Accurate ring strain energy (RSE) data for parent (CH2)(2)X rings are reported, where X are group 13-16 elements (El) in their lowest oxidation state, from the second to the sixth row, with their covalence completed by bonds to H. They are obtained from appropriate homodesmotic and hyper-homodesmotic reactions at different levels up to the CCSD(T) level, thus providing a benchmark of high-quality reference RSE values, as well as acceptably accurate faster lower-level options. Derivatives of indium, thallium, and lead cannot be properly described by a three-member ring connectivity, because they display a unique donor-acceptor structure from an ethylene pi(C=C) orbital to an empty p orbital of a metallylene subunit. The RSE of groups 13 and 14 heterocycles increases on descending in the group (except for Ga and Ge), while it decreases for groups 15 and 16. The latter is presumably due to a strain-releasing mechanism favored by the increase of pcharacter at the sp(3)-type atomic orbital used by El in the endocyclic El-C bonds, % p(El)(El-C), originated by the tendency of the El lone pairs in groups 15-16 to increase their s-character. This strain-releasing mechanism does not exist in heavier tetrels, which keep almost unchanged the p-character in the endocyclic bonds at El, whereas for triels the p-character is still lower owing to their sp(2)-like hybridization. Remarkable linear correlations were found between the RSE and either the above-mentioned %p(El)(El-C), the distal C-C bond distance or the relaxed force constants for the endocyclic bond angles.