Principles of Chemistry I
Principles of Chemistry I CHEM 1307
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This 31 page Class Notes was uploaded by Kara Dibbert on Thursday October 22, 2015. The Class Notes belongs to CHEM 1307 at Texas Tech University taught by Tamara Hanna in Fall. Since its upload, it has received 31 views. For similar materials see /class/226511/chem-1307-texas-tech-university in Chemistry at Texas Tech University.
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Date Created: 10/22/15
Orbital Hybridization amp Molecular Orbitals Chemical Bonding An orbital model can be used to describe the electrons in molecules Valence Bond Theory Developed by Linus Pauling Valence electrons are localized between atoms Halffilled atomic orbitals overlap to form bonds Provides a qualitative visual picture of molecular structure and bonding Molecular Orbital Theory Developed by Robert Mulliken Valence electrons are delocalized spread out over the molecule Provides a quantitative picture of bonding Valence Bond VB Theory Atomic orbitals on atoms combine to form chemical bonds Covalent Bond Valence electrons are shared Lewis structures Molecular shape can be predicted VSEPR Equilibrium exists between attraction and repulsion Valence Bond Theory Describes the overlap of 2 atomic orbitals Electrons pair in overlap region of space There is a higher probability of finding a bonding electron in the region of space between the two nuclei Describes the ground state of molecules lowest energy Orbital Overlap When two hydrogen atoms are brought together the electrons from one atom are attracted to the nucleus of the other atom o 9 a Signl i muluwmu rhere is an optimum distance at Wthh Samcmcrlan Noummp energy ls loweSt39 so JZCtion n amzctmn I 39 At a greater distance the overlap is less and the bond is weaker 435 lamol N ltbnWW At a smaller distance repulsion is too high Potential energy 74 bond mam Intemuclear disiance Orbital Overlap The covalent bond that forms from the overlap of two 5 orbitals is called a sigma 6 bond Two electrons of opposite spin can be accommodated in an orbital One electron comes from each atom The greater the orbital overlap the stronger the bond H F HF 15 orbital 2p orbital Overlap creates of hvdroqen of fluorine H F siqma 1r bond JD 9 F r F 2p orbital 2p orbital Overlap creates H of fluorine of fluon ne F F sigma 7 bond The 25 and 2p electrons not involved in the bond are lone pairs or nonbonding electrons Orbital Hybridization Let s consider the molecule methane C ls2 Zs2 2p2 2 unpaired electrons in p orbitals angle of 90 Lewis structure Molecular model Electronpair geometry All the C H bonds are the same how do we explain this Orbital Hybridization A new set of hybrid orbitals can be created by mixing the s p and cl orbitals on an atom Three important rules govern the outcome of the hybrid orbitals 1 The number of hybrid orbitals is always equal to the number of atomic orbitals that are mixed to create the hybrid set atomic orbitals in hybrid orbitals out 2 Hybrid orbitals are built by combining an s orbital with as many p and cl orbitals as needed to accommodate the bonding and lone pairs of the central atom 3 The hybrid orbitals are directed towards the terminal atoms leading to better orbital overlap and stronger bonds What is the hybridization on the carbon atom in methane Orbital Hybridization In methane four orbitals are needed to match the electron pair geometry of the central carbon atom By mixing the 2s and three 2p orbitals on carbon a new set of four sp3 hybrid orbitals can be formed Yhezsandlhe 11 1 13 mammals A spa orbitals have E K mm the same energy may 39 in any 2p 2 and shape 1 Orliitalhybndizatinani CH c f formed by overlap m quot 397 I a quot of a ls orbital from Fourspihybridovlyitals Fnp3vll2d H Hyhndizztiun prudms 4 5p hybrid orbitals ail having ma same energy from C Types of Hybrid Orbitals Consider BeCl2 1 Be EI One Zs