Ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal and is most commonly discussed for small rings such as cyclopropanes and cyclobutanes.
In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles are substantially smaller than the idealized value of approximately 109. 
