01 / The world before
Elements were known, but their relationships were harder to see.
02 / The discovery
Organizing elements by recurring properties created predictive structure.
03 / The people
Mendeleev and many chemists who revised the table.
04 / The obstacles
Some early placements needed correction as atomic knowledge improved.
05 / The world after
Chemistry gained a shared map of elements.
06 / Why it still matters
The table remains a working language for chemistry.