Atomic weights revision changes periodic table

By Adam Becker A magnificent period piece from mid-19th-century Russia has just received a bit of a renovation. Five elements at the heart of the periodic table will never look the same again, following an update to their atomic weights. “Your chemistry teacher probably said to you ‘atomic weights are constants of nature’, but nothing could be further from the truth,” says Tyler Coplen, director of the Reston Stable Isotope laboratory in Virginia. In fact, the atomic weight of some elements varies depending on where you are on Earth. Most of the mass of an atom resides in its nucleus, composed of protons and neutrons (with the exception of hydrogen, whose nucleus consists of a single proton). The number of protons in the nucleus determines what atom you are dealing with: all carbon atoms have 6 protons, all oxygen atoms have 8, and so on. But the number of neutrons can vary from atom to atom of the same element. Atoms of the same element but with different numbers of neutrons are called isotopes. Every element has a number of unstable isotopes, which break apart through radioactive decay, but some elements have more than one stable isotope too. This is where chemists run into problems when it comes to defining the atomic weight. Bromine, for example, has two stable isotopes that are roughly equally abundant on Earth. But the two are not equally dispersed: for instance, the heavier isotope of bromine is marginally more common in seawater and salts than it is in organic substances. Magnesium behaves in a similar way: there are three stable isotopes of this metal, and they vary slightly in abundance in different environments. This means that the average weight of a bromine or magnesium atom in these different environments varies very slightly too. This has led the guardians of the periodic table – the International Union of Pure and Applied Chemistry (IUPAC) – to decide that the atomic weights of bromine and magnesium are better accommodated by intervals than by single numbers. Bromine’s atomic weight has now switched from 79.904 to the interval [79.901, 79.907]. Magnesium’s atomic weight, meanwhile, was formerly 24.3050, but is now represented by the interval [24.304, 24.307]. While they were at it, the IUPAC also took the opportunity to tweak the atomic weights of three more elements: germanium, indium and mercury. This is not the first time that this has happened – and it is unlikely to be the last. Two years ago, the IUPAC replaced the atomic weights of 10 other elements with intervals, including some of the most common elements on Earth, such as hydrogen and carbon. Coplen, an author on the latest IUPAC report, says this will likely happen for other elements with more than one stable isotope in the future, as measurement devices get more sensitive. “The [IUPAC] commission is going to be meeting again this year in August,” says Coplen. “I expect there’s going to be several more elements that will change.” Journal reference: Pure and Applied Chemistry, More on these topics:
  • 首页
  • 游艇租赁
  • 电话
  • 关于我们