Nice piece from @GrrlScientist in the Guardian today (http://www.guardian.co.uk/science/punctuated-equilibrium/2011/jul/15/1?CMP=twt_fd), on one of the lesser-known elements, Scandium. She links to the @PeriodicVideos video on the element, where Martyn Poliakoff (who's worth watching for his hair, if nothing else) explains how Mendeleev predicted that an element with the properties of what is now known as scandium (Mendeleev called the undiscovered element eka-boron) would exist, based on the structure of his Periodic Table. The history of the makings of Mendeleev's table (published in 1869) is a wonderful statement of the need for creativity in science. Prior to Mendeleev's work, some chemists thought that the answer to ordering the elements lay with grouping by chemical properties, having observed similar reactions with certain triads of elements, such as the three known alkali metals (lithium, sodium, potassium). But this only worked with some of the elements, as lots were yet to be discovered. Dalton made an attempt to impose order on the unruly world of the elements based on mass and Berzelius set about measuring the relative atomic masses of all the known elements. However, other chemists disagreed with his values and it was not until Cannizzarro invented a new way of measuring atomic masses based on the new gas laws that chemists were really able to progress with ordering the elements. Despite this advancement, arranging the elements in order of relative atomic mass gave no discernable pattern in properties. However, in 1863, John Newlands noticed that the elements arranged by atomic mass showed repeating properties in octaves (8 elements). Newlands’ ideas were not well received and it took the work of Mendeleev to finally piece together an order for the elements. Critically - and here comes the genius of the man - Mendeleev’s approach was different from other scientists. Previous orderings were based either on atomic mass or properties - Mendeleev used both. The problem was that not all the elements had been discovered (only 63 by that time), but here came Mendeleev's second piece of genius: (apparently) he had a dream where he saw his completed table and from this, he realised that to make the table work, he had to leave spaces for elements that were unknown. This was an incredibly bold move and showed his trust in the predictive power of his Periodic Table. He was ultimately vindicated, through the discovery of elements such as scandium and gallium. Now his table hangs in nearly every science classroom in the world. Why? Well, for precisely the same reason as it became accepted by other chemists initially: its predictive power. You don't need to know the properties of every element, you simply find its place in the Periodic Table and you can work them out by knowing only a few basic trends in reactivity for Groups (columns) and Periods (rows). Thanks Mendeleev, for making chemistry easy!