plasmapy.atomic.standard_atomic_weight(element: plasmapy.atomic.particle_class.Particle) → astropy.units.quantity.Quantity

Return the standard (conventional) atomic weight of an element based on the relative abundances of isotopes in terrestrial environments.


element (str, int, or Particle) – A string representing an element or an integer representing an atomic number, or an instance of the Particle class.


atomic_weight – The standard atomic weight of an element based on values from NIST.

Return type:


  • InvalidElementError – If the argument is a valid particle but not a valid element.
  • InvalidParticleError – If the argument does not correspond to a valid particle.
  • TypeError – If the argument is not a str or int.

See also



Standard atomic weight data are most readily available for the terrestrial environment, so this function may not be wholly appropriate for space and astrophysical environments.

The relative abundances of different isotopes of an element sometimes vary naturally in different locations within the terrestrial environment. The CIAAW provides ranges for these element, which include H, Li, B, C, N, O, Mg, Si, S, Cl, Br, Tl. This function provides a single value from the CIAWW 2015 standard values when a single value is given, and the lower accuracy conventional value given by Meija et al. (2013, doi:10.1515/pac-2015-0305) for the elements where a range is given.


>>> standard_atomic_weight("H")
<Quantity 1.67382335e-27 kg>
>>> standard_atomic_weight("lead")
<Quantity 3.44063689e-25 kg>