Chemists Want You to Know That Atomic Weights Aren't Constant: Revisions Reflect Variations in Abundances of Isotopes

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Just as the weight listed on your driver's license doesn't necessarily reflect your actual poundage, the official atomic weights of most chemical elements are actually more like ballpark estimates than precise constants. To better reflect reality, the weights of 10 chemical elements will no longer be expressed as single numbers, but as ranges.

The atomic weights of oxygen, hydrogen, lithium, boron, carbon, nitrogen, silicon, sulfur, chlorine and thallium will now be noted as intervals with upper and lower bounds, rather than as specific values. The adjustments, published online December 12 in Pure and Applied Chemistry, come as the first phase in an overhaul of the atomic weight of almost every element on the periodic table.

"It should have been done a decade ago," says Tyler Coplen, head of the U.S. Geological Survey's Reston Stable Isotope Laboratory in Virginia and coauthor of several of the reports that led to the current overhaul.

Many elements exist in more than one stable form--atoms of the variants have different numbers of neutrons in their nuclei. For example oxygen, the most abundant element in the Earth's crust, typically comes with eight protons in its nucleus (which defines it as oxygen) and eight neutrons. But oxygen can gain an extra neutron or two, changing the element's weight. (Electrons are so light that their weight isn't taken into account.) The different versions of such elements are known as isotopes, from iso, Greek for same, and topos, meaning place, because they share the same square on the periodic table.


Until now, determining the atomic weight of an element with more than one stable isotope entailed averaging the relative amounts of the different versions, boosting the uncertainty around each number. While most elements do have a preferred, energetically stable form that dominates in nature, an ocean of data over the last 50 years or so has made it clear that the abundance of each isotope differs depending on when and where you look.

Elements undergo what's called physical and chemical fractionation during processes such as going from liquid to solid. For example oxygen-18, or "heavy" oxygen, prefers cold things. So the ice cubes in your freezer will have considerably more oxygen-18 than the liquid water from which they formed, notes Coplen.

Similarly, the intense evaporation experienced by citrus trees leaves their cellular water rich in "heavy" hydrogen, or deuterium, yielding orange juice that may be enriched by as much as 4 percent relative to the environment. …