1 SHAPES OF MOLECULES VSEPR MODEL Valence Shell Electron Pair Repulsion model Electron pairs surrounding atom spread out as to minimize repulsion Electron pairs can be bonding pairs including multiple bonds or nonbonding pairs Arrangement of all the atoms surrounding central atom depends on electron pairs surrounding central atom Electron domain is a better term than electron pair Two similar but different geometries 1 Electron domain geometry arrangement of e domain around central atom remember multiple bonds count as a single e domain 2 Molecular geometry arrangement of atoms around central atom A molecular geometry is decided only after an electron domain geometry has been determined need to write Lewis structure to determine number of electron domains Geometries with two e domains about central atom 1 electron domain geometry linear A A generic atom angle between e domains is 180 2 possible molecular geometries a Linear only linear geometry is possible with two electron domains Example BeCl2 Cl Be O C Cl Example CO2 O Note Only two electron domains around central atom since multiple bonds count as a single domain Example CO O C 2 Geometries with three e domains about central atom 1 electron domain geometry trigonal planar A generic atom A angle between e domains is 120 2 possible molecular geometries a Trigonal Planar all three electron domains are bonding pairs Examples BF3 and NO3F F O B N F O O b Bent V shaped two bonding pairs and one nonbonding pair Example dichlorocarbene C Cl Cl Nonbonding e pairs take up more room than bonding pairs Therefore bond angle between chlorine atoms is slightly less than 120 Geometries with four e domains about central atom 1 electron domain geometry tetrahedral tetrahedron is three dimensional object angle between electron domains is 109 4 A Caution Representation is 2 D not 3 D bond angle between epairs is not 90 3 2 possible molecular geometries a Tetrahedral all four electron domains are bonding pairs Example CH4 H H H C H C H HH H 2 D picture 3 D picture Example PO43O O O P O P O OO O b Trigonal Pyramidal 3 bonding pairs and 1 nonbonding pair Example NH3 H H N N H H H H Bond angle is 107 specifically for NH3 Bond angle is less than 109 4 because nonbonding pair takes more room than bonding pair Example ClO3O Cl O O Cl O O O 4 c Bent V shaped two bonding pairs and two nonbonding pairs Example H2O H O H Bond angle is 104 5 Redraw H2O to show tetrahedral angle H H O Example SF2 S F F Geometries with five e domains about central atom 1 electron domain geometry trigonal bipyramidal two different positions in a trigonal bipyramid axial two position along axis equatorial three positions along equator angle between equatorial positions is 120 axial positions are 90 from equator nonbonding pairs prefer equatorial position 2 possible molecular geometries a Trigonal Bipyramidal all five electron domains are bonding pairs Example PF5 F F P F F F F F F P F F Note axial bond lengths usually longer than equatorial bond lengths 5 b Seesaw four electron domains are bonding pairs and one nonbonding pair Example SF4 F F F S F S F F F F remember lone pairs prefer equatorial position c T shaped three electron domains are bonding pairs and two are nonbonding pairs Example ClF3 F Cl F F F F Cl F both lone pairs occupy an equatorial position d Linear two electron domains are bonding pairs and three are nonbonding pairs Example I3 triiodide ion I I I all three lone pairs occupy equatorial positions 6 Geometries with six e domains about central atom 1 electron domain geometry octahedral angle between electron domains is 90 2 possible molecular geometries a Octahedral all six electrons domains are bonding pairs Example SF6 F F S F F F F b Square pyramidal five electrons domains are bonding pairs with one lone pair Example BrF5 F F F Br F F note angles will be slightly less than 90 c Square planar four electrons domains are bonding pairs with two lone pairs Example XeF4 F F F Xe F What are angles between fluorine atoms 7 POLAR MOLECULES A polar molecule has one side slightly positive and the other slightly negative Two conditions must be met in a polar molecule 1 Polar covalent bonds 2 Correct geometry To emphasize necessity of correct geometry compare two examples GeO2 Ge 2 0 Cl 3 5 Ge O 1 5 H2O H 2 1 O 3 5 H O 1 4 Ge O H O indicates positive end of bond Ge O bond is more polar than H O bond however GeO2 is nonpolar molecule and H2O is a polar molecule Question How can this be Answer GeO2 has a linear geometry and H2O has a bent geometry H O Ge O H O total polarity adds to zero total polarity is nonzero Example Is either ammonia or methane a polar molecule N H H H H H C HH Answer Ammonia is a polar molecule but methane is a nonpolar molecule 8 Nonequivalent polar bonds can affect overall polarity CF4 is a nonpolar molecule CF3Cl is a polar molecule F F C Cl FF F C FF Dipole Moment When equal and opposite charges Q are separated by a distance d the dipole moment is defined as Q d Since a polar molecule has a separation of charge it has a dipole moment Polarity of molecule is usually considered via its dipole moment Scheme Chemical formula Lewis structure e pair geometry molecular geometry polarity VALENCE BOND THEORY Bond Overlap Molecular bonds form when atomic orbitals overlap Hydrogen H2 1s 1s Bond is overlap of two 1s orbitals Fluorine F2 2pz 2pz Bond is overlap of two 2pz orbitals Hydrogen Fluoride HF 1s 2pz Bond is overlap of a 1s orbital and a 2pz orbital 9 Concepts of bond overlap Energy of molecule is lowered when overlap of singly occupied orbitals occurs As atoms approach each other increasing overlap has a limit Nuclei start to repel each other Bond distance is a compromise between increasing overlap and increasing nuclear repulsion Energy Potential Energy Curve for Diatomic Bonding nuclear repulsion electron overlap zero interaction at R Atomic Distance R Equilibrium bond length HYBRIDIZATION Consider the orbital diagram of the ground state carbon atom 2p 2s 1s Since bonding occurs from the overlap of atomic orbitals one would na vely think that the carbon would form only two bonds coming from the overlap of the two 2p electrons with orbitals from other atoms However we know from a huge number of experiments that carbon forms four bonds How can this be 10 Perhaps a small input of energy causes one of the 2s electrons to enter the empty 2p orbital 2p 2s 1s Now the carbon atom can form four bonds however we have another problem In a compound such as