CHEM 10171 Lecture 3: Gen Chem Notes 3, Bonding and Geometry

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University of Notre Dame
Chemistry and Biochemistry
CHEM 10171

Bonding • Bond order = ½ (bonding electrons - antibonding) o No unpaired electrons: diamagnetic o Two unpaired electrons: paramagnetic • S-P mixing o Energetically, s & p orbitals in N are much closer than they are in O; maybe need to consider interaction of s with p orbitals ▪ S-p mixing doesn’t affect pi orbitals (purely p) • Heterodiatomic molecules o Effects on orbital composition • Consequence of lopsided orbital o Number of electrons around F > 7 o Number of electrons around H < 1 ▪ Net dipole moment Molecular Geometry • MO theory: distinction between sigma and pi bonding • Generalize to complicated molecules: o Represent using lewis structures, but include distinction between sigma and pi bonds ▪ Single bond: sigma bonds only ▪ Double bonds: sigma bond and pi bond ▪ Triple bond: sigma bond and two pi bonds o Atoms form sigma bonds using hybrids orbitals o atoms s form pi bonds using (pure) p orbitals o In the absence of resonance structures: ▪ Lone pairs are held in hybrid orbitals • Hybridization → geometry • Three causes: o 2p orbitals, 2 hybrid orbitals → s+p =--> sp hybridization o HCCH: 1 p orbital, 3 hybrid orbitals: s + 2p = sp2 hybridization ▪ Trigonal planar geometry o Dash: going back o Wedge: coming out o Unhybridized p
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