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Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure...

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Molecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond Theory Hybrid Orbitals Multiple Bonds VSEPR (Valence Shell Electron Pair Repulsion) Valence Bond Theory Overlap of atomic orbitals – is a covalent bond that joins atoms together to form a molecule Consider each atom to donate 1 e- to the pair which makes up a bond Filled Orbitals F 2 2p __ __ __ __ __ __ 2s __ __ 1s __ __ F F (http://www.hull.ac.uk/php/chsajb/concepts/ho_2.html ) (http://www.ausetute.com.au/lewisstr.html ) 1s 2 __ 1s 2 __ No empty orbitals (http://www.ausetute.com.au/lewisstr.html )
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Page 1: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

Molecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond Theory Hybrid Orbitals Multiple Bonds VSEPR (Valence Shell Electron Pair Repulsion) Valence Bond Theory Overlap of atomic orbitals – is a covalent bond that joins atoms together to form a molecule Consider each atom to donate 1 e- to the pair which makes up a bond Filled Orbitals F2 2p __ __ __ __ __ __ 2s __ __ 1s __ __ F F

(http://www.hull.ac.uk/php/chsajb/concepts/ho_2.html)

(http://www.ausetute.com.au/lewisstr.html)

1s2 __ 1s2 __ No empty orbitals

(http://www.ausetute.com.au/lewisstr.html)

Page 2: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

Types of Bonds Sigma overlap between nuclei examples include: s and s s and p p and p Pi bond overlap above and below nuclei with parallel p orbitals Overlap of p orbitals that are perpendicular to line through nuclei Sigma ( σ ) bond Filled orbitals do not contribute to bonding but do contribute to size

(http://classes.yale.edu/02-03/chem220a/studyaids.html)

Page 3: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

Pi (π ) Bond In atoms with double or triple bonds π Bond is weaker than σ since less overlap Hybridization and bond formation are simultaneous process Hybrid orbitals overlap more effectively Electron pairs of bonds are as far away from each other as possible thus there is a lower energy

Page 4: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

Lennard Jones Potential Illustrates energy that holds atoms together in a bond or molecules together in a liquid Y axis is energy and X axis is distance between atoms or molecules Lower energy when brought together but energy too high if pushed very close together Most stable position is one with lowest energy Atoms brought close together

(http://employees.csbsju.edu/hjakubowski/classes/ch331/protstructure/olunderstandconfo.html) Hybrid Orbitals Element Orbitals Bonds Expected Actual Bonds Be 1s22s2 0 2 B 1s22s22px

1 1 3 C 1s22s22px

12py1 2 4

Page 5: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

(http://www.platte1.k12.wy.us/Gowdy/Chemistry/chapter_13.htm) Mixing of different orbitals to form equivalent obitals is hybridization Be 2p __ __ __ 2p __ __ Linear 2s __ -----> sp __ __ 1s __ 1s __ B 2p __ __ __ -------> sp2 __ __ __ Trigonal Planar 2s __ ------> 1s __

Page 6: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

C 2p __ __ __ ------------> sp3 __ __ __ __ Tetrahedral 2s __ ------------> 1s __ Geometry differs from either isolated orbital Energy is also different from isolated atoms To Determine structure 1) Draw Lewis structure and find number of pairs of electrons 2) Determine electron pair geometry 3) Determine Molecular Geometry Electron Pair Geometry can be different or same as molecular geometry (illustrated below) Electron Pair Geometry of CH4: Tetrahedral Molecular Geometry of CH4: Tetrahedral

(http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/CH10/FG10_03b.JPG) Electron Pair Geometry of NH3: Tetrahedral Molecular Geometry of NH3: Pyramidal

Page 7: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

(http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/10.html) Electron Pair Geometry of H2O: Tetrahedral Molecular Geometry of H2O: Bent

(http://dbhs.wvusd.k12.ca.us/webdocs/Bonding/) Molecular Geometry (shapes)

Page 8: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

(http://www.elmhurst.edu/~chm/vchembook/202linear.html)

(http://www.elmhurst.edu/~chm/vchembook/202linear.html)

Page 9: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

(http://www.elmhurst.edu/~chm/vchembook/203trigplanar.html)

(http://www.elmhurst.edu/~chm/vchembook/205trigpyramid.html) Square Planar

Page 10: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

(http://www.up.ac.za/academic/chem/mol_geom/planar.htm)

(http://www.elmhurst.edu/~chm/vchembook/204tetrahedral.html)

(http://fia.coas.unf.edu/gchm/chime.html) Octahedral

Page 11: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

(http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/10.html) Summary of Molecular Geometry

Example No. of Electron Pairs

Electron Pair Geometry (Bond Angle)

No. of Pendant Atoms

Molecular Geometry Formula Image

Click below to show rotation.

