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09.26.2011 - Lecture 09 Today in Chemistry 260/261 Lecture...

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Lecture 09 9/26/11 1 Chemical bonding Lecture 9 9/26/2011 Today in Chemistry 260/261 Periodic trends: • Atomic/ionic radii • Ionization energy • Electron affinity • Electronegativity Valence bond model Hybrid atomic orbitals Resonance Later in Chemistry 260/261 • Reading: 6.1, 6.4 Periodic Properties Atomic/Ionic Radius non-bonding atomic/ ionic radius bonding atomic/ionic radius =1/2 bond length Ionization Energy Electron Affinity Electronegativity Amount of energy required to remove an electron from a gaseous atom X( g ) ! " ! X + ( g ) + e # Amount of energy released when a gaseous atom gains an electron X( g ) + e ! " # " X ! ( g ) Tendency of one atom to attract electrons from another atom to which it is bonded Atomic Size IN GENERAL Atomic radius increases with increasing principle quantum number Atomic radius decreases with increasing nuclear charge in any given shell/subshell. Atomic Size At Z= 30, 4s/4p electrons are well shielded by 3d electrons. Full 3d is shielding well – repelling more 5d elements not as big as expected f electrons are very inefficient shielders Z eff is thus larger than expected àฏ smaller atoms LANTHANIDE CONTRACTION Deviations of contracting size within a row are due to shielding effect (filled subshells). Ionic Size Cations shrink relative to the corresponding neutral atom Anions expand relative to the corresponding neutral atom Effective nuclear charge and e - e - repulsion explain both trends
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Lecture 09 9/26/11 2 ConcepTest One of these spheres represents a K + ion, the other a Cl - ion. True or False: The red sphere is the K + ion. ConcepTest One of these spheres represents a K + ion, the other a Cl - ion. True or False : The red sphere is the K + ion. Of course True, same shielding (electron count) of a larger nuclei, will be more contracted! Ionization Energy X( g ) ! " ! X + ( g ) + e # ° Outermost electron removed from atom to form cation n E ! " I.E. ! ! I.E. , as eff Z ( ) [ ] 2 2 n n Z eff = Koopman s approximation Frozen orbital approximation: Assumes that the orbital energies are the same before and after ionization Relaxation effects (1-3 eV) included in Hartree-Fock method Remember: Z eff for 2p < Z eff for 2s Ionization Energy X( g ) ! " ! X + ( g ) + e # 2p orbital is higher in energy than the 2s orbital (2p is more shielded than 2s) Generally, ionization energy increases with nuclear charge… but there are some kinks. electrons begin being paired in 2p ° more e - -e - repulsion Explanations must balance effective nuclear charge, shielding/penetration, and e - -e - repulsion. IP almost contant: increasing # of d electrons shields 4s from increasing Z eff (4S is ionized) Second Ionization Energy X + ( g ) ! " ! X 2 + ( g ) + e # X( g ) ! " ! X + ( g ) + e # Always takes a greater amount of energy to remove a 2nd electron relative to the first I.E.
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