Announcements – 9/13/00 Bonding: Ionization Energies n Labs begin TODAY! n Ionization Energy (IE) n Old exams on website -quantifies the tendency of an electron to leave an atom in the gas phase: n Problem Set Solutions? n Exam #1 X (g) X+ (g) + e- DE = IE -covers matl thru this Friday (Ch 1&2) IE: -always positive (energy ADDED) -email/contact me ASAP if you have a conflict with exam time -INCR across row -DECR down a group n Demo and Quiz on Friday 1 2 Bonding: Electron Affinity Bonding: Electronegativity n Electron Affinity (EA) n Electronegativity (EN) - -quantifies ability of an atom to attract an e in -combines IE and EA terms to give the the gas phase relative ability of an atom to attract e-’s to X (g) + e- ® X- (g) -DE = EA itself when bonded to another atom EA: -it’s the energy released upon addition of an electron to an atom EN: -INCR across a row -can be positive or negative -DECR down a group - (pos: atom wants the e -Best to consider DEN for a bond neg: atom happy as an atom) 3 4 1 EN: Examples Bond Polarity: Dipole Momement n NaCl: Na EN = 0.93 n HCl d+ d- ¬ partial charges DEN = 2.23 (ionic) H – Cl Cl EN = 3.16 2.2 3.2 Polar Covalent bond: share e-, but not equally n O2: O EN = 3.44 DEN = 0 (covalent) -Quantify via: DIPOLE MOMENT (µ) n HCl: H EN = 2.2 µ = d x d Bond length (m) DEN = 0.96 (?) 1 Debye (D) Amt of displaced charge (C) Cl EN = 3.16 (polar covalent) = 3.34 x 10-30 C-m 5 6 Dipole Moment Examples Visualizing Molecules n H2O O EN = 3.44 H EN = 2.2 DEN = 1.24 CH4 -each H-O bond is polar, but does the MOLECULE have a net dipole moment? n CH4 C EN = 2.55 H EN = 2.2 DEN = 0.35 -each C-H bond has a dipole moment, but does the entire MOLECULE have a net dipole moment? We need to know the STRUCTURE! How do we figure out the structure? 7 8 2 Lewis Bonding Theory Lewis Symbols n Based on some simple assumptions: n Place valence electrons around -valence electrons are the major players in element symbol: chemical bonding -ionic bonds form when electrons are transferred between atoms :X: <- element X (4 e-) -covalent bonds form when electrons are shared by atoms - pair electrons, when possible -the extent of electron transfer/sharing is - bonds represented by dashes (-) so as to give each atom a stable electron Example: · · · · configuration (usually an octet) :F· · - F:· · 9 10 Drawing Lewis Structures Example n The Quick and Dirty Method: Sulfur Dioxide (SO2): 1. Add up the total number of valence S & O: Group 6A -> 6 x 3 = 18 e- electrons (from Group #) · · 2. Draw skeleton structure with only -Draw Skeleton structure single bonds 2 e-/bond, leaves 14 e- S 3. Distribute remaining electrons around -Move e- to make atoms as non-bonding electrons multiple bonds 4. Redistribute non-bonded electrons into and octets :O: :O: multiple bonds so that each atom has an Done! · · · · octet 11 12 3 Getting Structures VSEPR Theory n Once we have the Lewis diagram, we can determine n Count electron pairs (include non- the structure by looking at the distribution of bonded!) bonded and non-bonded electron pairs about a central atom, using: 2 -> Linear (180o bond angle) Valence Shell Electron Pair Repulsion Theory 3 -> Trigonal Planar (120o bond angle) 4 -> Tetrahedral (109.5o bond angle) Ø electron pairs will repel each other and will distribute themselves about a central atom so as to 5 -> Trigonal Bipyramidal maximize their separation in 3-dimensional space 6 -> Octahedral 13 14 VSEPR Structures 15 4.