Cyclopentane
= Cyclopentane
<Cyclopentane> is the cyclic <alkane> $\mathrm{C_5H_{10}}$. Its five-membered ring puckers rather than remaining completely planar, reducing unfavorable eclipsing interactions among neighboring bonds.
Envelope-like conformations interconvert through ring motion. This illustrates why simply drawing a regular polygon does not specify the lowest-energy geometry of a saturated ring.