Nineteenth-century chemists faced a basic problem: dozens of known elements with no clear reason why some behaved alike while others didn't, and no agreement on how many elements even existed. Physicist Jim Al-Khalili traces the false starts and personal disputes that ran through this search for order, from early attempts to group elements by weight to the rival claimants who each thought they had cracked the pattern first. The centerpiece is Dmitri Mendeleev's periodic table, built not just to organize what was known but to predict elements nobody had found yet, leaving gaps that later discoveries filled almost exactly as he'd forecast. Al-Khalili visits period locations and handles historical apparatus and documents to walk through how the arguments actually played out among the scientists involved, rather than summarizing the outcome as inevitable. The episode treats the periodic table as a hard-won argument rather than a tidy chart, and it's presented as one installment in a longer series on the history of chemistry hosted by Al-Khalili for the BBC.