Atomic Elements Get a Weight Change

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Ten atomic elements are undergoing a change that will be listed on new periodic tables. Their atomic weights will be posted as intervals instead of a single value. For example, the atomic weight of boron (atomic number 5) is currently written as 10.811. On the new periodic table, it will be given as an interval—from 10.806 to 10.821. This might not seem like a big change—and it is very small—but such a change can be critical to calculations in scientific research and for industrial applications. Also, chemistry teachers and students will have to learn how to use the new weight intervals.

For the first time in history, the atomic weights of 10 elements, including nitrogen, will be expressed as intervals instead of single values.

The weights of the 10 following elements will be written as intervals: hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium. Notice that some of these are common in chemistry problems, so you will be hearing about these weight changes—if you haven’t already.

Why are these atomic weights now listed as intervals rather than a single number? Because of isotopes—atoms of the same element that have a different number of neutrons and so a different atomic weight. For example, carbon has three naturally occurring isotopes: carbon 12 (the most abundant), with six protons and six neutrons; carbon 13, with six protons and seven neutrons; and carbon 14, with six protons and eight neutrons.

The new atomic weight measurements not only account for the presence of isotopes but also consider their relative concentrations in the universe. Carbon 12 makes up 98.89% of all carbon, while carbon 13 is 1.11%, and the natural abundance of carbon 14 is 0.0000000001%. So, the weight interval for carbon will lean more heavily toward carbon 12 and range from 12.0096 to 12.0116. This range will replace the average atomic weight for carbon listed in any chemistry textbook, which is 12.011.

Why are such small differences important? For example, the atomic weight of carbon is smaller in performance-enhancing drugs than in natural testosterone. So, this difference can be used to test whether athletes used these drugs to improve their performance.

So, get ready for changes to the periodic table! Who knows what will change next?

—Roberta Baxter

Learn more:

Chemistry Concept

Periodic Table, Isotopes