09molecules - MOLECULES BONDS Ionic closed shell or open...

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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
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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 ) ( ) ( 0
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P461 - Molecules 3 Atomic Properties 0.1 nm 10 eV
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P461 - Molecules 4 NaCl potential vs R Pauli repulsion at small R barb2right shielding of nuclei becomes incomplete barb2right electrons’ wave functions overlap and electrons forced to higher states (2p barb2right 3s) 3 terms
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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 barb2right takes 5.16 eV to separate K+ and Cl- ions to infinity barb2right takes 3.82 eV to pull electron from Cl- to form neutral Cl barb2right gain 4.34 eV when K+ captures electron and forms neutral K 5.16+3.82-4.34 = 4.64 eV = energy required
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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 barb2right 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 barb2right 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 barb2right covalent bond “10% ionic and 90% covalent” (DH made up numbers) the nuclei will start to not be shielded from each other barb2right some ++ repulsion nm eV eVnm U e R 14 . 2 . 10 4 . 1 1 4 0 2 = = = πε
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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
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P461 - Molecules 8 Covalent Bonds - Hydrogen even if only 1 electron, bond is covalent look first at ionized diatomic H have repulsive potential between 2 protons depends on R = p-p separation (about 0.11 nm) but then have attractive energy V e for the electron.
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