Quantum mechanics is the branch of physics that describes the properties of individual atoms and molecules. Quantum mechanics shares many principles with classical mechanics, but many of its assertions run contrary to everyday experience. Nevertheless, it is a highly successful theory (this means that its predictions agree with experiment in every case), and there is no alternative physical theory for describing chemical systems.
The development of quantum mechanics began in the early 1900's when Planck and Einstein published papers describing light as a "quantized" or particle-like entity (light was widely regarded by physicists as a smooth, continuous wave). However, the current form of quantum mechanics was not discovered until the early 1920's by other physicists: Schrödinger, Heisenberg, Bohr, and Dirac.
The complexity of chemical systems makes it difficult to apply quantum mechanics to these systems. "Exact" predictions are possible in only a few cases and "approximate" predictions are required for all other cases. Consequently, the chemist's version of quantum mechanics is a theory that is filled with approximations and simplifications.
So-called "quantum chemists" have as their goals the development of useful (but necessarily approximate) quantum mechanical descriptions of chemical systems, and the discovery of which approximations are successful and which are not. The first useful quantum chemical models of molecules were published in the late 1920's, but many problems, experimental, theoretical, and computational, remain.
(last updated 5/31/97)