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Unformatted text preview: 02/01/2008 D ne o General Chemistry
Chem102 for Engineering Dr. Raafat
1 lecture Objectives Vale She Elle nce he e ll ctron Pair Re l EPR) ale air pulsion Mode (VS Mole cular shape s Line ar? Line Trigonal planar? Te trahe dral? Trigonal bipyram idal? Octahe dral Mole cular dipolem e om nts 2 Molecular Shape Threedimensional shape of molecule or three dimensional arrangement of atoms in a molecule Explains molecular interactions and chemical properties It is helpful to know how to predict the geometric shape of molecules… Linear? Vshaped? Trigonal planar? Tetrahedral? 3 The VSEPR Model Valence Shell Electron Pair Repulsion Model Make predictions about shape from Lewis structures Electronpairs will repel each other electrically Try to minimize electronpairs repulsion Thus, the electronpairs are arranged as far apart as possible around a central atom
4 I. Linear Structure, AX2 Two pairs of electrons surrounding the central atom 180o apart 5 Examples of Linear molecules AX2 Linear – central atom has no non-bonding electrons (no lone pairs) E. P. G. = Electron Pair Geometry
6 I. Trigonal Planar Three pairs of electrons around the central atom 120o apart 7 Examples of Trigonal planar ge e AX3 om try, Trigonal No non-bonding electrons on the central atom or Boron trifluoride BF3 Formaldehyde H C H O Trigonal geometry 8 Ozone AX2E – classification (one lone pair on central atom) O3 ; number of valence electrons = 18 electrons OOO OOO
Resonance structures Nonlinear or bent or Vshaped Mole cular ge e (nam om try e of m cular structure is ole ) base on theposition of d t heatom s
(E. P. G.)
9 I. Tetrahedral structure Four pairs of electrons on central atom 109.50 apart 10 10 Examples of tetrahedral molecules AX4 Tetrahedral – central atom has no non-bonding electrons H CH H H The hydrogen atoms are as far apart as possible at 109.5° bond angle. This is tetrahedral geometry. The molecule is three dimensional. 11 11 AX3E – classification (one lone pair on central atom) Ammoni a Trigonal pyramidal geometry H—N—H angle = 107° 12 12 Water H H O .. : AX2E2 – classification (two lone pairs on central atom) bent geometry H—O—H angle = 104.5° Vshape Nonlinear 13 13 Te trahe dral Ele ctron-pair Ge e om try H2O NH3 C4 H two lone pairs Angle 104.5° Molecular geometry: bent or Vshape one lone pair Angle 107° no lone pair Angle 109.5° Trigonal pyramidal Tetrahedral Te trahe anglem vary from109.5° (typical value dueto size dral ay ) diffe nce be e bonding and lonepair e ctron de re s twe n le nsitie s
bonding pair s ore • A lonepairs take up m spacethan a bonding pair
• Mutual re pulsion: two lonepairs > lonepair − bonding
lonepair pair > bonding pair − bonding pair.
14 14 I. Trigonal bipyramidal Five pairs of electrons on central atom Equatorial e ctrons: Thre pairs de an Equatorial le e fine Triangle(angle120°) Axial e ctrons: Two pairs lieaboveand be the le low t riangleplane Axial positions are90° f rom3 . e quatorial positions
15 15 Example of Trigonal bipyramidal molecules AX5
Trigonal bipyramidal – central atom has no non-bonding electrons
5 PC l Phosphorus pe ntachloride .. Cl .. .. Cl
P .. Cl .. . . . . .. Cl .. . .
Cl . . . .
4 .. . . . .
F S F F F
16 16 S ulfur Te trafluoride S ,F AX4E– classification (one lone pair on central atom) axial • • Lonepair is in thee quatorial position be cause Lone it re quire m room than a bond pair. s ore ..
F .. .. 3 86.20 Br The repulsion between the two equatorial lone pairs decreases the angle from the ideal 90°. 3- .. I Triiodide,
I I I .. AX2E3 .. 1800 Lone Pairs need to have more room and be further from other pairs than bonding pairs. 17 17 Linear .. Bromine trifluoride, BrF AX3E2 – classification (one lone pair on central atom) .. F .. .. .. ..
F .. . .
T-shaped . .. . .. .. .. .. I. Octahedral AX6 Six pairs of electrons on central atom Equatorial e ctrons: Four pairs de an le fine Equatorial square Axial e ctrons: Two pairs lieaboveand be le low t hesquareplane . Octahedral: all angles are 90° 18 18 Example of Octahedral molecules AX6
Octahedral – central atom has no non-bonding electrons Phosphorous hexachloride, PCl6 Sulfur hexafluoride SF6 Square Bipyramidal
19 19 AX5E – classification (one lone pair on central atom)
5 Bromine pentafluoride, BrF S quarePyram idal
20 20 Xenon Tetrafluoride
4 AX4E2 – classification (two lone pairs on central atom) XeF Seesaw high energy (unfavored) Square Planar low energy (favored) 21 21 Molecular Dipole Moments
δ- O δ+ C O δ- polyatomic molecule must have polar bonds This may not sufficient for the whole molecule to have net dipole moment. In order to determine if the molecule is polar 22 22 Molecular Dipole Moments
O C O Carbon dioxide has no dipole moment; µ = 0 D Carbon 23 23 C l4 C C 2C 2 Hl Carbon tetrachloride Dichloromethane µ=0D µ = 1.62 D 1.62
24 24 C l4 C Resultant of these two bond dipoles is Resultant of these two bond dipoles is µ=0D
Carbon tetrachloride has no dipole moment because all of the individual bond dipoles cancel.
25 25 C 2C 2 Hl Resultant of these two bond dipoles is Resultant of these two bond dipoles is µ = 1.62 D 1.62
The individual bond dipoles do not cancel in dichloromethane; it has a dipole moment.
26 26 ...
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