section_path: Characteristics/Fundamental properties
section_name: Fundamental properties
paragraph_text: Fundamental properties
  
  The invariant mass of an electron is approximately 9.109 × 10 kg, or 5.489 × 10 Da. Due to mass–energy equivalence, this corresponds to a rest energy of 0.511 MeV (8.19 × 10 J). The ratio between the mass of a proton and that of an electron is about 1836. Astronomical measurements show that the proton-to-electron mass ratio has held the same value, as is predicted by the Standard Model, for at least half the age of the universe.
  
  Electrons have an electric charge of −1.602 176 634 × 10 coulombs, which is used as a standard unit of charge for subatomic particles, and is also called the elementary charge. Within the limits of experimental accuracy, the electron charge is identical to the charge of a proton, but with the opposite sign. The electron is commonly symbolized by e, and the positron is symbolized by e.
  
  The electron has an intrinsic angular momentum or spin of ħ / 2. This property is usually stated by referring to the electron as a spin-1/2 particle. For such particles the spin magnitude is ħ / 2, while the result of the measurement of a projection of the spin on any axis can only be ± ħ / 2. In addition to spin, the electron has an intrinsic magnetic moment along its spin axis. It is approximately equal to one Bohr magneton, which is a physical constant that is equal to 9.274 010 0657 (29) × 10 J⋅T. The orientation of the spin with respect to the momentum of the electron defines the property of elementary particles known as helicity.
  
  The electron has no known substructure. Nevertheless, in condensed matter physics, spin–charge separation can occur in some materials. In such cases, electrons 'split' into three independent particles, the spinon, the orbiton and the holon (or chargon). The electron can always be theoretically considered as a bound state of the three, with the spinon carrying the spin of the electron, the orbiton carrying the orbital degree of freedom and the chargon carrying the charge, but in certain conditions they can behave as independent quasiparticles.
  
  The issue of the radius of the electron is a challenging problem of modern theoretical physics. The admission of the hypothesis of a finite radius of the electron is incompatible to the premises of the theory of relativity. On the other hand, a point-like electron (zero radius) generates serious mathematical difficulties due to the self-energy of the electron tending to infinity. Observation of a single electron in a Penning trap suggests the upper limit of the particle's radius to be 10 meters. The upper bound of the electron radius of 10 meters can be derived using the uncertainty relation in energy. There is also a physical constant called the " classical electron radius ", with the much larger value of 2.8179 × 10 m, greater than the radius of the proton. However, the terminology comes from a simplistic calculation that ignores the effects of quantum mechanics; in reality, the so-called classical electron radius has little to do with the true fundamental structure of the electron.
  
  There are elementary particles that spontaneously decay into less massive particles. An example is the muon, with a mean lifetime of 2.2 × 10 seconds, which decays into an electron, a muon neutrino and an electron antineutrino. The electron, on the other hand, is thought to be stable on theoretical grounds: the electron is the least massive particle with non-zero electric charge, so its decay would violate charge conservation. The experimental lower bound for the electron's mean lifetime is 6.6 × 10 years, at a 90% confidence level.