The atomic weights of a clutch of chemical elements are to be revised under an IUPAC directive. Since it was first introduced in 1869, the periodic table of the chemical elements has been added to on many occasions. But, the latest changes will see the atomic weight values for a ten elements being replaced with a range of values to better accommodate their isotopic distribution. The elements that will be affected are: hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium. Elements with only a single isotope or with very rare isotopes, such as fluorine and gold, will retain their fixed weights.
One-off periodic revision