An electron is a subatomic particle with a negative elementary electric charge. It is a fundamental constituent of matter and has an intrinsic spin of ½. Electrons are bound to the nucleus of an atom by the electromagnetic force, keeping all atoms electrically neutral. They are among the lightest particles in nature and play an important role in many natural phenomena, such as electricity, magnetism, chemistry and thermal conductivity.
The electrons in an atom are arranged in shells around the nucleus. The first shell can hold up to two electrons, the second shell can hold up to eight electrons and so on. The electrons in each shell are at different energy levels. The outermost shell is called the valence shell and determines how reactive an atom is. Atoms that have lost or gained electrons become ions. When atoms share or transfer electrons they form chemical bonds that hold molecules together.
Electrons were first discovered in 1897 by J.J Thomson during experiments with cathode rays. He found that these rays were deflected by magnets and concluded that they must be tiny charged particles (later called “corpuscles”). In 1911 Ernest Rutherford showed that atoms consisted mostly of empty space with a small dense nucleus containing most of the mass and all the positive charge (protons had not yet been discovered). He proposed that atoms were held together by electrostatic attraction between the positive nucleus and negative electrons orbiting around it like planets around a sun. This “solar system” model was later modified by Niels Bohr who proposed that electrons could only occupy certain allowed orbits around the nucleus (the Bohr model).
In 1925 Werner Heisenberg formulated his famous uncertainty principle which put strict limits on how well we can know both the momentum and position of a subatomic particle like an electron. According to this principle it is impossible to know both quantities simultaneously with absolute certainty – there is always some uncertainty or “fuzziness”. This lead to the development of quantum mechanics which describes the behavior of matter on very small scales where classical physics breaks down completely. In quantum mechanics particles like electrons do not have a definite location until they are observed or measured (the act of measurement itself changes their wave-like state into a more localized particle-like state).
Today we know that each electron has a specific set of quantum numbers which determine its unique identity (no two electrons can have exactly the same set). These numbers also specify various properties such as energy level, spin orientation etc.. We also now realize that although Rutherford’s solar system model was wrong, Bohr’s model was essentially correct – just substituting probability for certainty when describing where an electron might be found at any given time around its parent atom!