We study the critical temperature Tc and Debye temperature θD for 12 single-element superconductors and find a relation TcθD=const. for every two metals of each group in the periodic table of elements. Especially, even the Tc ratio of Hg to Zn is as high as 4.77 but their TcθD are still close to each other. This formula is beneficial to explain why the single-element insulator at room temperature will not show the superconducting characteristic since it has no free-electron gas. The relation reveals that Tc is inversely proportional to ωD or θD, and also inversely proportional to the acoustic velocity v and the momentum of the Fermi electrons PF. Furthermore, two special formulae are also found for all chosen transition metals and post-transition metals, respectively, and show an important thing that the critical temperature Tc is also related to the valence electrons or the largest positive oxidation number in those superconductors.