We have analyzed here covalency, atomicity, and ionicity as the fundamental bonding characteristics for hydrogen-based systems: (H + 2 , H 2 , and H - 2 ). The results provide a nuanced view of how bonds evolved between the different regimes: covalent, atomic, and ionic as the atoms move closer or separate. In this work, we tested and developed the conceptual framework that employs the formalism of combined firstand second-quantization. We discussed the exact solution of the extended Heitler-London model for the three systems which allowed us to precise quantification of bonding contributions coming from the kinetic exchange and effective Coulomb interactions.The analysis concentrates on the chemical bonding resulting from competition between those two energies. By examining these interactions vs. R we offer an intuitive understanding of bonding nature, and in particular, highlight the evolution from covalent to ionic character. Shorikov, A. O.; Belozerov, A. S. Firstprinciples definition of ionicity and covalency in molecules and solids. The Journal of Chemical Physics 2024, 160, 144113.