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Now this neat picture is being disrupted by superatoms - clusters of atoms of a particular chemical element that can take on the properties of entirely different elements. The chemical behaviour can be altered, sometimes drastically, by the addition of just one extra atom. "We can take one element and have it mimic several different elements in the periodic table," says Welford Castleman, an inorganic chemist at Pennsylvania State University who has studied the chemistry of aluminium superatoms.
It is a finding that is challenging our entire understanding of chemical reactivity. Adding superatoms to the periodic table would transform it from a flatland to a three-dimensional landscape in which each element is drawn out into a series of super-elements. Superatoms could have practical uses too: they could be combined into super-molecules to make new materials. And their unusual chemistry could be harnessed to make efficient fuels.
According to conventional thinking, the chemical properties of an atom depend on the way the electrons orbiting its nucleus are arranged in a series of shells. This in turn is determined by the number of electrons it possesses - just one in the case of hydrogen, for example, but up to 92 for an atom of the heavy metal uranium. The structure of the periodic table is explained by the gradual filling of the shells. Atoms with completely filled shells - the noble gases, such as helium, argon and xenon - are particularly unreactive. The most reactive elements are often those with atoms that are just one electron short of a filled shell and so occupy the column next to the noble gases in the periodic table, or those with one electron too many, which make up the left-most column of the table.