Periodic Law
The principle that the properties of the elements are periodic functions of their atomic numbers.
Unit
Dimensionless
In depth
Because the periodic table is built by repeatedly filling valence subshells, elements with matching valence configurations recur at regular intervals and show similar chemistry. Moseley's revision of Mendeleev's mass-based ordering to an atomic-number basis resolved the anomalous placements of argon–potassium and tellurium–iodine.
Examples in the real world
Ionisation energy, atomic radius and electronegativity all rise and fall in a repeating pattern across successive periods.