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Why is eclipsed ethane considered a transition state rather than a separate isomer?

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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.)

In chemistry, an isomer must be an energy minimum—a stable structure that resides in an energy well. Eclipsed ethane does not meet this requirement because it represents an energy maximum on the potential energy plot of the molecule’s rotation.1

When ethane rotates around its carbon-carbon bond, the eclipsed conformation occurs when the hydrogen atoms on adjacent carbons are as close as possible. In this state, the electrons in the carbon-hydrogen bonds experience significant repulsion, which increases the molecule's potential energy. Because this configuration sits at the peak of the energy barrier between two staggered conformations, it is classified as a transition state rather than a distinct isomer.1

Essentially, while staggered ethane represents a stable, local energy minimum, eclipsed ethane is merely a fleeting configuration that the molecule passes through during rotation. Therefore, ethane is considered to have only one form, as the eclipsed state is not a stable, isolable structure.12