ABSTRACT
The atom is the reference point for the whole of the science of materials, whatever we choose to call the particular discipline: physics, chemistry, geology, biochemistry, and so on. The atomic model as formulated by Dalton proposes that the chemical elements are composed of indivisible atoms, all the atoms of a given element possessing identical properties. The Bohr atom is important historically, but in the quantum mechanical model of the atom all the details proposed in the Bohr atom, including quantisation, arise without the need for any assumptions. Highly excited or energetic atoms, called ‘Rydberg atoms’ after a Swedish spectroscopist, provide a link between classical and quantum physics. The periodic orbits characteristic of classical systems still form useful components of quantum chaos models for helium and larger atoms. The situation as unstable periodic orbits embedded in a sea of more common nonperiodic orbits.
