The periodic table of the elements just got a tiny bit heavier! On September 24, the International Union of Pure and Applied Chemistry (IUPAC) and the U.S. Geological Survey announced that they have changed the standard atomic weights of 19 elements (substances that consist of only one type of atom, or tiny particle).
The changes came after new research measured the weights of the elements more accurately. An element’s standard atomic weight is the average mass of all the atoms in a normal sample of the element. Atoms of an element have different versions, called isotopes. Each isotope of an atom has a slightly different mass. To get an average weight for the atoms of an element, scientists must determine how much of each isotope is present. This weight appears on the periodic table. An element’s weight is measured in unified atomic mass units (u). One u is equal to one-twelfth the mass of the most abundant kind of carbon (carbon-12).
Officials changed the atomic weights of four elements—molybdenum, cadmium, selenium, and thorium—because of new calculations of the amounts of different isotopes present. (The weights of molybdenum and thorium decreased, while the other two weights increased.)
The weights of the other 15 elements were changed because of better measurements of atomic mass. Some of those elements, such as holmium and thulium, increased in weight. Others, including aluminum and arsenic, decreased in weight. But all the weight changes were very tiny, and they resulted in a total weight increase of a mere 0.003640021 u for the periodic table.
The atomic-weight changes won’t likely have any major effects on the basic principles of science, and probably only the most detailed textbooks will change the values. But the changes could be useful for research. The more accurate measurements of isotopes could help scientists better date archaeological findings or trace materials in forensic investigations, for example.
“Knowledge of atomic masses is important to understand the laws of physics,” says Juris Meija, Secretary of the IUPAC Commission on Isotopic Abundances and Atomic Weights.
For that reason, the commission meets every two years to recalculate the atomic weights in the periodic table. The new atomic-weight changes will appear in the “Table of Standard Atomic Weights 2013,” to be published in the journal Pure and Applied Chemistry.