Reviews The Periodic Table
Filling in the gaps
T
he Periodic Table is one of the ‘Big Ideas of Science’. Before the idea was conceived, chemists had no idea how many elements existed; something that was not finally clarified until Henry Moseley’s X-ray spectroscopy experiments in 1913. Some elements are so unreactive that they occur native or are easily obtained from mineral sources, so a small number were known to, and isolated by, the ancients, notably gold, silver, copper, iron, tin, sulfur, lead, carbon and mercury.
Up to the isolation of phosphorus in the late 17th century, a trickle of new discoveries occurred, before a steady stream from the mid-18th century. Often these discoveries accompanied a new scientific technique – the isolation of Group I and II metals, such as sodium and magnesium shortly after 1800, resulted from Humphry Davy’s application of electrolysis to decompose molten salts. Likewise, the application of spectral analysis in the second half of the 19th century awakened people to the existence of elements like rubidium and caesium. Laboratory techniques improved, allowing separations of the lanthanides, and the arrival of particle accelerators allowed nuclear synthesis to create elements unknown on Earth.
At the same time, the development of
more accurate balances led to chemists like John Dalton determining atomic weights of elements. Inevitably, the values suggested were generally not what we would now recognise; Dalton thought of water as HO, but his great achievement was to put in place a table of relative masses in 1804. Johann Döbereiner and later John Newlands were able to group elements with similar properties, but ultimately it was the work
of Lothar Meyer and Dmitri Ivanovich Mendeleev that produced the first recognisable Periodic Table of elements, organised by their chemical properties – even when that conflicted with mass order – a concept ultimately vindicated with Moseley’s work on atomic number. Mendeleev left spaces in the tables for elements yet to be discovered (as Scerri points out, so did Meyer); the properties of gap-fillers Ga (1875), Sc (1879) and Ge (1886) matched Mendeleev’s predictions, further support for the concept of periodicity, and by 1913 just seven gaps remained. A tale of 7 elements is concerned with these, in order of discovery: protactinium, hafnium, rhenium, technetium, francium, astatine and promethium.
Scerri sets out the personalities involved, their claims and why reputable scientists were often mistaken in their beliefs. Moseley had shown that there was a linear relationship between the frequency of X-ray emission from an element and the square of what we now call the atomic number. This meant that it was easy to predict the X-ray frequencies of the undiscovered elements in the gaps, and the ‘eye of faith’ sometimes saw lines, which no one else was able to detect, even for the non-radioactive Re and Hf. With the benefit of hindsight, we can see that often the possible abundance of the element in the ore
50 Chemistry&Industry • November 2013
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A tale of seven elements
Author Eric Scerri Publisher OUP Year 2013 Pages 2000 Price £12.99 ISBN 978-0-19- 539131-2
studied was so low that it could not have been detected. Of the remaining elements, Tc and Pm are radioactive metals, surrounded by stable metals, and this was not known to the researchers trying to find them. The final chapter is a useful, up- to-date summary of the elements after uranium, with claims now in place for the synthesis of all the elements up to 118 (and accepted with only three exceptions at present). The book would be a good read for any chemist.
Simon Cotton is an honorary senior lecturer in chemistry at the University of Birmingham, Edgbaston, UK
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