and one 2p orbital overlap to form two sp orbitals Two 2p orbitals are unused Overlapping sp Simplified view hybrid orbitals of hybrid orbitals 180 gt mix E a an gt 5 0065 Types of Hybrid Orbitals Consider BF3 0T0 B F One Zs and two 2p orbitals overlap to form three sp2 orbitals One 2p orbital is unused zpz is perpendicular to the plane containing the three sp2 orbitals 2p a a if B atom Types of Hybrid Orbitals Consider NH3 and H20 bonding electrons lone N atom ZS Although each compound has a different number of bonding and lone pairs the total number of electron pairs is four 39 lone bonding pairs electrons HR z H J mix I v i gt l ll 1 Lsp8 sp3 sp3 5pa O atom Types of Hybrid Orbitals Expanded Octet Consider PCI5 C39Z39I o Cl P o One of five sp3d orbitals g One Cl 3p orbital One 35 three 3p and one 3d orbitals overlap to form five sp3d hybrid orbitals B e m o 2 ad w m s m e n 0 Consider SF5 Types of Hybrid Orbitals Expanded Octet six sp3d 2 2p orbital One 35 three 3p and two 3d orbitals overlap One F to form six sp3d2 hybrid orbitals S atom Hybridization Predict the hybridization in the following molecules SF4 sp3d n0 a 25 nn sp3 Hybridization glycine HZNCHZCOOH HCVOV N C H II H o 0 spz sp3 H sp3 Bonds to Central Shape Hybrid Orbs Unused Orbs Atom 2 Linear sp 2 p orbs 3 Trigonal planar sp2 1 p orbs 4 Tetrahedral sp3 None 5 Trigonal bipyramidal sp3d 4 cl orbs 6 Octahedral sp3d2 3 cl orbs Types of Covalent Bonds VB theory states that bond formation occurs when orbitals on adjacent atoms overlap cbond endtoend overlap Higher 6 density along the bond axis between atoms One bonding region No nodal plane Free rotation around the bonds Primary bond H g H H H Types of Covalent Bonds nbond sidetoside overlap Overlap between p orbitals not hybridized orbitals p orbitals oriented in the same direction aligned Two bonding regions above and below cplane Different regions of electron density to avoid e39e39 repulsion Restricted rotation around the double bond sp2 Secondary bond A H Double bond one 5bond one nbond H CC H H one unhybridized p orbital on each carbon artibfi tls Types of Covalent Bonds Triple bond two sets of unhybridized p orbitals on each atom One cbond along zaXis One nbond Parallel to y axis Nodal plane X2 One nbond Parallel to X aXis Nodal plane yz The two nbonds are perpendicular to each other H CEC H Two lobes e of on 1 bond Two lobes Molecular Orbital MO Theory An alternative way to describe the placement of electrons in a molecule using molecular orbitals Atomic orbitals combine to produce delocalized molecular orbitals that are spread across multiple bonds Allow for the prediction of the energy and shape of molecules More accurate description of the magnetism of compounds Disadvantage hard to visualize Predict the magnetism of oxygen 02 The Lewis structure shows all the electrons paired but oxygen is actually paramagnetic Molecular Orbital MO Theory Electrons are assigned in order of increasing orbital energy First Principle of MO Theory The total number of molecular orbitals is equal to the total ENERGY l number of atomic orbitals that are combined When H2 is formed the two 1s orbitals combine to create two new molecular orbitals CH 15 15 gt 15 15 Nodal plane a 0 molecular orbital antibonding Subtraction of the two 1s orbitals forms an antibonding molecular orbital Addition of the two 1s orbitals forms a bonding molecular orbital 0 molecular orbital bonding Molecular Orbital MO Theory Bonding Molecular Orbitals Addition of the two atomic orbitals Increased probability of finding an electron in the region between the two nuclei If the electrons lie directly along the bond the orbital can be labeled a csorbital For Hz the bonding molecular orbital is labeled 615 AntiBonding Molecular Orbitals Subtraction of the two atomic orbitals Reduced probability of finding an electron in the region between the two nuclei Nodes exists between the orbitals For Hz the antibonding molecular