Since it is the electron in the electron cloud of an atom that determines its chemical behavior, the quantum mechanics description of electron configuration is necessary to understanding chemistry. The trajectory that we normally associate with macroscopic objects is replaced for electrons in electron clouds, with statistical descriptions that show, not the electron path, but the region where it is most likely to be found. Consequently, electron behavior is described differently than the behavior of normal sized particles. Our fact that we cannot accurately know both the position and the momentum of an electron at the same time causes an inability to predict a trajectory for an electron. The more accurately we measure either the energy or the position of an electron, the less we know about the other. The wave-particle duality of electrons within the electron cloud limits our ability to measure both the energy and the position of an electron simultaneously. Quantum theory has some mathematical development, often referred to as quantum mechanics, that offers explanations for the behavior of electrons inside the electron clouds of atoms.

Quantum theory suggests that energy comes in discrete packages called quanta (or, in the case of electromagnetic radiation, photons). These definitions essentially describe quantum theory as a theory in which both energy and matter have characteristics of waves under some conditions and characteristics of particles under other conditions. Most definitions of quantum theory and quantum mechanics offer the same description for both.

Erwin schrödinger atomic theory