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Atoms don鈥檛 always weigh the same
We like to think of the periodic table of the elements as immutable. It isn鈥檛. Its nether regions have for some time been filling up with new elements that physicists have forged from smaller atoms. Now even its more mundane areas, populated by familiar, everyday elements, are undergoing a fundamental change: elements are losing their precisely defined atomic weights.
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Atomic weight expresses the average mass of an element鈥檚 atoms relative to those of other elements. It is not to be confused with atomic number, the unvarying number of protons found in the nucleus of atoms of a particular element. The atomic weight adds the tally of neutrons to this, and that鈥檚 where the problems start: elements may come in different forms, isotopes, whose atoms contain different numbers of neutrons.
To reflect that, the guardians of the periodic table, the , calculated an average atomic weight based on the relative abundances of an element鈥檚 natural isotopes. Most hydrogen atoms, for example, have a nucleus that contains a single proton and nothing else, but a very few have one or two neutrons, too, leading to an official atomic weight of 1.00794 鈥� till now.
The problem with this approach, says Tyler Coplen of the US Geological Survey鈥檚 in Virginia, is that it perpetuates a misconception. 鈥淭eachers are teaching their students that atomic weights are fundamental constants of nature,鈥� he says. They are not: the ratio of the different isotopes of a particular element depends on the processes that created, transported or aggregated the material of which it forms part.
As water vapour circulates through Earth鈥檚 atmosphere from the equator to the poles, for example, molecules containing heavier isotopes of hydrogen fall back into the sea earlier. So the average weight of hydrogen atoms tends to be slightly higher in tropical waters than in seas near the poles. For different reasons, the average weight of the carbon atoms in a hydrocarbon called crocetane, seeping through the ocean floor off the coast of Alaska, is 0.01 per cent greater than the periodic table suggests it should be.
A continuous stream of new isotopic measurements meant . 鈥淚t was driving us crazy,鈥� says Coplen. And so it is all change. In December 2010 IUPAC stripped 10 of the most troublesome elements 鈥� including hydrogen, lithium, boron, carbon, sulphur and nitrogen 鈥� of their falsely precise atomic weights. Their weights now come as an upper and lower bound taking into account the spread in isotopic ratios in all known terrestrial samples. Hydrogen, for example, is 鈥淗 [1.00784; 1.00811]鈥� ().
Some elements will not be affected by this ongoing switch. Fluorine, aluminium, sodium, gold and 17 other elements have only one stable isotope, so their atomic weight really is a constant of nature. And some highly radioactive elements exist too fleetingly for their atomic weights even to be defined.
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