orbital is labeled 515 where the asterisk denotes antibonding Molecular Orbital MO Theory Electrons are assigned in order of increasing orbital energy Second Principle of MO Theory Bonding molecular orbitals are lower in energy than the parent orbitals while antibonding orbitals are higher in energy Chemical bonds stabilize the orbitals make them lower in E A I K I 1 gt It Gut1S 9 x 2 i t 1 LU f 15 x 015 I 18 I i n I AC IVIO A0 of H of H2 of H Molecular Orbital MO Theory Electrons are assigned in order of increasing orbital energy Third Principle of MO Theory Electrons are assigned to orbitals of successively higher energy according to the Pauli exclusion principle and Hund s rule When two electrons are assigned to an orbital they must have opposite spin In the case of H2 two electrons would fill the 615 orbital before filling the 515 orbital Fourth Principle of MO Theory Atomic orbitals combine to form molecular orbitals most effectively when the atomic orbitals are of similar energy This rule is very important for heavier molecules like 02 and N2 Bond Order Describes what type of bond exists between to atoms f girl lilir lifle What is the bond order of Hez Total of 4 electrons to fill into diagram Atomic Molecular Atomic orbital orbitals orbital A BO 12 2 12 2 o 39 a Is gt Therefore no bond exists between E H H the two helium atoms quotu He atom 139 He atom 15 i 39 15 Fractional bond orders do exist 015 Hez molecule 01520 102 Molecular Orbital MO Theory Let s consider Liz A lithium atom has electrons in the ls and Zs orbital Combining a ls and Zs orbital is possible in theory but the energy levels are too different so no interaction occurs ENERGY Fill in the core and valence electrons 95 3 Write out the MO configuration 72 512 152 522 quot3915 Determine the bond order 1 1 BO 12 4 2 1 DD DU single bond Atomic orbital 015 Atomic I orbitals Core electrons do not contribute to BO Molecular orbitals Molecular Orbital MO Theory Let s consider Bez C7152 ka152 C7252 6Zs2 core electrons c3292 6252 BO 124 4 0 no bond Let s consider Be2 C7152 ka152 C7252 6Zs1 core electrons c3292 6251 BO 124 3 05 exists under special circumstances Molecular Orbitals from p Orbitals Two p orbitals can combine to form a csbond The orbitals overlap end to end The combination of two p orbitals forms two molecular orbitals One 6 orbital is bonding lower E one 6 orbital is antibonding Nodal plane A i 00 H000 Zpl sz 172 z molecular orbital antibonding I Z r gt 2p 2P O39zpz molecular orbital bonding ENERGY Molecular Orbitals from p Orbitals Two p orbitals can combine to form a nbond The orbitals overlap side to side The combination of four p orbitals forms four molecular orbitals Two 7 orbitals are bonding lower E two 7 orbitals are antibonding ENERGY Nodal plane CE quot2 772px molecular orbital antibonding 2px 2px 77sz molecular orbital bondinq Each pair of orbitals has the same energy Molecular Orbital Diagrams To determine the bond order for diatomic compounds count the electrons and fill them into the MO diagram CAUTION Determine if core electrons are needed in the diagram If only valence electrons are needed you will not ll in the 615 or 615 orbitals valence e39 6 e39 8 e39 12 e39 52 c2 02 7 E I szp quot 2 El D3 77 T2 E El 772 7TH 02quot 2 0 as 72 0393 bond order 1 2 3 2 1 Fill the electrons into the lowest energy orbitals first and obey Hund s Rule Molecular Orbital Diagrams Heteronuclear We can also predict bond order and magnetism for heteronuclear compounds such as NO Valence electrons 11 Energy 256 25 1 25 AOOfN I AOOfO MD of NO Bond order 83 25 2 Molecular Orbital Diagrams Practice Determine the bond order for the following species F223 FZ39 F2 F2 F22 Count the valence electrons and fill them into the diagram F22 F2 F2 F F22 02p in I I C 1 1 mp 772p 2v 29 2p I I I I I 35 is I I I I I 25 28 I I I I I bond order 0 05 1 1 5 2 strongest F2 shortest F22
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