The atomic weights of ten elements will no longer be expressed as a single number. Instead, those atomic weights will be expressed as a range. According to chemists at the International Union of Pure and Applied Chemistry’s Commission on Isotopic Abundancies and Atomic Weights, this should more accurately reflect the way the elements are found in nature.
The atomic weight of an element is the ratio of the mass of one atom of that element to 1/12th the mass of a carbon-12 atom. Some elements, such as gold, only come in one isotope, whereas others, such as hydrogen, can have a variable number of neutrons. This means that while every sample of pure gold weighs the same as any other sample, this is not true of hydrogen samples. Therefore, giving a single atomic weight for an element like hydrogen was misleading.
The elements with the new designations are hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium. These elements will now have their atomic weights listed as a range with upper and lower limits. That should make chemistry problems so much more fun for undergrads.
By the way, 2011 is the International Year of Chemistry, so start working on your costumes.