Improved measurements of different elements and their isotopes have changed the official atomic weights of 19 elements, the International Union of Applied Chemistry and the U.S. Geological Survey announced today. The changes are relatively small, and they're part of a regular effort to update atomic weights.
Here's a vocab refresher in case you've forgotten your high school (or college) chemistry. Every atom of an element—let's take silver as an example—has the same number of protons. Silver has 47. However, not every atom of an element necessarily has the same number of neutrons. These different versions of an element's atoms are called isotopes. Silver occurs as silver-109 and silver-107. Chemists calculate the atomic weight of an element that you see on the periodic table from the masses of its isotopes, giving more common isotopes more weight than less common isotopes.