The change of bonding status,typically occurring only in chemical processes,could dramatically alter the material properties.Here,we show that a tunable breaking and forming of a diatomic bond can be achieved through physical means,i.e.,by a moderate biaxial strain,in the newly discovered MoN2 two-dimensional (2D) material.Based on first-principles calculations,we predict that as the lattice parameter is increased under strain,there exists an isostructural phase transition at which the N-N distance has a sudden drop,corresponding to the transition from a N-N nonbonding state to a N-N single bond state.Remarkably,the bonding change also induces a magnetic phase transition,during which the magnetic moments transfer from the N(2p) sublattice to the Mo(4d) sublattice,meanwhile the type of magnetic coupling is changed from ferromagnetic to anti-ferromagnetic.We provide a physical picture for understanding these striking effects.The discovery is not only of great scientific interest in exploring unusual phase transitions in low-dimensional systems,but it also reveals the great potential of the 2D MoN2 material in the nanoscale mechanical,electronic,and spintronic applications.