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How do staggered and eclipsed conformations of ethane differ?

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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 ethane, staggered and eclipsed conformations represent two limiting structures defined by the rotation of the molecule around its carbon-carbon bond. These differences are best visualized using a Newman projection, which allows one to sight down the carbon-carbon bond to observe the relative positions of the hydrogen atoms.12

Staggered Conformation: In this structure, the carbon-hydrogen bonds are positioned as far apart as possible. This configuration represents an energy minimum and is more stable than the eclipsed form.12
Eclipsed Conformation: In this structure, the carbon-hydrogen bonds are as close as possible. This proximity causes electrons in the bonds on opposite sides of the carbon-carbon bond to repel each other, creating torsional strain that increases the potential energy of the molecule by approximately 3 kcal/mol.12

Because the eclipsed form is an energy maximum—acting as a transition state between two staggered forms—it is not considered a stable isomer, and ethane is generally viewed as having only one form.12