Lewis Structures - Rules for Drawing Electron Dot
Diagrams
!
1.#Calculate#the#total###of#valence#electrons#(e4)#
a.!Consult*the*molecular*formula*and*sum*up*all*the*valence*electrons*from*the*separate*atoms.*
b.!Add*one*for*each*(7)*charge*(extra*electron);**
c.!Subtract*one*for*each*(+)*charge*(missing*electron).**
*
2.#Draw#a#molecular#arrangement#
a.!Choose*the*central*atom(s).*The*least&electronegative*atom*is*almost*always*the*central*atom.**
o!For*example,*in*ClO2,*the*Cl*is*the*central*atom;*in*SF5*the*S*is*the*central*atom.**
b.!Occasionally,*you*will*need*to*choose*the*unique*atom,*even*when*it*is*the*most*electronegative:**
o!For*example,*the*O*in*Cl2O.**
c.!A*wrong*choice*usually*will*be*signaled*by*your*being*unable*to*write*a*valid*structure.**
d.!Arrange*the*other*atoms*around*the*central*atom,*in*accord*with*the*normal*valences*of*the*atoms.*
That*is,*do*not*place*more*atoms*around*a*central*one*than*it*normally*can*bond*to.**
o!For*first*and*second*row*elements*the*maximum*valence*=*the*Group*number*through*Group*
IV;*after*that,*it*is:**8*7*(the*Group*number).**
o!The*difference*is*that*up*through*IV,*the*atoms*tend*to*donate*electrons*to*get*an*octet,*
whereas*beyond*IV,*they*accept*electrons.**
o!Recognition*of*exceptions*will*come*with*experience.**
e.!You*may*find*that*you*will*have*to*place*fewer*atoms*than*normal*around*a*central*one;*this*is*taken*
care*of*later.**
f.!Hydrogen*is*never*the*central*atom.*It*forms*only*one*bond,*so*it*must*generally*be*in*the*outer*layer*
of*atoms.*Therefore,*place*hydrogen*atoms*last.**
*
3.#Insert#bonding#electron#pairs#
a.!Insert*pairs*of*electrons*between*all*pairs*of*atoms*that*are*to*be*bonded*together.*If*this*uses*up*all*
available*electrons,*go*to*Rule*5.*
*
4.#Complete#octets#
a.!Place*any*remaining*electrons*on*peripheral*atoms*as*unshared*pairs,*starting*with*the*most*
electronegative*such*atom.**
o!Fill*this*atom*up*to*an*octet.**
o!Then*proceed*to*the*next*most*electronegative,*and*so*on.**
b.!Remember*that*hydrogen*can*only*have*two*electrons,*and*so*cannot*have*any*unshared*pairs.**
If*any*electrons*remain*unused,*place*them*on*the*central*atom*as*unshared*pairs.*
*
5.#Form#double#&#triple#bonds#
a.!If*the*central*atom*still*lacks*a*full*octet,*and*does*not*belong*to*a*group*that*doesn’t*require*a*full*
octet*(ex.*Be,*Al),*then*move*unshared*pair*of*electrons*from*the*peripheral*atoms*to*form*double*or*
triple*bonds*as*necessary.*
b.!Assign*a*formal*charge*if*it*is*a*polyatomic*ion,*and*enclose*the*structure*in*brackets.*
*
Exceptions#
There*are*exceptions*to*the*octet*rule.*
