Atomic Physics
Atomic physics is a branch of physics devoted to the study of the material composition and energetic processes of atoms and molecules.
Concepts of the atom and of atomic structure date to the ancient Greek philosopher Democritus who proposed that all things are made of atoms and that, although they are too small to be seen in their ceaseless motion, they could collide and accumulate.
The contemporary study of physics (i.e., the study of matter and energy and the interaction between them) is roughly divided into two categories: classical physics and modern physics. Classical physics includes the traditional branches of physics (e.g. mechanics) developed before the beginning of the 20th century. Most of classical physics is concerned with behavior of energy and matter on the macroscopic scale of observation. On the other hand, atomic, particle, and nuclear physics comprise areas of modern physics. Unlike classical physics, modern physics is concerned with energy and matter under extreme conditions or on the microscopic scale. On this small-scale ordinary, commonly accepted aspects of motion, space, matter and energy are no longer applicable. Accordingly, atomic physics relies on the principles of quantum mechanics to describe the discrete nature of matter at the atomic and subatomic levels. Atomic physics involves specifically investigations into the electronic parts of the atom.
Near the end of the 19th century, English physicist J. J. Thomson's (1856-1940) discovery of the electron proved that the atom was not an indivisible particle. Subsequent to Thomson's discovery, various models of atomic structure evolved to describe atomic structure. Thomson's model of the atom was a solid sphere that had electrons embedded in a positive part thought to be a fluid. He thought that the positive part made up the bulk of the atom's mass and volume. In 1909-1911 English physicist Ernest Rutherford (1871-1937) developed an atomic model based upon the observation of the deflection of alpha particles bombarding thin foils. Rutherford's model of the atom contained negatively charged particles surrounding a nucleus of tightly packed positive charges. Importantly, Rutherford proposed that most of the atom was empty space.
Rutherford's proposal led to the birth of atomic physics. Subsequent studies of the atom were divided into investigations of the electronic parts of the atom, atomic physics, and investigations of the nucleus itself, nuclear physics. The division arose because of the huge difference in energy required to produce nuclear as opposed to electronic changes.
Although Rutherford's atomic theory had two major problems it was the basis for studies involving the atom. In 1913 Danish physicist Niels Bohr (1885-1962), who had studied under Rutherford, corrected the problems with Rutherford's atomic theory. Instead of existing in arbitrary orbitals Bohr proposed that electrons are found only in certain states. Using German physicist Maxwell Planck's (1858-1947) theoretical insights, Planck's constant, Bohr was able to create a formula used to find the energy levels of different atoms. Although insightful and useful as a conceptual model, in reality the Bohr model could only be applied to the simple hydrogen atom. Detailed models of more complex atoms proved elusive and mathematically complex.
In 1923 French physicist Louis de Broglie (1892-1987) proposed that electrons have a particle-wave duality wherein they have properties of both particles and waves. This idea supported Planck and German-American physicist Albert Einstein's theory that the fundamental entity of electromagnetic radiation, the photon, has both particle and wave properties. In 1924, using Einstein's special theory of relativity, de Broglie showed that particles have waves associated with them and that electrons cannot be pictured as localized particles in space but rather should be thought of as clouds of negative charge spread out over the entire orbit. These clouds represent the regions around the nucleus where the probability of finding an electron is the largest. Electrons occupy different energy levels in atoms and have different wave properties. Because electrons behave both as particles and waves it became obvious that a specific type of partial differential equation should be used to describe the position and path of electrons.
In 1926, Austrian physicist Erwin Schrödinger (1887-1961) used partial differential equations and the Hamiltonian function to develop a powerful equation that relates the energy of the electron and the energy of the electric field in which it is situated. This equation, the Schrödinger equation, relates the energy of a system to its wave properties and allows one to predict the energy of the electron and its future behavior ( i.e. the probability of finding the electron in a particular region around the nucleus). Shortly after the advancement of the Schrödinger equation German physicist Max Born postulated that the wave function could be used to determine the probability of finding a particle in a particular region at a specific time. In 1927 German physicist Werner Heisenberg developed a formula describing atoms in terms of the frequencies of their atomic spectral lines and set forth the Heisenberg uncertainty principle that states that one cannot know both the position and the velocity of a particle simultaneously.
Subject to the limitations of the Heisenberg uncertainty principle, the advancement of atomic physics and quantum physics allowed increasingly accurate descriptions of complex atoms. The delineation of atomic substructure and mechanisms of subatomic processes evolved into the modern study of particle physics.
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