Chem 102 lec 2a W’11 UPDATE on office hours: TR 330-430pm Last time: orientation 1) rules for Lewis structures. 2) hydrocarbon line structures 3) multiple covalent bonds 4) resonance 5) bond polarity 6) VSEPR model and electron domain and molec geometry 7) molecular polarity Today: 1) review VSEPR and molecular geometry 2) hybridization 3) sigma and pi bonds Lecture 2a:  (1) Review VSEPR Model: VSEPR Model. 2 e domains = linear; 180° 3 = trigonal planar; 120° 4 = tetrahedral; 109.5° 5 = trigonal bipyramidal; 120° and 90° 6 = octahedral; 90° (2) Refer to relevant tables in the book: Terms used for molecular geometry: Linear, triangular (or “trigonal”) planar, tetrahedral, triangular pyramidal, angular (or “bent”), triangular bipyramidal, seesaw, T-shaped, octahedral, square pyramidal, square planar.

(3) Note that lone e pairs repel more than lone pairs. So the actual angles differ from the ideal. Know the implications for bipyramidal and octahedral electron domain structures. (4) Do examples: Draw the Lewis structures. From that determine the electron domain shapes, the molecular shapes, the angles between the bonds, the polarity of bonds and the polarity of the molecules themselves. PF5 SF4 (4) Hybridization: quick preview a) relate e pair geometry to hybridization last time we gave a preview: electron domain hybridization linear sp trigonal planar sp2 tetrahedral sp3 bipyramidal sp3d octahedral sp3d2 (5) understanding hybridization; drawing the shapes Hybrid orbitals. Valence bond theory (VBT)

1) draw H2 molec. 1s & 1s combine: sigma (σ) bond. Electron density is along internuclear axis

2) H-F bond: 2p and 1s overlap: also sigma bond 3) draw F-F molec: 2p & 2p combine. : sigma bond

also 2p-2p sigma 4) draw O=O molec: double bond. 2p and 2p but

now have: sigma bond and : pi (π) bond (describe properties of a pi bond: more easily broken. Not really 2 equally strong bonds. Sigma bonds can be rotated. But not pi bonds. Pi bonds are only found in double and triple bonds. Sigma bonds always.)

5) draw CH4: problem! need to invoke a new thing: hybrid orbitals.

Hybrid orbitals: formed from s and p (and d). # hybrid orbitals = # orbitals hybridized. P| p| p P | P| => 2sp| 2 sp| 2s || 2s if we consider a C atom: 1s2, 2s2 2p2 can have: two sp suborbitals and two 2 p suborbitals: sp suborbitals are linearly placed. can have three sp2 suborbitals and 1 2p orbital: sp2 suborbitals are trig planar placed can have four sp3 suborbitals. Tetrahedral. Know: sp orbital sp2 sp3 sp3d sp3d2 NEXT LECTURE: (1) Intermolecular interactions: a) know various types intermolecular interactions – b) know to relate bond lengths to the orbital overlaps. – you read this