Bond Order Lengths and Energies of bonds between two atoms may not always be similar but show di erent stereotypes Benzene C6H6 Bond length 1 397 Ed 505 kJ mol 1 intermediate Bond Order is 1 5 Here the bonding depends on the nature of the whole molecule Bond force constant determined from vibraOonal spectroscopy higher bond order sO er bond shorter bond Polar Covalent Bonding ElectronegaOvity and Dipole Moment Most bonds are neither 100 ionic or covalent Bonds with parOal charge transfer are polar covalent Absolute value of indicates the bond polarity 1 2 2 0 ionic 0 4 covalent Examples check them out from H Cl polar C O polar H H nonpolar Cl Br slightly polar F F nonpolar Cl F slightly polar H F very polar Ag F slightly ionic K F very ionic Dipole Moments and Percentage Ionic Character x 100 KF almost pure ionic 3 HF 2 0 H F means a fracOon of the full charge is at the nucleus Dipole Moment q R q R q Units C m Common unit Debye 1D 3 336 10 30C m 1 D e e separated by 0 2082 q e R equilibrium bond length Percent ionic 100 0 2082 R ExcepOons CO expect C O but experiment indicates C O See later on formal charge and lone pair Ionic Character calculated from dipole moment vs Simple examples of Lewis dot diagrams Octet stable except H where 2 electron shell stable Basic Lewis structures and bond order Bond length and strength N2 O2 F2 O2 double bond 6 electrons total 12 stable octets N2 Triple bonds 5 electrons total 10 stable octets Ethane ethylene and acetylene single double triple bonds Formal charge Formal charge is de ned as the di erence between the valence electrons in an isolated atom i e free atom and the number of electrons assigned to that atom in a Lewis structure Not a charge as in ion formaOon Used to check probability of a structure Formal charge valence electrons on atom lone pair e s bonding electrons 2 If C and O equally share bonding electrons then it has a formal charge of 1 Formal charge on C is 4 2 6 2 1 O is 6 2 6 2 1 Total 0 for neural molecule and has a value of the ion charge for caOons and anions Lewis dot diagrams Check videos hsp www kentchemistry com links bonding lewisdotstruct htm Add up the total number of valence electrons add or subtract charges if anion or caOon Draw out skeleton structure of molecule Draw in bonds with 2 electrons per bond What s residual electrons Fill in the electrons on atoms outside as dots Fill in any in central atoms lone pairs Work out of mulOple bonds needed if you don t have enough electrons share them out If octet not saOs ed check formal charge Check if atoms follows the octet rule or not ExcepOons N 5 O 6 11 double bond uses 4 7 lew 4 on O and 3 on N BF3 BeF2 BeF2 Be 2 valence electrons 2 7x2 16 4 for bonds 12 F want 6 each 12 Be atom doesn t obey octet rule Formal charge on F would be 1 7 4 1 2x4 1 Note F is very electronegaOve so Be cant pull it over Be 2 4 2 2 F Be F Azide ion N3 terminal central terminal Period 3 onwards Resonance Forms Some molecules have two or more equivalent Lewis diagrams But both bonds are same length 1 28 Bond length between H2O 1 49 and O2 1 21 H O H O O We use the concept of resonance hybrid Resonance Hybrid Lewis forms Note there is no oscillaOng to and fro between two Lewis representaOons Rather electrons are delocalized over more than one bond Charge of Ion put sign outside of brackets THE VALENCE SHELL ELECTRON PAIR REPULSION VSEPR THEORY Lewis concept is unable to explain the shapes of molecules This theory provides a simple procedure to predict the shapes of covalent molecules Sidgwick and Powell in 1940 proposed a simple theory based on the repulsive interacOons of the electron pairs in the valence shell of the atoms It was further developed and rede ned by Nyholm and Gillespie 1957 Main postulates of VSEPR theory The shape of a molecule depends upon the number of valence shell electron pairs bonded or nonbonded around the central atom Pairs of electrons in the valence shell repel one another since their electron clouds are negaOvely charged These pairs of electrons tend to occupy such posiOons in space that minimise repulsion and thus maximise distance between them The valence shell is taken as a sphere with the electron pairs localising on the spherical surface at maximum distance from one another A mul7ple bond is treated as if it is a single electron pair and the two or three electron pairs of a mul7ple bond are treated as a single super pair Where two or more resonance structures can represent a molecule the VSEPR model is applicable to any such structure First steps Use Lewis diagram to determine the number of bonds and lone pairs Basic Geometries no lone pairs Linear trigonal tetrahedral trigonal pyramidal SN and geometric forms SN 1 6 DIY VESPR with Balloons You can use a bigger balloon for lone pairs CO32 the Lewis diagram shows carbon at the center with no lone electron pairs Carbonate ion carbon at the center with no lone electron pairs SN 3 0 Trigonal Planar The Lewis diagram is as follows C 4 e O 6e x 3 18e 2 charge 2e Total electrons 24 One double bond needed for carbon octet Resonance hybrid First Determine the steric number SN C has 4 valence electrons shared with 4 H 8 stable octet SN 4 0 4 Form is tetrahedral Bond angle 109 470 First Determine the steric number SN CO2 C has 4 4 from O SN 2 0 2 Linear bond angle 1800 Repulsive e ect The repulsive interacOon of electron pairs decrease in the order Lone pair lp Lone pair lp Lone pair lp Bond pair bp Bond pair bp Bond pair bp NH3 H2O CH3Cl SN 3 1 2 2 4 angle 109 47 for a tetrahedron bond a ngle reduced by 2 5 degrees per lone pair 0 Trigonal Pyramidal bent distorted tetrahedral First Determine the steric number SN IF4 central I 8e s F 7 1 from I 8 4 I valence electrons from I leaves 4 on I 2 lone pairs SN 4 2 6 6 Octahedral bond angles 900 First Determine the steric number SN IF4 central I 8e s F 7 1 from I 8 4 I valence electrons from I leaves 4 on I 2 lone pairs SN 4 2 6 6 Octahedral bond angles 900 Fluorides PF5 SF4 XeF2 All have SN 5 SN 5 0 4 1 2 3 What s their shapes Fluoride SF4 6 4x7 34 electrons of which F 4 x 2 8 used in bonds ll in F e s 24 total 32 leaves lone pair 2 on …