Theory of electron structure The ground state of electrons: chemical bonding and electron density

covalent bonding involves a transition from an isolated atomic or ionic state to a bonding state in a solid where the electron band is just filling the band gap. Because of the directional nature of covalent bonds, covalent crystals have a loose structure, unlike the close packing of other types of structures. One of the most successful theories is that of semiconductors and their conversion under high pressure to more densely packed metal systems.

Hydrogen bonding is formed due to the special case of hydrogen, because it is the only active element without inner electrons, so there is no repulsive force of inner electrons when protons and electrons attract each other, as in other elements. The properties of water and ice are greatly affected by this property, because protons can be shared between different water molecules. In addition to this, hydrogen bonding is crucial in many molecules, especially in biological activities, which is a huge challenge in its application in complex materials. Of course, we will not focus too much on the complexity of hydrogen bonds, just know that the electronic structure method can also be used to describe the strength of hydrogen bonds in different situations.

Bonding in actual materials is usually a combination of several of the above mentioned. For example, in a metal there can also be directional covalent bonds or ionic bonds caused by charge transfer. Molecular crystals can contain strong covalent and intramolecular ionic bonds, as well as weaker intermolecular van der Waals bonds. The list goes on.

Now let's talk about another concept of the ground state, electron density.

It plays a fundamental role in the theory of electronic systems and can be measured by X-rays or diffraction of high-energy electrons. Since the total electron density of all but the lightest atoms is largely contributed by the core (generally the part of the atom other than the valence electrons), electron density can reveal the following properties:

The density of nuclei is essentially the density of atoms

debye-waller factor, which describes the effect of thermal motion and zero-point energy on the average density of diffraction (also dominated by nuclei)

Density change due to bonding or charge transfer