Please give a short, concise explanation of why cyclopentane puckers and cyclohexane form a chair rather than lying flat.

The compounds which have same molecular formula and different structures are known as conformers. They are formed by rotating molecule at single bond.In the flat structure of cyclopentane and cyclohexane, the bond angle between carbon atoms is 108°C and 120°C, respectively, as shown below. The hybridization of each carbon atom in cyclopentane and cyclohexane is sp3. The geometry of carbon atom is tetrahedral when the hybridization is sp3. However, in flat structure of cyclopentane and cyclohexane geometry of carbon atoms is not tetrahedral.The bond angle in flat structure of cyclohexane is more than tetrahedral bond angle (109.5°C) and each carbon atom eclipse other carbon atom due to which an angle strain and torsional strain is observed in it. In flat structure of cyclopentane the bond angle is approximately equal to tetrahedral bond angle hence angle strain is not observed in it. However, torsional strain is still observed in it because each carbon atom eclipses other carbon atom. In order to overcome torsional strain and angle strain, puckered and chair conformation of cyclopentane and cyclohexane, respectively, is observed. In these conformations minimal or no angle strain and torsional strain is observed.Answer: The flat form of cyclopentane and cyclohexane shows angle strain and torsional strain. In order to reduce these strains cyclopentane forms puckered conformation and cyclohexane forms chair conformation.