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Why do methane, ethane, and propane have no constitutional isomers?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — isomerism Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)

Methane, ethane, and propane do not have constitutional isomers because there is only one possible way to connect their constituent carbon and hydrogen atoms while adhering to the fundamental rules of chemical bonding—specifically, the tetravalency of carbon (forming four bonds) and the univalency of hydrogen (forming one bond).12

Constitutional isomers are molecules that share the same molecular formula but differ in the connectivity of their atoms. For the first three alkanes, the limited number of carbon atoms prevents the formation of alternative structures, such as branching, which only becomes possible starting with butane ( $C_{4} H_{10}$ ). As the number of carbon atoms in a hydrocarbon chain increases, the number of potential ways to arrange those atoms grows significantly, leading to a sharp rise in the number of possible constitutional isomers.12