The finding of the periodic law is a milestone in chemical science. The periodicity of light elements in the Periodic Table is fully accounted for by quantum mechanics. Here we report that relativistic effects change the bond multiplicity of the group 6 diatomic molecules M-2 (M = Cr, Mo, W, Sg) from hextuple bonds for Cr-2, Mo-2, W-2 to quadruple bonds for Sg(2), thus breaking the periodicity in the nonrelativistic domain. The same trend is also found for other superheavy-element diatomics Rf(2), Db(2), Bh(2), and Hs(2).