometry – arrangement of electron domains around the central atom while Molecular geometry – arrangement of the atoms around the central atom.- Electron-domain geometry is often not the shape of the molecule.- We assume the electron pairs are placed as far as possible from each other. Shapes of Electron Domain and Molecular Geometries:- Linear: molecular geometry = linear, 2 bonding domains (180)- Trigonal Planar: molecular geometry = trigonal planar, 3 bonding domains & bent, 2 bonding domains and 1 nonbonding domain (120)- Tetrahedral: molecular geometry = tetrahedral, 4 bonding domains & trigonal planar, 3 bonding domains and one nonbonding domain & bent, 2 bonding and 2 nonbonding domains (109.5)- Trigonal Bipyramidal: has two distinct positions in this geometry (axial and equatorial), has four distinct molecular geometries in the domain: trigonal bipyramidal, seesaw, t-shaped, andlinear, lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry. (90 and 120) - Octahedral: all positions are equivalent in the domain, there are three molecular geometries: octahedral, square pyramidal, and square planarPolarity of molecules – consider both bond polarity and molecular shape to predict polarity of Molecules- Electronegativity – it increases as you move right and up the periodic table- If the outside atoms are more electronegative than the central atom than the molecule is = nonpolar- Molecules with the same atoms like H-H are nonpolar bonds and nonpolar molecules- Molecules with different atoms like F-Cl are polar bonds and polar molecules- Polar – molecules being pulled in all equally- Nonpolar – molecules being pulled out equally - Steps to predict the polarity of molecules:o Draw a Lewis Structure.o Use VSEPR theory to predict molecular shapeo Use electronegativity values to predict bond dipoleso Determine polarity of a molecule based on overall dipole moment of the molecule. - Nonbonding pairs are physically larger than bonding pairs, thus they have greater repulsions that tend to decrease bond angles in a molecule.- Multiple bonds place greater electron density on one side of the central atom than do single bonds; they also affect bond angles.Valence-bond (VB) theoryAtomic orbital overlap - Increased overlap-bring the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion. However, if atoms get too close, the intermolecular repulsion greatly raises the energy Hybrid orbitals - sp, sp2, sp3, sp3d, sp3d2 hybrid orbitals and their geometries Describe hybridization and geometry for the central atom of molecules- We mix orbitals so that the number of hybrids equals the number of domains - We think of covalent bonds forming through the sharing of electrons by adjacent atoms- Valence bond theory – says that a covalent bond is formed when atomic orbitals overlap - How to determine hybrid orbitals:1. Draw Lewis structure for molecule or ion.2. Determine electron-domain geometry using VSEPR model.3. Specify the hybrid orbitals needed to accommodate the electron pairs based on their geometry arrangement Multiple bonds and bonds – sigma bonds remain localized between two atoms; pi bonds can become delocalized between more than two atomsResonance structures – the p orbitals on all three oxygen’s overlap with the p orbital on the central nitrogen, delocalized  bonding – means the pie electrons are not localized between the nitrogen and one of the oxygen’s,but rather are delocalized throughout the ionDescribe bonding (, and delocalized  bonding) in moleculesMolecular-orbital (MO) theoryConcepts - bonding orbitals (box at the bottom of the energy-level diagram) and anti-bonding orbitals (box at the top of the energy-level diagram)Molecular orbital diagram (energy-level diagram) – The relative energies of molecular orbitals are represented by the energy-level diagram; this includes bonding electrons and anti-bonding electrons-Examples: H2 = (1/2)(2-0) = 1; He2 = (1/2)(2-2) = 0 DNE; Li2 = (1/2)(4-2) = 1; Be2 = (1/2)(4-4) = 0 DNE- According to the MO theory, overlap of two s atomic orbitals produces one bonding molecular orbital and one anti-bonding molecular orbitalBond order – (1/2)(number of bonding electrons – number of anti-bonding electrons). A bond order of ½ is possible. If the bond order = 0 then it does not exist. Chapter 24 (24.1-24.5):Hydrocarbons – compounds composed only of carbon and hydrogen, four basic types: alkanes, alkenes, alkynes, aromatic hydrocarbonsSaturated such as alkanes and unsaturated such as alkenes (have fewer than the maximum number of hydrogens) hydrocarbonsNomenclature of simple hydrocarbons – IUPAC names – three parts:1. Base – number of carbons in the longest continuous chain2. Suffix – tells what type of compound it is3. Prefix – tells what groups are attached to the chainDetermining names – 1. Find the longest chain in the molecule or the “parent chain”2. Number the chain from the end nearest the first substituent encountered.3. List the substituents as a prefix along with the number(s) of the carbons(s) to which they are attached.4. Name the molecule. If there is more than one type of substituent in the molecule, list them alphabetically. Ex: 3-Ethyl-2,4,5-trimethylheptaneStructures of organic compounds:Hybridization – three common states and geometries: sp^3 tetrahedral, sp^2 trigonal planar, sp linearstructural formula - condensed formulaIsomers - structural isomers and geometric isomers, occur when a molecule cannot rotate freely and when alkenes with two different groups on each carbon of the double bond- cis-isomers = have the carbons in the chain on the same side of the molecule- trans-isomers = have the carbons in the chain on opposite side of the moleculeAlkanes – Cn H2n+2, sp^3 hybrids, tetrahedral geometry, 109.5 bond angles, only sigma bonds, free rotation about the C-C bondsAlkenes – contain at least one C=C double bond, unsaturated, the simplest =