Quantum states in atoms and molecules

Abstract

The main characteristics of atoms and molecules are their structure and their energy spectrum . Under the term structure of an arbitrary particle we usually understand the size and the distribution of its mass and its charge in space. In quantum physics such a distribution is defined by the square of the modulus of the wavefunction of a particle. The particle itself may consist of a system of other particles that are bound by a certain type of coupling. For example, an atom consists of a nucleus and a system of interacting electrons, a molecule consists of a system of interacting atoms, and so on. If we consider an atom, the nucleus of the atom is assumed to be at rest (the so-called adiabatic approximation ). This assumption can be made because the nucleus has a much larger mass than that of an electron (a proton's mass is 1836 times larger than the mass of an electron). Then, the square of the modulus of the wavefunction, ∣ψ( r 1 , r 2 , …, r n )∣ 2 , for the system of electrons defines the probability density of finding the j th electron at the point r j . Graphically it is very convenient to depict ∣ψ∣ 2 in the form of an electron cloud , which can be considered as an averaged distribution of matter added to the mass of the nucleus located at the center of an atom.