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.
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.
The description of the physical and chemical properties of matter is a central issue that has occupied the minds of scientists since the age of the ancient Greeks. In their route to dissect matter down to what cannot be divided any further, they coined the term atom , the indivisible . Matter became then a collection of atoms. More than twenty centuries had to pass until the development of a more precise concept of atom, thanks, amongst others, to the systematic studies of Mendeleyev and the establishment in 1869 of the periodic table of the elements (Mendeleyev, 1869). The discovery of the electron in 1897 and the first modern model of the atomic structure by Sir Joseph Thomson were soon refined by his student, Sir Ernest Rutherford, who in 1910 showed that an atom was made of a positively charged small nucleus and a number of negatively charged electrons that neutralize the nuclear charge. Much in the spirit of planetary systems, and drawing from the analogy between gravitational and electrostatic interactions, scientists in the beginning of the twentieth century built an image of the atom that consisted of a number Z of electrons – of elementary charge – e – orbiting around the nucleus of charge Ze . A number of experimental observations, though, were incompatible with this idea of orbiting electrons. In particular, according to the successful electromagnetic theory, charged electrons in orbital (radially accelerated) motion should radiate energy, thus decelerating and eventually collapsing onto the nucleus. Clearly, such a picture would imply that matter is essentially unstable, in flagrant contradiction with our everyday experience of the very existence of matter.
The whole realm of phenomena at the atomic or subatomic level is the special province of quantum theory. When isolated from radiation and other atoms, most atoms remain stable indefinitely: they neither collapse nor explode. When atoms are excited electrically or by collisions or otherwise, they emit radiation of discrete wavelengths characteristic of the kind of atoms excited. The simplest model of the atom is a hard, tiny, electrically neutral sphere—just the smallest possible fragment of the bulk material that possesses the identity of a given chemical element. Neils Bohr's theory was built primarily on the photon concept of radiation, which was understood as an hypothesis but was not yet unambiguously verified when Bohr did his early work. Bohr's first solution of the hydrogen-atom problem in his historic 1913 paper depended on a rather arbitrary combination of classical and quantum ideas.
Among philosophers in particular the principle of complementarity has been considered an unreasonable limitation of natural science, and Niels Bohr’s exposition of the inevitability of a statistical description when accounting for atomic phenomena has been regarded as a renouncement directly opposed to the very spirit of scientific investigation. Indeed, fundamental limitations in the range of applicability of the causal spacetime description of physical phenomena do exist, but quantum physics is precisely the generalization of the conceptual frame of classical physics which has converted these limitations into a springboard for investigations which are among the most fruitful in the history of science. Far from raising a barrier for scientific investigation, quantum physics has extended the scope of natural science to a degree that was unimaginable before its advent. Natural science now encompasses such different objects as the constituents of atoms, processes in remote galaxies, and psycho-chemical phenomena in man. Biochemistry and psychopharmacology, for instance, both rely on one of the most important results of quantum physics: the periodical system of the elements, which is fundamental to all modern chemistry. Furthermore atomic physics and chemistry are basic sciences for technology and thus contribute to the continuous development of instruments of observation for all branches of natural science.
Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. Atomic physics typically refers to the study of atomic structure and the interaction between atoms. It is primarily concerned with the way in which electrons are arranged around the nucleus and the processes by which these arrangements change. This comprises ions, neutral atoms and, unless otherwise stated, it can be assumed that the term atom includes ions. The term atomic physics can be associated with nuclear power and nuclear weapons, due to the synonymous use of atomic and nuclear in Standard English. Physicists distinguish between atomic physics -which deals with the atom as a system consisting of a nucleus and electrons- and nuclear physics, which studies nuclear reactions and special properties of atomic nuclei. As with many scientific fields, strict delineation can be highly contrived and atomic physics is often considered in the wider context of atomic, molecular, and optical physics. Physics research groups are usually so classified. Atomic physics primarily considers atoms in isolation. Atomic models will consist of a single nucleus that may be surrounded by one or more bound electrons. It is not concerned with the formation of molecules (although much of the physics is identical), nor does it examine atoms in a solid state as condensed matter. It is concerned with processes such as ionization and excitation by photons or collisions with atomic particles. While modelling atoms in isolation may not seem realistic, if one considers atoms in a gas or plasma then the time-scales for atom-atom interactions are huge in comparison to the atomic processes that are generally considered. This means that the individual atoms can be treated as if each were in isolation, as the vast majority of the time they are. By this consideration atomic physics provides the underlying theory in plasma physics and atmospheric physics, even though both deal with very large numbers of atoms. Electrons form notional shells around the nucleus. These are normally in a ground state but can be excited by the absorption of energy from light (photons), magnetic fields, or interaction with a colliding particle (typically ions or other electrons). Electrons that populate a shell are said to be in a bound state. The energy necessary to remove an electron from its shell (taking it to infinity) is called the binding energy. Any quantity of energy absorbed by the electron in excess of this amount is converted to kinetic energy according to the conservation of energy. The atom is said to have undergone the process of ionization. If the electron absorbs a quantity of energy less than the binding energy, it will be transferred to an excited state. After a certain time, the electron in an excited state will "jump" (undergo a transition) to a lower state. In a neutral atom, the system will emit a photon of the difference in energy, since energy is conserved. If an inner electron has absorbed more than the binding energy (so that the atom ionizes), then a more outer electron may undergo a transition to fill the inner orbital. In this case, a visible photon or a characteristic x-ray is emitted, or a phenomenon known as the Auger effect may take place, where the released energy is transferred to another bound electron, causing it to go into the continuum. The Auger effect allows one to multiply ionize an atom with a single photon. There are rather strict selection rules as to the electronic configurations that can be reached by excitation by light — however there are no such rules for excitation by collision processes. One of the earliest steps towards atomic physics was the recognition that matter was composed of atoms. It forms a part of the texts written in 6th century BC to 2nd century BC such as those of Democritus or Vaisheshika Sutra written by Kanad. This theory was later developed in the modern sense of the basic unit of a chemical element by the British chemist and physicist John Dalton in the 18th century. At this stage, it wasn't clear what atoms were although they could be described and classified by their properties (in bulk). The invention of the periodic system of elements by Mendeleev was another great step forward. The true beginning of atomic physics is marked by the discovery of spectral lines and attempts to describe the phenomenon, most notably by Joseph von Fraunhofer. The study of these lines led to the Bohr atom model and to the birth of quantum mechanics. In seeking to explain atomic spectra an entirely new mathematical model of matter was revealed. As far as atoms and their electron shells were concerned, not only did this yield a better overall description, i.e. the atomic orbital model, but it also provided a new theoretical basis for chemistry (quantum chemistry) and spectroscopy. Since the Second World War, both theoretical and experimental fields have advanced at a rapid pace. This can be attributed to progress in computing technology, which has allowed larger and more sophisticated models of atomic structure and associated collision processes. Similar technological advances in accelerators, detectors, magnetic field generation and lasers have greatly assisted experimental work.
By now the “chaos revolution” has reached nearly every branch of the natural sciences. In fact, chaos is everywhere. To name but a few examples, we talk about chaotic weather patterns, chaotic chemical reactions and the chaotic evolution of insect populations. Atomic and molecular physics are no exceptions. At first glance this is surprising since atoms and molecules are well described by the linear laws of quantum mechanics, while an essential ingredient of chaos is nonlinearity in the dynamic equations. Thus, chaos and atomic physics seem to have little to do with each other. But recently, atomic and molecular physicists have pushed the limits of their experiments to such high quantum numbers that it starts to make sense, in the spirit of Bohr's correspondence principle, to compare the results of atomic physics experiments with the predictions of classical mechanics, which, for the most part, show complexity and chaos. The most striking observation in recent years has been that quantum systems seem to “know” whether their classical counterparts display regular or chaotic motion. This fact can be understood intuitively on the basis of Feynman's version of quantum mechanics. In 1948 Feynman showed that quantum mechanics can be formulated on the basis of classical mechanics with the help of path integrals. Therefore it is expected that the quantum mechanics of an atom or molecule is profoundly influenced, but of course not completely determined, by the qualitative behaviour of its underlying classical mechanics.
A whole community of physicists is under the spell of the oversized atoms called Rydberg atoms. Created when one or sometimes two of their electrons is excited to very high energy levels, far from the nuclear core, Rydberg atoms can detect subtle electric fields and probe chemical reactions that ordinarily proceed too fast for study. And because their outermost electrons sometimes orbit the nucleus like planets orbiting the sun—a behavior reminiscent of classical physics, not the quantum physics of the atom—physicists are using Rydberg atoms to study the border between the quantum and classical worlds.