09molecules - P461 Molecules 1 MOLECULES BONDS • Ionic...

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Unformatted text preview: P461 - Molecules 1 MOLECULES BONDS • Ionic: closed shell (+) or open shell (-) • Covalent: both open shells neutral (“share” e) • Other (skip): van der Waals (He- He)…Hydrogen bonds (in DNA, proteins, etc) ENERGY LEVELS • electronic • vibrational • rotational P461 - Molecules 2 Ionic Bonds - NaCl • First approximation. Both atoms are ions (no electron sharing) and bond due to EM force between two charged bodies Atom valence ionization ~effZ radius Na 3s 5.1 eV 1.8 .17 nm Cl 3s 2 3p 5 13 eV 2.9 .07 nm Ar 3s 2 3p 6 16 eV 3.3 .07 nm • Ar more tightly bound than Cl. But Cl- “looks” like Ar and more tightly bound then neutral Cl Has effective Z ~ 3 − + ⇒ ⇐ Cl bond Na 2 2 6 . 13 n Z E eff − = − + − + → + − − = Cl Na Cl Na eV need eV Cl E Cl E ) 8 . 3 1 . 5 ( : 8 . 3 ) ( ) ( P461 - Molecules 3 Atomic Properties 0.1 nm 10 eV P461 - Molecules 4 NaCl potential vs R Pauli repulsion at small R b shielding of nuclei becomes incomplete b electrons’ wave functions overlap and electrons forced to higher states (2p b 3s) 3 terms P461 - Molecules 5 Ionic Bonds - KCl • another example. What is the energy required to dissociate a KCl molecule into K atom and Cl atom given • Ionization energy K = 4.34 eV • electron affinity Cl = 3.82 eV • separation distance R = 2.79 angstroms • attractive EM potential for 2.79 A = -5.16 eV b takes 5.16 eV to separate K+ and Cl- ions to infinity b takes 3.82 eV to pull electron from Cl- to form neutral Cl b gain 4.34 eV when K+ captures electron and forms neutral K 5.16+3.82-4.34 = 4.64 eV = energy required P461 - Molecules 6 Ionic vs Covalent • As R >> 0.05 (size of 2p orbit), there is little overlap in the electron wave function between the Na and Cl ions b mostly ionic bond “94% ionic and 6% covalent” (DH makes up numbers) • look at HFl molecule H ionization energy = 13.6 eV Fl electron affinity = 3.4 eV b need 10.2 eV in electrostatic energy • as the size of filled 2p in Fl is about 0.05 nm and the nominal 1s in an H atom is 0.05 nm, the electrons are attached to both atoms b covalent bond “10% ionic and 90% covalent” (DH made up numbers) • the nuclei will start to not be shielded from each other b some ++ repulsion nm eV eVnm U e R 14 . 2 . 10 4 . 1 1 4 2 = − = − = πε P461 - Molecules 7 Covalent Bonds - Diatomic Molecules • assume all valence electrons are shared • often S=0 lowest energy but not always (Oxygen is S=1) • if both atoms are the same then | ψ| 2 same if switch atom(1) and atom(2) --- electron densities around each atom are the same (even sort of holds if different atoms like CO) H(1s) <-- very far apart ---> H(1s) close together H(“1s”)H(“1s”) electron wavefunctions overlap -“shared” • two energy levels (S=0,1) which have 2 2 | ) 1 , 2 ( | | ) 2 , 1 ( | ψ ψ = E R=infinity (atoms) 1s*1s Vib and rot bands P461 - Molecules 8 Covalent Bonds - Hydrogen • even if only 1 electron, bond is covalent • look first at ionized diatomic H...
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This note was uploaded on 10/02/2009 for the course PHYS 460 taught by Professor Johnson,c during the Spring '08 term at Northern Illinois University.

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09molecules - P461 Molecules 1 MOLECULES BONDS • Ionic...

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