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 112, 114, & 116 and isotopic compositions or abundances are given when appropriate. The atomic weights data were published by Coplen [2] in Atomic Weights of the Elements 1999, (and include changes reported from the 2001 review in Chem. Int., 23, 179 (2001)) 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 [6] 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]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.

Coursey. J. S., Schwab. D. J, and Dragoset. R. A., NIST Atomic Weights and Isotopic Composition Database.