**elementary charge**

## SI defining constant |
|||||

Name | Symbol | Value | Unit | Expressed in terms of SI base units | |

elementary charge | e |
1.602 176 634 × 10^{−19} |
C | s A | |

## DefinitionThe elementary charge, symbol The numerical value of the elementary charge, symbol |

The elementary charge, *e*, together with the hyperfine transition frequency of the caesium 133 atom, Δ*ν*_{Cs}, forms the basis for the definition of the ampere.

### Particle physics

Atoms are composed of subatomic particles called protons, neutrons and electrons. These particles have electric charge of either +*e*, –*e* or 0.

Particle | Charge in e |
Charge in SI units |

electron | -1 | -1.602 176 634 × 10^{−19} C |

proton | +1 | 1.602 176 634 × 10^{−19} C |

neutron | 0 | 0 C |

The Standard Model of particle physics describes 6 subatomic particles called quarks. These particles have electric charge of either +⅔*e* or -⅓*e*. The exact numerical value of the electric charge of these particles expressed in coulombs follows from the definition of the value of *e*:

Quark | Charge in e |
Charge in SI units |

up | +⅔ | 1.068 117 756 × 10^{−19} C |

down | -⅓ | -0.534 058 878 × 10^{−19} C |

charm | +⅔ | 1.068 117 756 × 10^{−19} C |

strange | -⅓ | -0.534 058 878 × 10^{−19} C |

top | +⅔ | 1.068 117 756 × 10^{−19} C |

bottom | -⅓ | -0.534 058 878 × 10^{−19} C |

A proton is composed of two up quarks of charge +⅔*e* and one down quark of charge –⅓*e*, giving a total charge of +1*e*. A neutron is composed of one up quark of charge +⅔*e* and two down quarks of charge –⅓*e*, giving a total charge of 0*e*.

### Nature

The elementary charge *e* corresponds to a coupling strength of the electromagnetic force via the fine-structure constant *α* = *h*^{2}/(2c ε_{0}*h*) where ε_{0} is the vacuum electric permittivity or electric constant. Some theories predict a variation of *α* over time. The experimental limits of the maximum possible variation in *α* are so low, however, that any effect on foreseeable practical measurements can be excluded.