Skip to main content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.


The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Atomic Weights and Isotopic Compositions with Relative Atomic Masses

Version History and References | Disclaimer

Developers and Contributors:

J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset
NIST Physical Measurement Laboratory

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 J. Meija et al in Atomic Weights of the Elements 2013, and the isotopic compositions data were published by M. Berglund and M.E. Wieser in Isotopic Compositions of the Elements 2009. The relative atomic masses of the isotopes data were published by M. Wang, G. Audi, A.H. Wapstra, F.G. Kondev, M. MacCormick, X. Xu1, and B. Pfeiffer in The AME2012 Atomic Mass Evaluation.

These data have been compiled from the above sources for the user's convenience and does not represent a critical evaluation by the NIST Physical Measurement Laboratory.

Search the Database

 Atomic Symbol or Number or  All Elements

Output Type HTML Table
Pre-formatted ASCII Table
Linearized ASCII Output
Isotopes  Most common isotopes
 All isotopes


Related Links:

Development of this database was funded in part by NIST's Systems Integration for Manufacturing Applications (SIMA) Program.
Online: September 1999 | Last update: January 2015


Created August 23, 2009, Updated November 25, 2019