In alkanes, optimum overlap of atomic orbitals is achieved at 109.5°.  The most common cyclic compounds have five or six carbons in their ring. Adolf von Baeyer received a Nobel Prize in 1905 for the discovery of the Baeyer strain theory, which was an explanation of the relative stabilities of cyclic molecules in 1885.
Angle strain occurs when bond angles deviate from the ideal bond angles to achieve maximum bond strength in a specific chemical conformation. Angle strain typically affects cyclic molecules, which lack the flexibility of acyclic molecules.
Angle strain destabilizes a molecule, as manifested in higher reactivity and elevated heat of combustion. Maximum bond strength results from effective overlap of atomic orbitals in a chemical bond.  A quantitative measure for angle strain is strain energy. Angle strain and torsional strain combine to create ring strain that affects cyclic molecules.
Normalized energies that allow comparison of ring strains are obtained by measuring per methylene group (CH) of the molar heat of combustion in the cycloalkanes.
: per CH &minus; 658.6 kJ  = strain per CH
The value 658.6 kJ per mole is obtained from an unstrained long-chain alkane.
Cycloalkanes generally have less ring strain than cycloalkenes, which is seen when comparing cyclopropane and cyclopropene.