# How is the atomic mass of an element calculated?

##### 1 Answer
Jun 5, 2017

By comparison of its composition and mass to a “standard” Carbon-12 atom.

#### Explanation:

The current system of atomic masses was instituted in 1961 and is based on the mass of ${C}^{12}$ (read carbon twelve). By definition the atomic mass of a single 12C atom is exactly 12 atomic mass units (denoted by the abbreviation amu or u). The masses of all other elements are based on this standard.
Ref: http://www.chemistry.wustl.edu/~coursedev/Online%20tutorials/Atomic%20Mass.htm

"An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of 12C" in its nuclear and electronic ground state. (Refs. 1 and 2).

The "1999 values" were published in 2001 (ref. 12) and are entitled "Atomic Weights of the Elements 1999". These are available online (Refs. 13 and 14). Minor modifications (The "2001 values") to these are available online (Refs. 15 and 14).

Literature sources
1. P. de Bievre and H.S. Peiser for IUPAC in Pure & Appl. Chem., 1992, 64, 1535.
2. N.N. Greenwood and A. Earnshaw in Chemistry of the Elements, 2nd edition, Butterworth, UK, 1997.
3. Pure & Appl. Chem., 1999, 71, 1594-1607.
4. IUPAC 2005 Standard Atomic Weights
5. IUPAC 2007 Standard Atomic Weights Revised
old.iupac.org/news/archives/2007/atomic-weights_revised07.html
6. IUPAC 2005 Standard Atomic Weights Revised
7. Elements 116 and 118 (see retraction)
8. Elements 116 and 118
9. Elements 116 and 118
10. Element 115
11. Pure & Appl. Chem., 1996, 68, 2339-2359.
12. Pure & Appl. Chem., 1994, 66, 2423.
13. Pure & Appl. Chem., 1992, 64, 1519.
14. T.B. Coplen and H.S. Peiser for IUPAC in Pure & Appl. Chem., 1998, 70, 237.
15. T.B. Coplen for IUPAC, "Atomic Weights of the Elements 1999", in Pure & Appl. Chem., 2001, 73, 667-683.
16. http://www.chem.qmw.ac.uk/iupac/AtWt/.
17. Summary of Minutes of Commission Meeting at IUPAC General Assembly, Brisbane, Australia, 30 June-2 July 2001.

Retrieved from: https://www.webelements.com/periodicity/atomic_weight/