Revised Periodic Table

Revised Periodic Table Photo Credit: Clipart.com

The Periodic Table of the Elements gets its first significant revision in over half a century.


Revamping a classroom icon. I'm Bob Hirshon and this is Science Update.

The Periodic Table of the Elements, seen on the walls of chemistry classrooms everywhere, is in for a historic change. For the first time, the atomic weights of ten elements, including hydrogen, carbon, and oxygen, will be stated as a range instead of a single number. Ty Coplen of the U.S. Geological Survey says that's because some elements exist in different forms, called isotopes, some of which are more abundant than others.

What we want to do is make that clear to students and teachers, that in fact these variations exist, and that atomic weights are not constants of nature.

The "we" he's referring to is the International Union of Pure and Applied Chemistry. Advances in research and technology have made it possible to pinpoint the upper and lower limits of atomic weight ranges, rather than just an average. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

Chances are, there's a Periodic Table of the Elements hanging in your science classroom. It may also be on the inside cover of your science textbook. As you've probably learned, it's a chart that includes an atomic number for each element (the number of protons it has), as well as an atomic weight. The definition of "atomic weight" is a little more complicated: it's the ratio of the mass of the total number of atoms in a given element, from a chosen source, to 1/12 the weight of carbon-12 atom. 

You may also know from science class that some elements exist in different forms, called isotopes. Isotopes have the same number of protons—and therefore are the same element—but different numbers of neutrons. A carbon atom, for instance, most commonly has 12 protons and 12 neutrons, but isotopes with 13 and 14 neutrons also exist in nature. And if an element has more than one stable isotope, its precise atomic weight may vary from one sample to another. So it's up to the International Union of Pure and Applied Chemistry (IUPAC) to set standard, average atomic weights for each element. These are the weights published in the Periodic Table, and are used as the basis for all sorts of scientific calculations. 

Now, as a step toward being more precise, IUPAC has decided to publish the full range of possible atomic weights for some of Earth's most abundant elements: hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine, and thallium. Scientists have always known these elements existed in a range of weights; what's new is that they can precisely state what the upper and lower limits of that range are. For example, sulfur's standard atomic weight is 32.065. However, its actual atomic weight can be anywhere between 32.059 and 32.076, depending on where the element is found. Different samples of elements have different ratios of isotopes within them; a sample with a slightly higher proportion of "heavy" isotopes will have a higher atomic weight.

The IUPAC scientists decided to do this, in part, for educational reasons; they want students to understand that atomic weights for some elements are not constant. The challenge will be for students to choose which atomic weight to use when making a calculation. If they make the right choice, they'll get a better answer than they would have using the standard weight.

Now try and answer these questions:

  1. What is the periodic table? What is an atomic weight? How does it differ from conventional weight (like the weight of a person)?
  2. Why is the atomic weight of some elements now stated as a range?
  3. Why was this not done before?
  4. What is the value of making this change to the table?

Going Further

For Educators

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Isotopes of Pennies
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Monogamous Voles
6-12 | Audio

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