Gallium

Mendeleev predicted gallium in 1871 as eka-aluminium, giving its atomic weight and density in advance. When Lecoq de Boisbaudran found it spectroscopically in 1875 and reported a density of 4.7 g/cm³, Mendeleev wrote to tell him the measurement was wrong and should be near 5.9; Lecoq repurified his sample and got 5.9. That exchange did more than anything else to convince chemists the periodic table was a law rather than a table. Gallium melts at 29.76 °C, so it liquefies in a warm palm, and the gap to its 2400 °C boiling point is the widest liquid range of any element. Its compounds matter more than the metal: gallium arsenide for high-frequency and space-grade solar cells, and gallium nitride for the blue LED that earned Nakamura, Akasaki and Amano the 2014 Nobel Prize and made white LED lighting possible.