A s defined by Linus Pauling, “Chemistry is the science of substances: theirstructure, their properties, and the reactions that change them into other substances” . The first two aspects, structure and properties, are clearly associated to the arrangement of atoms in a molecule, i.e., the chemical bond. These bonds determine chemical reactivity —the third aspect-and their visualization allows chemists to understand how atoms or molecules bond at a most fundamental level. A mechanistic understanding of chemical and biological functions and the structures of solid materials depends on knowing the geometric structures and the natures of bonds. But, despite the fact that the chemical bond is a fundamental concept in chemistry, “what is a chemical bond?” still remains a critical question for the chemical community because of the lack of a unique definition and inadequate understanding of its physical nature.