A constitutional isomer refers to compounds that have the same molecular formula but differ in the connectivity of their atoms. This means that the arrangement of atoms in the molecule is different, leading to distinct chemical properties and structures. For example, C4H10 can exist as both butane and isobutane, which are constitutional isomers of each other. These isomers can vary in physical properties such as boiling points, melting points, and solubility. The differences arise because the arrangement of atoms affects how the molecules interact with each other and with other substances. Understanding constitutional isomers is important in fields like organic chemistry and pharmaceuticals.