Although the concept of a wave function is intrinsic to physics, the molecular orbital (MO) resides in the realm of chemistry, especially when building them from atomic orbitals (MO-LCAO). What is in the realm of philosophy is the question of whether the MO really exists or not, considering that it is not a quantum observable. Even if they may not be real, analysis such as photoelectron spectroscopy, Koopmans theorem, Fukui function, Dyson orbitals, and atomic resolution spectroscopy techniques indicate that molecules behave as if MOs were real. This is a fascinating debate, but here we will focus only on its utility as a quantitative and qualitative chemical tool. The MOs approach to the chemical bond is simple. We count the orbitals, and if the number of bonding ones is larger than antibonding ones, then there is a chemical bond, as shown in the interaction diagram of Fig. for the triple bond of CO. Since each MO can hold up to two electrons, we have a direct connection with Lewis' shared electron pair model. Can this work as a general definition of the chemical bond? Well, most chemistry books think so. It certainly has a lot of appeal in terms of explanatory and predictive power, much more than the previously discussed structural and energetic approaches, models that show the existence of bonds but have many difficulties explaining their raison d'être. For example, we can build the final orbitals from its fragments in interaction diagrams, allowing better comprehension of the interaction. The structural and energetic definitions only consider what happens in the final product, or in other words, they work on the chemical bond, but not on the chemical bonding. and therefore can be bonding between one pair of atoms, antibonding between another pair, and nonbonding for the rest (see Fig. ). While bond orders can be mathematically obtained from the canonical Mos, these methods loose the simple visual connection between a bond and the orbitals.