Atomic Weights to Change

The atomic weights of 10 elements are to be changed in order to more accurately reflect how these elements are found in nature. These 10 elements are:

• hydrogen
• lithium
• boron
• carbon
• nitrogen
• oxygen
• silicon
• sulfur
• chlorine
• thallium

The atomic weights of these 10 elements will now be expressed as intervals, having upper and lower bounds.
For example, sulfur is commonly known to have a standard atomic weight of 32.065. However, its actual atomic weight can be anywhere between 32.059 and 32.076, depending on where the element is found. In sports doping investigations, performance-enhancing testosterone can be identified in the human body because the atomic weight of carbon in natural human testosterone is higher than that in pharmaceutical testosterone.

Elements with only one stable isotope do not exhibit variations in their atomic weights. For example, the standard atomic weights for fluorine, aluminum, sodium and gold are constant, and their values are known to better than six decimal places.

IUPAC will feature the change in the standard atomic weights table as part of associated International Year of Chemistry activities in 2011.

Reference
Michael E. Wieser, Tyler B. Coplen. Atomic weights of the elements 2009 (IUPAC Technical Report). Pure and Applied Chemistry, 2010; 1 DOI: 10.1351/PAC-REP-10-09-14

Further Reading
http://www.ausetute.com.au/isotopes.html
http://www.ausetute.com.au/atomicmass.html

Study Questions

1. What is meant by the term isotope?
2. How is atomic weight calculated?
3. If naturally occurring hydrogen contains 99.99% hydrogen-1 and 0.01% deuterium, what is the atomic weight of naturally occurring hydrogen?
4. If you took an air sample from a planet on which there was 100 times more deuterium than on earth, what would you expect the atomic weight of hydrogen to be then?
5. Why is tritium not included in the calculation of the atomic weight of hydrogen?
   
6. Why would the atomic weight of an element with only one stable isotope be more consistent than the atomic weight of an element with two or more stable isotopes?
7. Why are unstable isotope abundances not used when calculating the atomic mass of a naturally occurring element?
8. Why would the atomic weight of carbon in natural human testosterone be higher than that in pharmaceutical testosterone?