Gold, 18 Elements Get a New Atomic Weight Along with 18 other elements By Ruth Brown, Newser Staff Posted Sep 25, 2013 7:28 AM CDT Updated Sep 25, 2013 8:10 AM CDT 21 comments Comments (Shutterstock) (Newser) – Scientists made a weighty announcement this week: 19 elements on the Periodic Table, including gold, arsenic, and aluminum, have been assigned new atomic weights. Remember the glory days when cadmium was 112.411(8) atomic mass units? No longer. The International Union of Pure and Applied Chemistry says it's now 112.414(4) amu, reports LiveScience. Fans of the nonmetal selenium may note that this is the first time the element's weight has been revised since 1934. The adjustments were made thanks to better calculations and more precise measurements, and though they may seem tiny, they're a big deal to scientists. "Knowledge of atomic masses is important to understand the laws of physics," says an IUPAC spokesperson. "A good recent example of that was in 2005 when high-precision measurements of atomic masses allowed researchers to test the validity of the iconic 'E=mc^2.'" A full list of revised elements and their new weights is at the IUPAC.