periodic law

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Chemistry The principle that the properties of the elements recur periodically as their atomic numbers increase.

Wiktionary

1. n. chemistry The principle that chemical properties of the elements are periodic functions of their atomic numbers

GNU Webster's 1913

1. n. (Chem.) See under Periodic.
2. adj. (Chem.) the generalization that the properties of the chemical elements are periodic functions of their atomic wieghts. “In other words, if the elements are grouped in the order of their atomic weights, it will be found that nearly the same properties recur periodically throughout the entire series.” The following tabular arrangement of the atomic weights shows the regular recurrence of groups (under I., II., III., IV., etc.), each consisting of members of the same natural family. The gaps in the table indicate the probable existence of unknown elements.

WordNet 3.0

1. n. (chemistry) the principle that chemical properties of the elements are periodic functions of their atomic numbers