WS 5-bond order & strength.docx Page 1 of 6 Alscher
WS#5:Bond#Order#and#Bond#strength1#
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In#covalent#bonding,#atoms#fill#their#outer#valence#shells#by#sharing#electrons#with#neighboring#
atoms.##Each#atom#in#a#covalent#bond#“counts”#the#shared#electrons#as#belonging#entirely#to#itself.##The#
bond#order#is#the#number#of#electron#pairs#being#shared#by#a#pair#of#bonded#atoms.###
The#covalent#bond#in#H2,#HCl,#or#Cl2#is#a#single#bond.##This#bond#consists#of#a#single#pair#of#
electrons.##A#single#bond#has#a#bond#order#of#1.##Single#bonds#are#the#most#common#type#of#bond,#but#
many#molecules#(and#polyatomic#ions)#contain#multiple#bonds.##Multiple#bonds#most#frequently#involve#
C,#O,#N,#and#sometimes#P#and#S.##There#are#two#types#of#multiple#bonds:#double#and#triple.##A#double#
bond#consists#of#two#bonding#electron#pairs,#four#electrons#shared#between#two#atoms,#so#the#bond#order#
is#2.##Ethylene#(C2H4)#is#a#simple#hydrocarbon#that#contains#a#carbonPcarbon#double#bond.##A#triple#bond#
consists#of#three#bonding#pairs;#two#atoms#share#six#electrons#so#the#bond#order#is#3.##In#the#N2#molecule,#
the#atoms#are#held#together#by#a#triple#bond.##Six#shared#electrons#and#two#unshared#electrons#give#each#
nitrogen#an#octet.##Bond#numbers#can#be#fractions#as#well#as#whole#numbers.##(Think#resonance!)#
The#strength#of#a#covalent#bond#depends#on#the#magnitude#of#the#mutual#attraction#between#
bonded#nuclei#and#shared#electrons.##The#more#electron#density#that#is#located#between#the#atoms,#the#
stronger#the#bond.#The#bond#energy#bond#enthalpy#or#bond#strength#is#the#energy#required#to#overcome#
this#attraction#and#is#defined#as#the#standard#enthalpy#change#for#breaking#the#bond#in#mole#of#gaseous#
molecules.##Bond#breaking#is#always#positive.#
A—B(g)#®#A(g)#+#B(g)#∆H°Bond#breaking#=BE(A—B);#+BE(A—B)#>0.###
The#same#amount#of#energy#that#is#absorbed#to#break#the#bond#is#the#energy#released#when#the#
bond#forms.##A(g)#+#B(g)#®#A—B(g)#∆H°bond#forming#=–BE(A—B);#BE(A—B)#<0.#
Bond#energies#depend#on#characteristics#of#the#bonded#atoms,#their#electron#configurations,#
nuclear#charges,#and#atomic#radii;#each#bond#type#has#its#own#energy.##Stronger#bonds#are#larger#in#
energy#(have#a#deeper#energy#well),#weaker#bonds#are#lower#in#energy#(have#shallower#energy#wells).##
The#energy#of#a#given#type#of#bond#varies#slightly#from#moleculePtoPmolecule,#and#even#with#in#the#same#
molecule.##The#bond#energies#given#below#are#averages#(Table#2)#
# #
1 Background reading for this handout can be found in sections 8.8 & 9.7 of Brown and LeMay.
