The heaviest element of all, oganesson (118), was created in 2002, though only named in 2016
. Though Dalton invented atomic theory, it was Berzelius who embedded it at the heart of the subject.
It had been apparent from the time of their discovery that sodium and potassium were similar, as were calcium, strontium and barium. Lithium, when discovered, proved similar to sodium and potassium. Likewise, bromine and iodine proved similar to chlorine. In 1829 Johann Dobereiner, a German, noticed a curiosity about these trios , and also another triplet that shared similar properties: sulphur, selenium and tellurium.
For a time, Mendeleev had worked in Germany with Bunsen and Kirchhoff, but he had fallen out with them and returned home. In 1869 he was professor of general chemistry at the University of St Petersburg and was writing a Russian-language textbook on the subject. On February 14th of the Julian calendar then in use in Russia , having addressed halogens and alkali metals, he was racking his brains for an organising principle to act as a template for the rest.
Crucially, Mendeleev was not constrained, as Newlands had been, by preconceptions about how things ought to be. At points where the octave rule did not work, he let the grid burst out of its corset. This can be seen at both the top and the bottom of the published table. The following year, J.J. Thomson worked out that “cathode rays” emitted into a vacuum by a negative electrode were electrically charged particles that weighed far less than any atom. Then, in 1899, Ernest Rutherford, a former student of Thomson’s, showed that Becquerel’s radiation had two components, which he dubbed “alpha” and “beta” .Becquerel himself, in 1900, showed that beta particles were the same as Thomson’s cathode rays.
The Bohr-Rutherford model of the atom had a problem, though. Electrostatic forces should pull the electrons into the nucleus rather than keeping them in orbit. Here, the new science of quantum mechanics came to the rescue. Quantum theory requires objects to be both particles and waves. The wavelike aspect of electrons means that when they circle an atomic nucleus they settle into self-reinforcing three-dimensional standing waves, called orbitals.
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