Dmitri Mendeleev

Dmitri Mendeleev was a Russian chemist who in 1869 arranged the known chemical elements into a table ordered by atomic weight, grouping elements with similar chemical behavior into columns. Where the pattern demanded an element nobody had yet found, he left a gap and predicted the missing element's properties from its position rather than smoothing the gap away; three of those predictions, for elements he called eka-aluminium, eka-boron and eka-silicon, were matched within his lifetime by the discoveries of gallium in 1875, scandium in 1879 and germanium in 1886, whose measured densities, atomic weights and reactivity lined up closely with what he had forecast years earlier. That predictive success, rather than the classification itself, is why historians of science treat 1869 as a genuine theoretical achievement and not just a filing system. He never won a Nobel Prize, losing the 1906 chemistry prize by a single vote in a contested final round.