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In this Chapter we reexamine the concept of $\textit{covalency}$ and$\textit{ionicity}$ on example of the simplest molecules. First, starting fromthe exact expression for the two-particle wave function in the case of $H_2$molecule within the Heitler-London model, we demonstrate an unphysical behaviorof the covalency at large interatomic distance which, within standarddefinition, reaches the maximal value in the limit of separated atoms. Second,we correct this deficiency by introducing the concept of $\textit{atomicity}$,with the help of which, we define the $\textit{true}$ (intrinsic)$\textit{covalency}$, as well as retain the precise concept of$\textit{ionicity}$. We connect the introduced atomicity to the onset ofMott-Hubbard localization, adopted here from the well established notion in thecondensed matter. The evolution from the molecular to atomic states developsrapidly with interatomic distance beyond the localization threshold. This briefoverview is intented as pedagogical in nature, nonetheless is analyzedquantitatively for the case of $H_2$ molecule. At the end, we outline asimilar-type of model for the related case of the hydrogen bond on example ofadenine-tymine pair. Methodologically, the approach is based on combining thefirst- (wave mechanics) and second-quantization into a single scheme of formaldescription.

Interparticle correlations and chemical bonding from physical side: Covalency $\textit{versus}$ atomicity and ionicity