Compounds with the same number and type of atoms but with different arrangements.
 
  Molecular
 Formula C5H12
 
  Condensed
 structural formulas.
 
 CH3CH2CH2CH2CH3                  pentane
 
 CH3CH(CH3)CH2CH3 2-methylbutane
 
 (CH3)4C 2,2-dimethylpropane
 
  All are constitutional
 isomers of C5H12.