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<p>The electron (e−, or β− in nuclear reactions) is a <a href="/facts/Subatomic_particle/tMEzcTWQ">subatomic particle</a> with a negative one <a href="/facts/Elementary_charge/lPcj6CoQ">elementary electric charge</a>. </p><p><a href="/facts/Ordinary_matter/osyc04UN">Ordinary matter</a> is composed of <a href="/facts/Atom/FoQ4kGN5">atoms</a>. Each atom consists of a positively charged <a href="/facts/Atomic_nucleus/orpIm5Xl">nucleus</a> surrounded by a number of electrons that neutralize its charge. Electrons are bound to the nucleus to different degrees: the least bound electrons are responsible for the bonds between atoms present in <a href="/facts/Molecule/N9ljSfFq">molecules</a> and <a href="/facts/Crystal/e2R2mk3E">crystals</a>, and for all types of <a href="/facts/Chemical_reaction/GpqWlKrz">chemical reactions</a>, while the rest make up the <a href="/facts/Atomic_core/LQuGLKcG">atomic core</a>. </p><p>Electrons play an essential role in numerous <a href="/facts/Physics/a7aH2y08">physical</a> phenomena. In <a href="/facts/Metal/PT8hNX6R">metals</a>, highly mobile electrons are responsible for their high electrical and thermal conductivity. In <a href="/facts/Semiconductor/zFf3mGTE">semiconductors</a>, the number of mobile electrons is easily influenced by the material's composition, temperature, applied voltage or irradiation. This is the basis of <a href="/facts/Semiconductor_electronics/6W3ryr50">semiconductor electronics</a>. Electrons can escape from their atoms and materials, and as <i>free electrons</i> form particle beams in a vacuum, be further accelerated and focused. This has enabled the development of the <a href="/facts/Cathode-ray_tube/8NVGx5C7">cathode-ray tubes</a>, <a href="/facts/X-ray_tube/tx0dTDqp">X-ray tubes</a>, <a href="/facts/Electron_microscope/XtaBktwF">electron microscopes</a>, <a href="/facts/Electron_beam_welding/4WqClz4q">electron beam welding</a>, research by means of <a href="/facts/Particle_accelerator/Pkuc08xC">particle accelerators</a> and the generation of <a href="/facts/Synchrotron_radiation/mTsSKmRz">synchrotron radiation</a>. </p>