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Lecture 13

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University of Toronto Mississauga
Thottackad Radhakrishnan

Lecture 13 Wednesday, 5 February, 2014 11:01 AM Today's Topics: Announcements: Homework:  Bond dipoles  Midterms to be handed Quiz 2  Orbital hybridization back in tutorials this week Problem set 3  Confirmations Lecture Topic: Molecular polarity • Polyatomicmolecules - molecular dipole momentis the sum of the bond dipole vectors ○ Molecular dipole and molecular shape Molecule Dipole moment CO 2 0  BCl3 0 -30 SO 2 5.42x10 PCl 2.3x10-30 3  Dipole momentdepends on polarity of the bond and shape of the molecule  In CO2and BCl 3 the bonds are polar but the linear/trigonal planar shape cancel out the bonds ○ Molecular dipole and boiling point Molecule Dipole moment Boiling point SiH4 0 -185°C -30  PH 3 1.80x10 -88 -30 SH 2 3.21x10 -62 CIH 3.43x10 -30 -35  As the dipole momentincreases the intermolecularinteraction increases and boiling point increases  Higher dipole moment= higher intermolecularattraction = moreenergy needed to separate them = higher boiling point • Ex: which has a higher dipole moment,NF or NH ? 3 3 Molecule μ (Cm) Bond angle ○ NF 3 0.7x10-30 102° NH 3 4.88x10 -30107° ○ Typical thought process: NF 3as higher electronegativityso has higher dipole moment (NF3EN: 1.0, NH 3N: 0.8) ○ NH a3tually has a higher dipole momentbecause of the contribution of the lone pair of electrons - in N3 the ligands and lone pair act together, in the same direction ○ In NF 3he ligands and lone pair act in opposite directions ○ Bond angles in a trigonal pyramid is expected to be 109.5°  F is more electronegativethan H and it draws the bonded pair towards F allowing the lone pair to push the three F atoms closer and hence the bond angle is reduced to 102° Element Electron configuration Unpaired e Compound Valence Li 2s1 1 LiH 1 Be 2s2 0 BeH 2 2 B 2s 2p x1 1 (BH3 2 3 • C 2s 2p xp y1 2 CH 4 4 N 2s 2p xp 2y1 z 3 NH 3 3 O 2s 2p xp 2y1 z 2 OH 2 2 F 2s 2p xp 2y2 z 1 HF 1 Ne 2s 2p 6 0 - 0 ○ Note the pattern of valence electrons:12343210and the pattern of unpaired electrons:10123210 ○ Unpaired electronsaffects number of bonds that can be made  BUT Carbon has only 2 unpaired electronsand yet will always make CH inste4d of CH2- why? Orbital hybridization • Hybridization is a mathematicalprocess by which the wave functions fo
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