Electron

The electron (e−, or β− in nuclear reactions) is a subatomic particle whose electric charge is negative one elementary charge. It is an elementary particle that comprises the ordinary matter that makes up the universe, along with up and down quarks. Electrons are extremely lightweight particles. In atoms, an electron's matter wave occupies atomic orbitals around a positively charged atomic nucleus. The configuration and energy levels of an atom's electrons determine the atom's chemical properties. Electrons are bound to the nucleus to different degrees. The outermost or valence electrons are the least tightly bound and are responsible for the formation of chemical bonds between atoms to create molecules and crystals. These valence electrons also facilitate all types of chemical reactions by being transferred or shared between atoms. The inner electron shells make up the atomic core. The concept of electrons explains numerous physical phenomena. In metals, the outermost electrons are delocalised and able to move freely, accounting for the high electrical and thermal conductivity of metals. In semiconductors, the number of mobile charge carriers (electrons and holes) can be finely tuned by doping, temperature, voltage and radiation – the basis of modern electronics. Electrons can exist as free particles. As particle beams in a vacuum, free electrons can be accelerated, focused, and used for applications such as cathode ray tubes, electron microscopes, electron beam welding, lithography, and particle accelerators that generate synchrotron radiation.