A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. A molecule is formed by a set of same atoms (in the case of the elements) or different from each other (in the case of the compounds).
In the condensed state (solid and liquid) the molecules interact with each other by intermolecular forces, while in the gaseous state the molecules are almost independent of each other.
According to the molecular theory, the chemical and chemical-physical properties of a substance can be interpreted based on the nature of its molecules, more precisely of the nature of the atoms forming the molecule, of their numerical combination of relationships, of their adjacency relationships, the directionality of bonds and the overall spatial structure.
The term molecule entered the scientific language in the 17th century with the meaning of small mass and was initially confused with the atom. J. Dalton used the term for compounds, but only with Gay-Lussac and especially with A. Avogadro, the atomic-molecular theory received a precise enunciation, which, however, found acceptance and development only after S. Cannizzaro, taking up Avogadro’s ideas, had established a system of atomic weights in accordance with the experimental acquisitions.
In chemistry, particularly in quantum chemistry, a molecular orbital is the spatial distribution of electrons in a molecule. The molecular orbital characterizes the electron configuration of a molecule, defining the spatial distribution and energy of electrons, and was introduced by Friedrich Hund and Robert S. Mulliken in 1927 and 1928.
A molecular orbital is represented by a wave function whose square describes the probability distribution relative to the position of the electron. This wave function is obtained from the wave equation that describes the entire molecule, which in general is not easy to solve: this problem is solved by an approximation which consists of writing the molecular orbital as a linear combination of the orbitals atomics of individual atoms. The theory of molecular orbitals describes this approximation.