Electron
Name:
Electron
Detailed information:
Discovered by J. J. Thompson in 1896, the electron is a fundamental particle of matter that can exist either as a constituent of an atom or in the free state. It has a negative electric charge (4.8 × 10−10 esu) and a mass 1/1837 that of a proton, equivalent to 9.1 × 10−28 gram. The number of electrons in an atom of any element is the same as the number of protons in the nucleus, i.e., the atomic number. Thus, the range is from one electron in hydrogen to 103 in lawrencium. As the negative charge of the electrons equals the positive charge of the protons, all atoms are electrically neutral. Electrons are arranged in from one to seven shells around the nucleus; the maximum number of electrons in each shell is strictly limited by the laws of physics. The tendency of electrons to form complete outer shells accounts for the valence of an element, and they play an essential part in chemical bonding. The outer shells are not always filled: sodium has two electrons in the first shell, eight in the second, and only one in the third. A single electron in the outer shell may be attracted into an incomplete shell of another element, leaving the original atom with a net positive charge. Each atom then is called an ion. Valence electrons are those that can be captured by or shared with another atom. Electrons can be removed from the atoms of metals and some other elements by heat, light, electric energy, or bombardment with high-energy particles (see Radiation, Ionizing). In such cases, they are totally free from the atomic orbit and their energy can be utilized by means of a conductor (electricity) or a vacuum tube or semiconductor. Current is generated by detaching the electrons of a metallic conductor (silver, copper) by means of an electric or magnetic field; the electrons then flow along the conductor to a positively charged terminal. The science of electronics was made possible by the ability of a heated metal cathode to emit a continuous stream of electrons in a vacuum tube. Free electrons, called β particles, are spontaneously emitted by decaying radioactive nuclei; they have comparatively low energy but can be accelerated to velocities approaching that of light. The basic nature of the electron has been the subject of much research of the highest order of mathematical rigor. In simplest terms, the electron has the properties of both a particle and a wave, i.e., a standing wave is associated with an electron moving in its orbit. The energy state of any electron in an atom is described by four quantum numbers.See Shell; Atom; Orbital Theory; Lewis Electron Theory.
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