2 linear

(180o) 2 linear BeH2 Show rotation

s3 trigonal planar (120o)

3 trigonal planar CO32-

Show rotation

2 bent NO2-

Show rotation

Page 12: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

4 tetrahedral (109.5o)

4 tetrahedral CH4

Show rotation

3 trigonal pyramidal

NH3

Show rotation

2 bent H2O Show rotation

5 trigonal bipyramidal (90o, 120o)

5 trigonal bipyramidal

PCl5

Show rotation

4 see-saw SF4

Show rotation

3 T-shaped BrF3

Show rotation

2 linear ICl2- Show rotation

6 octahedral (90o)

6 octahedral SF6

Show rotation

5 square pyramidal

BrF5

Show rotation

4 square planar ICl4-

Show rotation

(http://www.molecules.org/VSEPR_table.html) Multiple Bonds Ethane

Page 13: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

(http://www-theor.ch.cam.ac.uk/people/ross/thesis/node143.html) Bond Angles ~ 109.5o Valence Carbon Electrons Ethane Carbon p __ __ __ ---> sp3 __ __ __ __ Tetrahedral

s __ Ethene Carbon p __ sp2 __ __ __ Trigonal Ethyne (Acetylene) p __ __ sp __ __ Linear Hybridization in Multiple Bonds p __ __ __ p __ p __

Page 14: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

-------> -------> sp2 __ __ __ or sp2 __ __ __ s __ Ethylene (Ethene)

(http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/10.html) 5 σ bonds 1 π bond Ethyne (Acetylene) p __ __ __ p __ __ p __ __

Page 15: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

s __ sp __ __ or sp __ __ Ethyne 3-D Orbitals

Page 16: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

(http://wps.prenhall.com/wps/media/objects/724/741576/chapter_01.html) Bond Length (A) 1.54 Carbon to Carbon single bond 1.34 Carbon to Carbon double bond 1.20 Carbon to Carbon triple bond Removal of Electrons Electrons do not come off the same order they go on Examples: Fe 3s23p63d64s2 Fe2+ 3s23p63d6 (4s2 comes off) Fe3+ 3s23p63d5 (3d off) Co [Ar] 4s23d7 Co2+ 3s23p63d7 Co3+ 3s23p63d6 Cu [Ar] 3d104s1 Cu+ 3s23p63d10 Cu2+ 3s23p63d9

Page 17: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

VSEPR minimize electron repulsion Ammonia NH3 1s22s22px

12py12pz

1

(http://www.uyseg.org/greener_industry/pages/ammonia/1AmmoniaAPQ.htm) σ bond from p orbital H—N—H 107.3o Could explain by sp3 hybridization because it is close to tetrahedral angle 109.5o Water H2O 1s22s22px

22py12pz

1

(http://www.hrw.com/science/si-science/chemistry/atomic_structure/molecules/02mol.html) H—O—H 104.5o Could explain by sp3 hybridization because it is close to tetrahedral angle 109.5o

Page 18: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

Valence Shell Electron Pair Repulsion (VSEPR) Model Compound Angle Bonding Pair Lone Pair CH4 109.5o 4 0 NH3 107.3o 3 1 H2O 104.5o 2 2 Bonding Pair charge is smaller Lone Pair charge cloud is larger so repulsion is greater Order of Repulsion LP—LP > LP—BP > BP—BP Polar Molecules and Electronegativity Bond Ionic metal (cation) and nonmetal (anion) Na+Cl- Pure Covalent is with identical atoms Cl—Cl Polar Covalent is the partial transfer or uneven sharing

(http://www.teachmetuition.co.uk/Chemistry/Intermolecular/dipole.htm)

(http://faculty.njcu.edu/tpamer/gilbert-lessons/lesson5/gilbert6-4.htm) Dipole moment is a positive and negative side Electron pair is more toward the Cl atom Electronegativity decides what type of bonding Electronegativity is the measure of ability of an atom to pull an electron toward it Basically the strength of attraction of electrons

Page 19: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

Linus Pauling Scale

(http://www.webelements.com/webelements/properties/text/image-balls/electroneg-pauling.html)

(http://www.bcpl.net/~kdrews/properties/properties2.html) Electronegativity F > O > Cl ~ N > Br > I ~ C ~ S ~ Se > P H ~ P F is the most electronegative Cs is the least electronegative

Page 20: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

If there is more than one atom in a compound then sum up bond moments can imagine it is like a molecular tug of war with atoms pulling electron pair

(http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/10.html) But with CO2 the dipole moment for the molecule results in 0 The reason is that there are equal pull from both directions

(http://www.hull.ac.uk/php/chsajb/symmetry&spectroscopy/ho_2.html) These diagrams show that negative (more e-) and positive side of molecules helps in predicting reactions In the compound H—Cl when the bond breaks the electron will go with Chlorine because it is more electronegative

Page 21: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

Understand Bond Strength correlates with Electronegativity difference Compound Electronegativity

Difference Bond Energy (kJ/mol)

H—F 1.9 570 H—Cl 0.9 430 H—Br 0.7 360 H—I 0.4 300 Greater the Electronegativity Difference then stronger the bond Representative VSEPR Structures Orbital geometry: Describes the geometry of the orbitals, takes the nonbonding electron pairs into account because they must be in an orbital. The steric number and the hybridization will give the orbital geometry (electron-pair geometry). Therefore, there are only 5 possible orbital geometries:

Page 22: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

Octahedral (sp3d2) Trigonal Bipyrimidal (sp3d) Tetrahedral (sp3) Trigonal Planar (sp2) Linear (sp) Molecular Geometry: uses the nonbonding electron pairs to describe the geometry of the molecule

(http://library.tedankara.k12.tr/chemistry/vol3/Molecular%20geometry%20and%20hybridization/z94.gif)

Steps for Determining Geometry: Draw Lewis structure and find number of pairs of electron Determine electron pair geometry Determine molecular geometry

Page 23: Molecular Structure - University of Tennessee at Chattanooga · PDF fileMolecular Structure Topics 3-D structure shape (location of atoms in space) Molecular Geometry Valence Bond

Note: Electron pair geometry can be different than molecular geometry For Example: Tetrahedral Electron geometry can have 3 different molecular geometry (tetrahedral, pyramidal and bent)


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