Final answer:
Five-membered rings have angle and torsional strain despite near-ideal bond angles. Angle strain occurs when bond angles deviate from the ideal, and torsional strain arises from eclipsing hydrogen interactions in planar conformations. Rings often pucker to relieve torsional strain.
Step-by-step explanation:
Despite the bond angles in a five-membered ring being close to 109.5 degrees, there is still some strain due to both angle strain and torsional strain. The term angle strain refers to the strain caused by bond angles deviating from the ideal tetrahedral angles of 109.5º. In the case of a five-membered ring like cyclopentane, while the bond angles are about 108º and thus close to the ideal, the planarity forces hydrogens on adjacent carbon atoms to be in closer proximity than is energetically favored, leading to torsional strain from eclipsing interactions between the hydrogens. In cycloalkanes, to relieve strain, the rings often adopt non-planar puckered conformations which help to reduce torsional strain, though they may introduce some angle strain.