Periodic Table

Definition

Mendeleev’s 1869 arrangement of the elements by mass and behavior that revealed a periodic, repeating rhythm of chemical properties.

Mendeleev arranged the roughly 63 known elements using only measured masses and observed behaviors, leaving gaps for elements that should exist, whose later discovery matched his predictions. We now know atomic number determines electron structure and valence electrons drive chemistry, making the table a map of behavior rather than a catalogue. Periods (rows) represent filling a new electron shell and groups (columns) collect elements with matching valence configurations and therefore matching chemistry.

Related concepts

Valence electrons · Atomic number · Periods and groups

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Part of The Blueprint of Water · 🧪 chemistry