Electron rest mass
The electron rest mass (symbol: me) is the mass of a stationary electron. It is one of the fundamental constants of physics and is also very important in chemistry because of its relation to the Avogadro constant. It has a value of about 9.11×10−31 kilograms or about 5.486×10−4 atomic mass units, equivalent to an energy of about 8.19×10−14 joules or about 0.511 megaelectronvolts.
The term "rest mass" + kinetic energy comes from the need to take account of the effects of special relativity on the apparent (or "observed") mass of an electron. It is impossible to "weigh" a stationary electron, and so all practical measurements must be carried out on moving electrons. The same is true with any other sub-atomic particle. For particles like photons or gluons the situation is even more problematic since the very concept of a stationary or "at rest" massless particle lacks meaning.
The electron rest mass in kilograms is calculated from the definition of the Rydberg constant R∞:
where α is the fine structure constant and h is the Planck constant. The relative uncertainty, 5×10−8 in the 2006 CODATA recommended value, is due entirely to the uncertainty in the value of the Planck constant.
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