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)
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)
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
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 1s 2 2s 2 0 2 B 1s 2 2s 2 1 2p x 1 3 C 1s 2 2s 2 2p 1 1 x 2p y 2 4
(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 1s -----> sp 1s B 2p -------> 2s ------> 1s sp 2 Trigonal Planar
C 2p ------------> 2s ------------> 1s sp 3 Tetrahedral 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 CH 4 : Tetrahedral Molecular Geometry of CH 4 : Tetrahedral (http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/ch10/fg1 0_03b.JPG) Electron Pair Geometry of NH 3 : Tetrahedral Molecular Geometry of NH 3 : Pyramidal
(http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/10.html) Electron Pair Geometry of H 2 O: Tetrahedral Molecular Geometry of H 2 O: Bent (http://dbhs.wvusd.k12.ca.us/webdocs/bonding/) Molecular Geometry (shapes)
(http://www.elmhurst.edu/~chm/vchembook/202linear.html) (http://www.elmhurst.edu/~chm/vchembook/202linear.html)
(http://www.elmhurst.edu/~chm/vchembook/203trigplanar.html) (http://www.elmhurst.edu/~chm/vchembook/205trigpyramid.html) Square Planar
(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
(http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/10.html) Summary of Molecular Geometry No. of Electron Pairs Electron Pair Geometry (Bond Angle) No. of Pendant Atoms Molecular Geometry Example Formula Image Click below to show rotation. 2 linear (180 o ) s3 trigonal planar (120 o ) 2 linear BeH 2 Show rotation 3 trigonal planar CO 3 2- Show rotation 2 bent NO 2 - Show rotation
4 tetrahedral (109.5 o ) 4 tetrahedral CH 4 Show rotation 3 trigonal pyramidal NH 3 Show rotation 2 bent H 2 O Show rotation 5 trigonal bipyramidal (90 o, 120 o ) 5 trigonal bipyramidal PCl 5 Show rotation 4 see-saw SF 4 Show rotation 3 T-shaped BrF 3 Show rotation 6 octahedral (90 o ) 2 linear ICl 2 - Show rotation 6 octahedral SF 6 Show rotation 5 square pyramidal BrF 5 Show rotation 4 square planar ICl 4 - Show rotation (http://www.molecules.org/vsepr_table.html) Multiple Bonds Ethane
(http://www-theor.ch.cam.ac.uk/people/ross/thesis/node143.html) Bond Angles ~ 109.5 o Valence Carbon Electrons Ethane Carbon p ---> sp 3 Tetrahedral s Ethene Carbon p sp 2 Trigonal Ethyne (Acetylene) p sp Linear Hybridization in Multiple Bonds p p p
s -------> -------> sp2 or sp2 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
s sp or sp Ethyne 3-D Orbitals
(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 3s 2 3p 6 3d 6 4s 2 Fe 2+ 3s 2 3p 6 3d 6 (4s 2 comes off) Fe 3+ 3s 2 3p 6 3d 5 (3d off) Co [Ar] 4s 2 3d 7 Co 2+ 3s 2 3p 6 3d 7 Co 3+ 3s 2 3p 6 3d 6 Cu [Ar] 3d 10 4s 1 Cu + 3s 2 3p 6 3d 10 Cu 2+ 3s 2 3p 6 3d 9
VSEPR minimize electron repulsion Ammonia NH 3 1s 2 2s 2 2p x 1 2p y 1 2p z 1 (http://www.uyseg.org/greener_industry/pages/ammonia/1ammoniaapq.htm) σ bond from p orbital H N H 107.3 o Could explain by sp 3 hybridization because it is close to tetrahedral angle 109.5 o Water H 2 O 1s 2 2s 2 2p x 2 2p y 1 2p z 1 (http://www.hrw.com/science/si-science/chemistry/atomic_structure/molecules/02mol.html) H O H 104.5 o Could explain by sp 3 hybridization because it is close to tetrahedral angle 109.5 o
Valence Shell Electron Pair Repulsion (VSEPR) Model Compound Angle Bonding Pair Lone Pair CH 4 109.5 o 4 0 NH 3 107.3 o 3 1 H 2 O 104.5 o 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
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
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
Understand Bond Strength correlates with Electronegativity difference Compound Electronegativity Bond Energy (kj/mol) Difference 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:
Octahedral (sp 3 d 2 ) Trigonal Bipyrimidal (sp 3 d) Tetrahedral (sp 3 ) Trigonal Planar (sp 2 ) 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
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)