The Longest world in English Language is "SMILES".
Because the distance between First S and the last S is a mile.
While if you want to know which word is having maximum letters. Then for your reference
The longest word currently in the Oxford English Dictionary is the supposed lung-disease "pneumonoultramicroscopicsilicovolcanoconiosis" (45 letters) but some editions of the Guiness Book of Records mention "aequeosalinocalcalinoceraceoaluminosocupreovitriolic" (52 letters), which is even longer.
longest chemical word for C1289H2051N343O375S8 is 0f (1909letters)
it is
methionylglutaminylarginyltyrosylglutamylserylleucylphenylalanylalanylglutaminylleucyllysylglutamylarginy
llysylglutamylglycylalanylphenylalanylvalyl
prolylphenylalanylvalylthreonylleucylglycylaspartylprolylglycylisoleucylglutamylglutaminylserylleucyllysyl
isoleucylaspartylthreonylleucylisoleucylglutamylalanylglycylalanylaspartylalanylleucylglutamylleucylglycyl
isoleucylprolylphenylalanylserylaspartylprolylleucylalanylaspartylglycylprolylthreonylisoleucylglutaminyla
sparaginylalanylthreonylleucylarginylalanylphenylalanylalanylalanylglycylvalylthreonylprolylalanylglutamin
ylcysteinylphenylalanylglutamylmethionylleucylalanylleucylisoleucylarginylglutaminyllysylhistidylprolylthre
onylisoleucylprolylisoleucylglycylleucylleucylmethionyltyrosylalanylasparaginylleucylvalylphenylalanylasp
araginyllysylglycylisoleucylaspartylglutamylphenylalanyltyrosylalanylglutaminylcysteinylglutamyllysylvalyl
glycylvalylaspartylserylvalylleucylvalylalanylaspartylvalylprolylvalylglutaminylglutamylserylalanylprolylph
enylalanylarginylglutaminylalanylalanylleucylarginylhistidylasparaginylvalylalanylprolylisoleucylphenylala
nylisoleucylcysteinylprolylprolylaspartylalanylaspartylaspartylaspartylleucylleucylarginylglutaminylisoleu
cylalanylseryltyrosylglycylarginylglycyltyrosylthreonyltyrosylleucylleucylserylarginylalanylglycylvalylthre
onylglycylalanylglutamylasparaginylarginylalanylalanylleucylprolylleucylasparaginylhistidylleucylvalylala
nyllysylleucyllysylglutamyltyrosylasparaginylalanylalanylprolylprolylleucylglutaminylglycylphenylalanylg
lycylisoleucylserylalanylprolylaspartylglutaminylvalyllysylalanylalanylisoleucylaspartylalanylglycylalanyla
lanylglycylalanylisoleucylserylglycylserylalanylisoleucylvalyllysylisoleucylisoleucylglutamylglutaminylhisti
dylasparaginylisoleucylglutamylprolylglutamyllysylmethionylleucylalanylalanylleucyllysylvalylphenylalany
lvalylglutaminylprolylmethionyllysylalanylalanylthreonylarginylserine