The electron rest mass, symbolized m e , is the mass of an electron as measured when its speed is zero relative to an observer. The electron has extremely small mass -- it's almost 2000 times less massive than the proton -- at approximately 9.10956 x 10 -31 kilogram (kg) or 9.10956 x 10 -28 gram (g). Every known electron at rest has the same mass as every other known electron at rest.
When an electron is accelerated to high speed, its mass increases because of relativistic effects. This increase is not significant until the speed of the particle, relative to the observer, becomes an appreciable fraction of the speed of light . If v is the speed of an electron in meters per second (m/s) and c is the speed of light (approximately 2.99792 x 10 8 m/s), then the mass of an electron in motion, m e* , is
m e* = m e / (1 - v 2 /c 2 ) 1/2 = 9.10956 x 10 -31 / (1 - v 2 /c 2 ) 1/2 kg
The relativistic mass increase of high-speed electrons is significant in real-world scenarios, because it increases the effects of the electrons on objects they strike. High-speed electrons are known as beta particle s.
Also see Table of Physical Units and Constants.