There is a chemical bond when an electrostatic force holds together multiple atoms in a chemical species (strong bonds, or primary or intramolecular) or more molecules in a substance in the condensed state (weak, or secondary, or intermolecular bonds).
The nature of the chemical bond can be explained by observing the coulombic forces interacting between the molecules. Take for example the H2+ cation: it consists of two hydrogen nuclei H and an electron. We denote by Ha the first hydrogen nucleus and with Hb the other hydrogen nucleus. Each of the two nuclei is associated with an electronic wave function, respectively 1sa and 1sb, whose linear combination forms the molecular orbital Ψ.
The molecular orbital Ψ will have low values between the two nuclei, while it will grow closer to them and then decrease further away from them. Therefore if we consider an electron, that is a negative charge placed between the two nuclei, it will be subjected to attractive forces by the two nuclei that will be counterbalanced by the repulsive ones until the stability of the system has been reached; then the electron will be fallen into a potential well from which he will be difficult to escape. In this way, a chemical bond was formed.
Primary chemical bonds
The primary chemical bonds are the forces that hold together the atoms that form the molecules. A primary bond is implemented by the sharing or transfer of electrons between atoms and by the electrostatic attraction between protons and electrons. These bonds generate the transfer of an integer number of electrons, called the bond order, even if in some systems there are intermediate quantities of charge, as in benzene, in which the binding order is 1.5 for each carbon atom. Primary bonds are generally classified into three classes, in order of increasing polarity:
Secondary chemical bonds
Secondary chemical bonds are those of the molecular dipoles, which can create the intermolecular attractive forces. The intermolecular bonds are essentially constituted by the mutual attraction between static dipoles (in the case of polar molecules) or between dipoles and ions (this is the case, for example, of a salt which dissolves in water).
In the case of noble gases or compounds formed by apolar molecules the ability to liquefy is explained by the random formation of a temporary dipole when the electrons, in their orbit, are randomly concentrated on one side of the molecule; this dipole induces in the molecules close to itself an imbalance of electric charge (the so-called induced dipole) which generates mutual attraction and causes the gas condensation. The bond is then produced by these particular attraction forces called dispersion or Van der Waals forces.
A particular case of intermolecular bonding, which can also be intramolecular when the geometry of the molecule allows it, is the hydrogen bond.
A hydrogen atom bound to an oxygen (or fluorine) atom, due to its positive polarization and its small size, attracts at relatively high intensity the atoms of oxygen (and fluorine and, to a lesser extent, of nitrogen) neighbors.
This bond, although weak, is responsible for the spatial conformation of proteins and nucleic acids, the conformation on which the biological activity of the compounds depends.