: In cyclopropane, the angle strain is maximum, hence it is a highly strained molecule and consequently most unstable. The angle strain in cyclobutane is less than that in cyclopropane, hence cyclobutane is more stable than cyclopropane. This stability increases upto 6 membered rings, then decreases from 7 to 11 membered rings and from the 12 membered onwards attains the stability of 6 membered ring. Heat of combustion is a method of measuring chemical stability. Hence cyclohexane has the lowest heat of combustion per - group.