Adds average mass for the elements:

  • mass, mass_units (u)
    The molar mass averaged over natural isotope abundance.

Adds mass and abundance information for isotopes:

  • mass, mass_units (u)
    The molar mass of the individual isotope.
  • abundance, abundance_units (%)
    Natural abundance for the isotope.

Atomic Weights and Isotopic Composition [1].

The atomic weights are available for elements 1 through 118 and isotopic compositions or abundances are given when appropriate. The atomic weights data were published by Wang [6] in The AME 2020 atomic mass evaluation (II). Hashes # after values indicate vlaues which are not solely derived from experiment, see the paper for details. Trailing zeros do not necessarily indicate uncertainties quoted in the reference. Tables, graphs and references, and the isotopic compositions data were published by Rosman [3] and Taylor [4] in Isotopic Compositions of the Elements 1997. The relative atomic masses of the isotopes data were published by Audi [5] and Wapstra [7] in the 1995 Update To The Atomic Mass Evaluation.

This data has been compiled from the above sources for the user’s convenience and does not represent a critical evaluation by the NIST Physics Laboratory. http://physics.nist.gov/PhysRefData/Compositions/

Neutron mass from NIST Reference on Constants, Units, and Uncertainty http://physics.nist.gov/cuu/index.html

[1]Coursey. J. S., Schwab. D. J., and Dragoset. R. A., NIST, Physics Laboratory, Office of Electronic Commerce in Scientific and Engineering Data.
[2]Coplen. T. B. : U.S. Geological Survey, Reston, Virginia, USA.
[3]Rosman. K. J. R. : Department of Applied Physics, Curtin University of Technology, Australia.
[4]Taylor. P. D. P. : Institute for Reference Materials and Measurements, European Commission, Belgium.
[5]Audi. G. : Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, Orsay Campus, France.
[6]Wang. M., Huang. W. J., Kondev. F. G., Audi. G., Naimi. S., Institute of Modern Physics, Chinese Academy of Sciences,
[7]Wapstra. A. H. : National Institute of Nuclear Physics and High-Energy Physics, Amsterdam, The Netherlands.

Natural abundance.

Parameters:isotope : Isotope
Returns:abundance : float | %
Coursey. J. S., Schwab. D. J, and Dragoset. R. A., NIST Atomic Weights and Isotopic Composition Database.
periodictable.mass.init(table, reload=False)

Add mass attribute to period table elements and isotopes


Atomic weight.


isotope : Isotope

mass : float | u

Atomic weight of the element.

*Wang. M., Huang. W. J., Kondev. F. G., Audi. G., Naimi. S., The AME 2020 atomic mass evaluation (II). Tables, graphs and references *