The rather presumptious title assumes the laws and fundamental constants of physics are the same everywhere (they may not be). With this constraint (and without yet defining what is meant by strongest), consider the three molecules: Property  (CCSD/aug-cc-pVTZ) N≡N (H-N≡N) ⁺ (H-N≡N-H) ²⁺ NN length, Å 1.0967 1.0915 1.0795 NN stretch, cm ^-1 2418.8 2356.4