Constitutional (Structural) Isomers. Constitutional isomers have different atomic connectivities.

Examples:

n-butane, CH₃CH₂CH₂CH₃; 2-methylpropane, (CH₃)₃CH

ethanol, CH₃CH₂OH; methyl ether, CH₃OCH₃

These molecules have the same kinds and numbers of atoms but different 'backbone' arrangements.