The periodic table's shape encodes real physical law, not arbitrary organization

Kean explains that Dmitri Mendeleev's 19th-century arrangement of elements by increasing atomic weight, later refined to atomic number, wasn't just a convenient filing system but revealed genuine patterns in how elements behave. Elements in the same column, or group, share similar numbers of outer electrons, which drives similar chemical behavior — explaining why sodium and potassium both react violently with water despite being different elements entirely.

Mendeleev's boldest move was leaving gaps in his table for elements not yet discovered, correctly predicting their approximate weights and properties based on the pattern alone. When gallium and germanium were later found and matched his predictions closely, it validated the table as reflecting something real about atomic structure, not merely convenient bookkeeping.

Kean treats this predictive power as one of science's most satisfying validations: a pattern noticed in existing data successfully forecasting the properties of things nobody had yet seen. Takeaway: a truly good scientific model doesn't just explain what's known, it predicts what's still missing.