Electron
Electron ( e-)
The electron [e ː lɛktrɔn, elɛk, elɛktro ː n] ( from Ancient Greek ἤλεκτρον electron Bernstein ', was observed on the electricity for the first time; coined in 1874 by Stoney and Helmholtz ) is a negatively charged elementary particles. Its symbol is e -. The alternative name Negatron is rarely used and is in use at most in beta spectroscopy.
In atoms and ions, electrons form the electron shell. The overall chemistry is essentially based on the properties and interactions of these bound electrons. The state of each of the bound electrons can thereby clearly by four quantum numbers ( principal quantum number, azimuthal quantum number, magnetic quantum number of the angular momentum and spin quantum number ) describe (see also Pauli principle). The free mobility of some of the electrons in metals is the cause of the electrical conductivity of metal conductors. Regardless of the atomic shell, an electron is re-generated and emitted during beta -minus decay of an atomic nucleus.
The experimental detection of electrons was first in 1897 by the British, Joseph John Thomson.
History of the discovery of the electron
The concept of a smallest, indivisible amount of electric charge was around the middle of the 19th century various proposals, among others, Richard Laming, Wilhelm Weber and Hermann von Helmholtz.
George Johnstone Stoney proposed in 1874 before the existence of electrical charge carriers, which should be associated with the atoms. Starting from the electrolysis he estimated from the magnitude of the electron charge, but received a factor of about 20 is too low. At the meeting of the British Association at Belfast, he proposed to use the elementary charge, together with the gravitational constant and the speed of light as a fundamental constants of nature as the basis of physical measurement systems. Stoney also coined, together with Helmholtz the name electron for the " atom of electricity".
The electron elementary particles as was demonstrated in 1897 by Joseph John Thomson ( he described it first as corpuscule ). He was able to show that the nature of the particles is independent of the cathode material, and the residual gas in the cathode ray tube of said cathode ray. During this time, the bound in the atomic electrons were detected by means of the Zeeman effect.
The elementary charge in 1909 by Robert Millikan measured.
Properties
The electron is the lightest of the electrically charged elementary particles. If the conservation laws apply for charge and energy - the equivalent of all physical experience - must electrons therefore be stable. In fact, there has been no experimental evidence to a decay electrons.
Electrons belong to the leptons and, like all leptons, a spin of 1/2. As particles with half-integer spin they belong to the class of fermions, ie, are particularly subject to the Pauli principle. Their antiparticles are the positron, e symbol with which they correspond to their electric charge in all properties.
Some of the basic properties of the electron, which are listed in the table on the right are connected to each other by the magnetic moment of the electron spin:
It is the magnetic moment of the electron spin, the mass of the electron, its charge and spin. called Landé g - factor or. The term front, which is the ratio of magnetic moment to the spinning is called a gyromagnetic ratio of the electron. For the electron, the theoretical value of exactly equal to 2 effects of quantum electrodynamics, according to the Dirac theory ( relativistic quantum mechanics) but cause a (slight ) deviation of the value of from 2 caused thereby deviation of the magnetic moment is as anomalous magnetic moment of the electron respectively.
Classic radius and Punktförmigkeit
Shortly after the discovery of the electron tried to estimate its extent, especially because of the classical notion of small billiard balls that collide in scattering experiments. The argument boiled down to the fact that the concentration of the electron charge to a very small extension of the electron energy need which must be stuck in the mass of the electron after the equivalence principle. Assuming that the energy of an electron is equal to the self- alone energy of the electrons in the charge 's electric field, it receives the classical electron radius
: Elementary charge: Circle number: Electric field constant: electron mass, : velocity of light: fine structure constant: Bohr radius.
The self-energy separates mentally electric charge and electric field of the electron. Putting the charge e in the potential, whereby they are thought uniformly distributed, for example, on a spherical surface of the radius, as this energy is required, the self- power. However, there were certainly also other derivations for a possible extension of the electron, which came to other values .
Today is the view regarding an expansion of the electron to another: In previously possible experiments show electrons neither extension nor internal structure and can be accepted in so far as a point. The experimental upper limit on the size of the electron is currently about 10-19 m. Nevertheless, the classical electron radius in many formulas occurs, is in which of the stationary properties of the electron size of dimension length (or area, etc. ) formed in order to explain experimental results. For example, the theoretical formulas for the cross sections of the photo and the Compton effect included the square of.
The search for an electric dipole moment of the electron has so far remained without positive result. A dipole moment would occur if in a non- point-shaped electron of gravity of the mass would not at the same time the center of gravity of the load. Something like this is predicted by theories of supersymmetry, which go beyond the standard model of elementary particles. A measurement in October 2013 that takes advantage of the strong electric field in a polar molecule, has revealed that a possible dipole moment with a 90% confidence level no greater than 8.7 • 10-31 m. Clearly this means that the charge and the center of mass of the electron, look no further than about 10-30 m can apart. Theoretical approaches, according to which higher values were predicted are refuted.
Cross-section
To be distinguished from the extension of the electron is its cross section for interaction processes. In the scattering of X-rays by electrons to obtain, for example, a cross -section which would correspond to an effective electron radius of about 3 × 10-15 m, which agrees reasonably well with the classical radius of an electron as described above. The total scattering cross section of photons of electrons is in the limit of smaller photon energies (see Thomson scattering and Compton effect).
Interactions
Many physical phenomena such as electricity, electromagnetism and electromagnetic radiation are essentially based on interactions of electrons. Electrons in an electrical conductor to be moved by a changing magnetic field and an electrical voltage is induced. The electrons in a current-carrying conductor producing a magnetic field. An accelerated electron - of course the case of curvilinear motion - emits photons, the so-called bremsstrahlung ( Hertzian dipole, synchrotron radiation, free-electron laser).
In a solid state, the electron undergoes interaction with the crystal lattice. Its behavior can be described by the different effective mass is used instead of the electron mass, which is also dependent on the direction of motion of the electron then.
Electrons that have dissolved in polar solvents such as water or alcohols from their atoms are called solvated electrons. For solution of alkali metals in ammonia they are responsible for the intense blue color.
An electron is a quantum object, that is, at it described by the Heisenberg uncertainty principle the position and momentum of focus is in the measurable range, so that, as with light, both wave and particle properties can be observed, also known as wave -particle duality will. In an atom the electron can be considered as a standing matter wave.
Experiments
The ratio e / m of the electron charge, the electron mass can be determined as a school experiment with the fine beam tube. The direct determination of the elementary charge succeeded by the Millikan experiment.
Wherein electrons, whose speed is not negligibly small in relation to the speed of light, the non-linear contribution to the pulse by the theory of relativity must be considered. Electrons with their small mass are relatively easy to accelerate to such high speeds; already with a kinetic energy of 80 keV has an electron half the speed of light. The pulse can be measured by the deflection in a magnetic field. The deviation of the pulse from the calculated value according to classical mechanics was first established by Walter Kaufmann in 1901 and first described after the discovery of relativity theory with the concept of " relativistic mass increase ," but which is now considered obsolete.
Free electrons
In the cathode ray tube ( cathode ray tube ) electrons are liberated from a heated hot cathode and the vacuum by an electric field along the field direction accelerated ( towards the positive anode). Magnetic fields the electrons are deflected perpendicular to the field direction and perpendicular to the instantaneous flight direction ( Lorentz force). These properties of the electrons have made possible the development of the oscilloscope, the TV and the computer monitor.
Other applications of free electrons are eg the X-ray tube, the electron microscope, electron beam welding, basic research in physics and particle accelerators by the generation of synchrotron radiation for research and industrial purposes.