The cyclobutane molecule has been found by electron diffraction to have the following bond distances and bond angles: C–C, 1.568±0.02A; C–H, 1.098±0.04A; ∠HCH, 114±8°. On the average the ring is nonplanar, with dihedral angle 20° (+10°, −20°), but the equilibrium symmetry may be either D_(2d) (puckered ring) or D_(4h) (planar ring with low rigidity leading to large amplitude of out‐of‐plane bending). This point is discussed in connection with earlier spectroscopic work. The long bond distances found in four‐membered rings are contrasted against the short distances in three‐membered rings, and the strain energies, bond distances, and HCH angles of cycloalkanes are discussed in terms of modern valence concepts. It is suggested that the potential energy arising from a repulsion of the nonbonded carbon atoms may contribute significantly to the apparently anomalously high strain energy of cyclobutane. The repulsive force associated with such a potential is shown to account satisfactorily for the long C–C distances
