Scandium's importance far exceeds its modest uses, because its discovery vindicated the periodic table itself. In 1871 Dmitri Mendeleev, confident in the pattern of his system, had left a gap and predicted the existence of an undiscovered element he called eka-boron, specifying its likely atomic weight, oxide formula and properties.
When Nilson and, decisively, Per Teodor Cleve characterised scandium, its properties matched Mendeleev's forecast closely enough to be recognised at once as the missing eka-boron. Together with gallium and germanium, scandium became one of the celebrated confirmations that turned the periodic table from a useful arrangement into a predictive law of nature. Credit for scandium is properly split: Nilson isolated the new element and gave it its name, but it was his sometime colleague Cleve who worked out its atomic weight and formally identified it with Mendeleev's predicted eka-boron, sending his own confirming letter to the periodic table's author. Cleve had an extraordinarily productive year in 1879, also announcing the discovery of holmium and thulium within months of Nilson's scandium, so that three new elements emerged from Scandinavian rare-earth chemistry in a single burst. Mendeleev, for his part, treated the scandium match as decisive vindication of a table many contemporaries had still regarded as no more than a convenient filing system.