Periodic table gets some flex
Committee’s revisions give chemical isotopes their due
By Rachel Ehrenberg
RESETTING THE TABLE In a new version of the periodic table, the atomic weights of elements with more than one stable form, such as chlorine, are shown as a range, while elements with one stable form, like arsenic, have an exact weight.
Just as the weight listed on your driver’s license doesn’t necessarily reflect your actual poundage, the official atomic weights of the chemical elements are actually more like ballpark estimates than precise constants. In acknowledgment of this natural variation, the official weights of 10 chemical elements will no longer be expressed as single numbers, but as ranges. The adjustments, published online December 12 inPure and Applied Chemistry, are the first in an overhaul of the atomic weight of almost every element on the periodic table.
Instead of being described by a single fuzzy number, the atomic weights of oxygen, hydrogen, lithium, boron, carbon, nitrogen, silicon, sulfur, chlorine and thallium will now be expressed as intervals. The change brings long overdue recognition to isotopes, versions of the 118 elements that are lighter or heavier than their standard, more common counterparts.
Most elements have a preferred, energetically stable form that dominates in nature. For example, oxygen, the most abundant element in the Earth’s crust, is most comfortable having eight neutrons and eight protons in its nucleus (the latter of which defines it as oxygen). But oxygen can gain an extra neutron or two, changing the element’s weight (electrons are also variable but so light that their weight isn’t taken into account). These heavier versions, or isotopes, have been presented as existing in constant quantities no matter the source. For example, it’s commonly said that more than 99 percent of oxygen is the normal eight-neutron variety — called oxygen-16 — while the heavier versions exist in fractions of one percent.
But those proportions aren’t set in stone, and the new adjustment to the official weights acknowledges that, says Tyler Coplen, head of the U.S. Geological Survey’s Reston Stable Isotope Laboratory in Virginia.
For example, ratios of the three oxygen isotopes will differ depending on whether the oxygen is in air, groundwater, fruit juice or bone. This variation is what makes isotopes such a powerful scientific tool: the relative ratios of the different carbon isotopes can tell scientists if ivory came from an elephant that ate shrubby savanna plants or woody jungle trees. Similarly, testosterone supplements are plant-derived and have a different isotopic carbon signature than testosterone made by the body (to Tour de France cyclist Floyd Landis’ chagrin).
“Isotope studies extend from studies of previous climates to dating artifacts to weapons programs and biomedical applications,” says James Adelstein, a professor at Harvard Medical School and coeditor of a National Research Council report on isotopes in medicine and the life sciences.
Previously, a given element’s official atomic weight was actually an average of this variation. But as the number of discovered isotopes grew — there are more than 2,000, but only 118 elements — weights kept needing adjustment. These numerical tweaks implied that the numbers couldn’t be pinned down with precision, when in fact such measurements are more precise than ever, says Coplen, who headed the international task force charged with surveying various isotope abundances in nature so that the numbers could be revised.
“It should have been done a decade ago,” Coplen says.
Now that it has completed the initial round, the International Union of Pure and Applied Chemistry’s commission in charge of atomic weights will reassess the rest of the elements in the coming years. Gold, fluorine, aluminum and sodium, each of which exists in only one stable version, will be left alone.