IUPAC name stem comes from the longest chain of carbon atoms, in this case five atoms giving pentan-, and the suffix comes from the most important functional group in the molecule, in this case the carboxylic acid. Since there are no other functional groups present, this gives the full name Pentanoic acidThe displayed formula has every atom drawn with their symbols. In this case there is a double bond to one oxygen in the carboxylic acid group and all other bonds are single bonds.