Resumen de Electron Densities, Atomic Charges, and Ionic, Covalent, and Polar Bonds
Electron density is a real, observable property of a molecule that can be determined by experiment. In contrast, an orbital is an abstract mathematical concept that, because it is not physically observable, cannot be observed experimentally. Good quality electron densities are, moreover, readily obtained from ab initio calculations. It is important therefore to see what information about bonding and geometry can be obtained directly from the electron density as an alternative to basing discussions on orbital descriptions. The atoms in molecules (AIM) theory provides a method for doing this. We discuss the molecules BF3 and SiF4 as examples and show that the atoms have large charges so that the bonds can be described as having a large ionic character. But the analysis also shows that the amount of shared density in the bonds of these molecules is high, so that the bonds can also be described as having a considerable covalent character. The terms ionic and covalent character are, however, vague, qualitative, and ill-defined. In contrast, the analysis of the electron density by the AIM theory leads to clearly defined quantitative properties such as the charges on the atoms and the electron density at the bond critical point that provide a sound basis for discussing bonding and geometry.
