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. 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 in Pure and Applied Chemistry, are the first in an overhaul of the atomic weight of almost every element on the periodic table.
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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, long overdue, explicitly acknowledges the fact that most of the 118 elements come in multiple forms of varying weight.
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 isotope — called oxygen-16 — while the heavier versions exist in fractions of one percent.